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Production Ecology of Three Short-Rotation Hardwood Species Across a Resource Gradient

Permanent Link: http://ufdc.ufl.edu/UFE0042317/00001

Material Information

Title: Production Ecology of Three Short-Rotation Hardwood Species Across a Resource Gradient
Physical Description: 1 online resource (135 p.)
Language: english
Creator: Henderson, Dawn
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: anpp, biomass, bottomland, content, efficiency, fertigation, fertilization, hardwood, lai, nue, nutrient, photosynthesis, platanus, populus, production, quercus, resorption, short, sla, srwc
Forest Resources and Conservation -- Dissertations, Academic -- UF
Genre: Forest Resources and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: PRODUCTION ECOLOGY OF THREE SHORT-ROTATION HARDWOOD SPECIES ACROSS A RESOURCE GRADIENT By Dawn Elizabeth Henderson December 2010 Chair: Shibu Jose Major: Forest Resources and Conservation The objective of this study was to examine the production ecology of three economically important hardwood species, Populus deltoides, Quercus pagoda, and Platanus occidentalis, to varying levels of resource application. Specifically, we wanted to determine how biomass production was altered, physiologically supported, and how nutrient uptake and storage within various biomass components were influenced by the applications of irrigation and the combined treatment of irrigation and nitrogen application at 56, 112, and 224 kg N ha-1 yr-1. Allometric equations were used for woody biomass production and litter traps were used to determine actual foliar biomass. Overall, our results suggest that biomass accrual is highly correlated with LAI; however, the relationship is species specific. Maximum biomass was reached well below the maximum LAI for P. deltoides, Q. pagoda and P. occidentalis. To determine how much light the canopy was capturing and the extent of which the treatments were influencing physiological mechanisms, photosynthesis data was collected for all three species. Leaves of which photosynthesis were measured were harvested, scanned, weighed, dried, and analyzed for nitrogen content. As expected, SLA, SLN, LAI, and Amax varied across the supplied resource gradient for all three species. Irrigation alone was sufficient in P. deltoides and P. occidentalis to increase SLA whereas SLA responded to both irrigation and fertilization in Q. pagoda. Amax reached peak rates for all species in the IRR+112 treatment. Finally, we wanted to determine the aboveground nutrient content and use efficiencies for these three species and understand the relationship between nitrogen, phosphorus, and potassium as it pertained to the resource gradient. We found that nutrient content, resorption efficiency and proficiency, leaf- and canopy-level nutrient use efficiency were not necessarily influenced by increased resource availability. While many plants have adaptations to conserve nutrients when nutrient levels are low, the available resources supplied by an abandoned agricultural field appear to be sufficient as to not alter the mechanism for nutrient conservation. Additionally, we found that maximum biomass production was not necessarily tied to maximum nutrient input. Understanding the interactions between the short-rotation woody species and intensive practices will assist development dedicated energy plantations.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Dawn Henderson.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Jose, Shibu.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042317:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042317/00001

Material Information

Title: Production Ecology of Three Short-Rotation Hardwood Species Across a Resource Gradient
Physical Description: 1 online resource (135 p.)
Language: english
Creator: Henderson, Dawn
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: anpp, biomass, bottomland, content, efficiency, fertigation, fertilization, hardwood, lai, nue, nutrient, photosynthesis, platanus, populus, production, quercus, resorption, short, sla, srwc
Forest Resources and Conservation -- Dissertations, Academic -- UF
Genre: Forest Resources and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: PRODUCTION ECOLOGY OF THREE SHORT-ROTATION HARDWOOD SPECIES ACROSS A RESOURCE GRADIENT By Dawn Elizabeth Henderson December 2010 Chair: Shibu Jose Major: Forest Resources and Conservation The objective of this study was to examine the production ecology of three economically important hardwood species, Populus deltoides, Quercus pagoda, and Platanus occidentalis, to varying levels of resource application. Specifically, we wanted to determine how biomass production was altered, physiologically supported, and how nutrient uptake and storage within various biomass components were influenced by the applications of irrigation and the combined treatment of irrigation and nitrogen application at 56, 112, and 224 kg N ha-1 yr-1. Allometric equations were used for woody biomass production and litter traps were used to determine actual foliar biomass. Overall, our results suggest that biomass accrual is highly correlated with LAI; however, the relationship is species specific. Maximum biomass was reached well below the maximum LAI for P. deltoides, Q. pagoda and P. occidentalis. To determine how much light the canopy was capturing and the extent of which the treatments were influencing physiological mechanisms, photosynthesis data was collected for all three species. Leaves of which photosynthesis were measured were harvested, scanned, weighed, dried, and analyzed for nitrogen content. As expected, SLA, SLN, LAI, and Amax varied across the supplied resource gradient for all three species. Irrigation alone was sufficient in P. deltoides and P. occidentalis to increase SLA whereas SLA responded to both irrigation and fertilization in Q. pagoda. Amax reached peak rates for all species in the IRR+112 treatment. Finally, we wanted to determine the aboveground nutrient content and use efficiencies for these three species and understand the relationship between nitrogen, phosphorus, and potassium as it pertained to the resource gradient. We found that nutrient content, resorption efficiency and proficiency, leaf- and canopy-level nutrient use efficiency were not necessarily influenced by increased resource availability. While many plants have adaptations to conserve nutrients when nutrient levels are low, the available resources supplied by an abandoned agricultural field appear to be sufficient as to not alter the mechanism for nutrient conservation. Additionally, we found that maximum biomass production was not necessarily tied to maximum nutrient input. Understanding the interactions between the short-rotation woody species and intensive practices will assist development dedicated energy plantations.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Dawn Henderson.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Jose, Shibu.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042317:00001


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1 PRODUCTION ECOLOGY OF THREE SHORT ROTATION HARDWOOD SPECIES ACROSS A RESOURCE GRADIENT By DAWN E LIZABETH HENDERSON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Dawn Elizabeth Henderson

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3 I want to t hank those women that have influenced my life become role models mentors friends, and have taught me to expect more f rom and for myself.

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4 ACKNOWLEDGMENTS I would like to thank my graduate committ ee chair Shibu Jose for so grac iously giving his time lending support, and not giving up on me. I would also like to thank my committee members (Dr s Eric Jokela, Hank Stelzer Kimberly Bohn, Francisco Escobedo, and Debbie Miller) for their contributions to this body of work and allowing me to pursue my degree I am grateful to my former f e llow graduate students that have moved on to new horizons : Dr. Diomy Zamora Sanjaya Ran asinghe M aheteme Gebremedhin Robert Wanvestraut, and Andrew Ruth for their assistance with field work Special thanks to Sara Merrit t Cathy Hardin LeWayne White and Chris Addison for their assistance with sample processing and to Dr s Kyehan Lee and Craig Ramsey for sharing their experiences and expertise with me. I would like to thank my husband Rob, for his understanding, love, and putting up with the life of a student and to my sister and best friend Greta Weatherly for her encouragement, gui dance and moral support This project was funded in part by the School of Natural Resources and Environment at the University of Florida, the William Paul Shelly Sr. Memorial Fund at the School of Forest Resources and Conservation and International Paper

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 A BSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 B ACKGROUND ................................ ................................ ................................ ...... 11 Introduction ................................ ................................ ................................ ............. 11 Literature Review ................................ ................................ ................................ .... 13 Environmental Limits on Productivity: An Overview ................................ ......... 13 Resource Availability and Ecophysiology ................................ ......................... 18 LAI Productivity Relationships ................................ ................................ ........ 21 2 B IOMASS PRODUCTION POTENTIAL OF THREE SHORT ROTATION WOODY CROP SPECIES UNDER VARYING NITROGEN AND WATER AVAILABILITY ................................ ................................ ................................ ........ 25 Methods ................................ ................................ ................................ .................. 28 Study Site ................................ ................................ ................................ ......... 28 Data Collection ................................ ................................ ................................ 29 Statistical Analysis ................................ ................................ ............................ 30 Results ................................ ................................ ................................ .................... 31 Survival ................................ ................................ ................................ ............. 31 Basal Area and Volume ................................ ................................ .................... 32 Foliar Biomass ................................ ................................ ................................ .. 33 Woody Biomass ................................ ................................ ............................... 33 Total Standing Biomass ................................ ................................ .................... 34 LAI ................................ ................................ ................................ .................... 35 ANPP ................................ ................................ ................................ ................ 35 Growth Efficiency ................................ ................................ ............................. 36 Discussion ................................ ................................ ................................ .............. 37 3 P RODUCTION PHYSIOLOGY OF THREE FAST GROWING HARDWOOD SPECIES UNDER VARYING NITROGEN AND WATER AVAILABILITY ............... 56 Methods and Materials ................................ ................................ ............................ 58 Study Site ................................ ................................ ................................ ......... 58 Data Collection ................................ ................................ ................................ 60 Statistical Analysis ................................ ................................ ............................ 62

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6 Results ................................ ................................ ................................ .................... 63 A max ................................ ................................ ................................ .................. 63 SLA ................................ ................................ ................................ .................. 63 SLN ................................ ................................ ................................ .................. 64 SLN vs. A max ................................ ................................ ................................ ..... 64 LAI ................................ ................................ ................................ .................... 64 ANPP and Canopy A max ................................ ................................ ................... 65 Discussion ................................ ................................ ................................ .............. 65 4 N UTRIENT USE EFFICIENCY OF THREE FAST GROWING HARDWOOD SPECIES ACROSS A RESOURCE GRADIENT ................................ .................... 78 Methods ................................ ................................ ................................ .................. 82 Study Site ................................ ................................ ................................ ......... 82 Data Collection ................................ ................................ ................................ 84 Analysis ................................ ................................ ................................ ............ 87 Results ................................ ................................ ................................ .................... 87 Nutrient Content ................................ ................................ ............................... 87 Nutrient Use, Re sorption Efficiency and Proficiency ................................ ........ 90 N:P ................................ ................................ ................................ ................... 90 N:P and ANPP ................................ ................................ ................................ .. 90 Discussion ................................ ................................ ................................ .............. 91 5 SUMMARY OF CONCLUSI ONS ................................ ................................ .......... 109 LIST OF REFERENCES ................................ ................................ ............................. 114 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 135

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7 LIST OF TABLES Table page 2 1 Volume, woody, and foliar biomass for all three species for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ............... 47 2 2 ANPP, LAI, and growth efficiency (GE) for P. deltoides ), Q. pagoda and P. occidentalis ................................ ................................ ................................ ........ 48 2 3 Aboveground net primary productivity (ANPP) of plantations and naturally occurring stands of P. deltoides Q. pagoda and P. occidentalis ....................... 49 3 1 Year six specific leaf area (SLA) for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ ........ 71 3 2 Year six ANPP of P. deltoides Q. pagoda and P. occidentalis .......................... 72 4 1 Average nutrient content for P. deltoides Q. pagoda and P. occidentalis ...... 101 4 2 Average % nutrie nt resorption efficiency and leaf level nutrient use efficiency for P. deltoides Q. pagoda and P. occidentalis ................................ ............... 104 4 3 Canopy nutrient use efficiency for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ ..... 105

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8 LIST OF FIGURES Figure page 2 1 Percent survival for P. deltoides Q. pagoda and P. occidentalis ...................... 50 2 2 Annual basal area accret ion for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ .......................... 51 2 3 Total (woody + foliar) standing biomass for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ ........ 52 2 4 ANPP and LAI for P. deltoides Q. pagoda and P. occidentalis ........................ 53 2 5 Growth Efficiency for P. deltoides Q. pagoda and P. occidentalis ................... 54 2 6 Yearly rainfall averages during the study, 1995 thro ugh 2003. ........................... 55 3 1 Average light saturated photosynthesis for P. deltoides Q. pagoda and P. occidentalis leaves. ................................ ................................ ............................ 73 3 2 Average specific leaf nitrogen for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ ........ 74 3 3 S pecific leaf nitrogen and light saturated photosynthesis for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ............... 75 3 4 Average leaf area index for P. deltoides Q. pagoda and P. occidentalis ......... 76 3 5 C anopy photosynthesis and aboveground net primary productivity for 6 year old P. deltoides Q. pagoda and P. occidentalis ................................ ............... 77 4 1 Average nutrient resorption proficiency of litterfall for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ .......................... 106 4 2 N:P foliar ratios for P. deltoides Q. pagoda and P. occidentalis f. ................... 107 4 3 Biomass production and foliar N:P for P. deltoides Q. pagoda and P. occidentalis ................................ ................................ ................................ ...... 108

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9 A bstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PRODUCTION ECOLOGY OF THREE SHORT ROTATION HARDWOOD SPECIES ACROSS A RESOURCE GRADIENT By Dawn Elizabeth Henderson D ecember 2010 Chair: Shibu Jose Major: Forest Resources and Conservation The objective of this study was to examine the production ecology of three economically important hardwood species, Populus deltoides Quercus pagoda and Platanus occidentalis to varying levels of resource application. Specifically, we wanted to determi ne how biomass production was altered, physiologically supported, and how nutrient uptake and storage within various biomass components were influenced by the applications of irrigation and the combined treatment of irrigation and nitrogen application at 5 6, 112, and 224 kg N ha 1 yr 1 Allometric equations were used for woody biomass production and litter traps were used to determine actual foliar biomass. Overall, our results suggest that biomass accrual is highly correlated with LAI; however, the relat ionship is species specific. Maximum biomass was reached well below the maximum LAI for P. deltoides Q. pagoda and P. occidentalis To determine how much light the canopy was capturing and the extent of which the treatments were influencing physiologica l mechanisms, photosynthesis data was collected for all three species. Leaves of which photosynthesis were measured were harvested, scanned, weighed, dried, and analyzed for nitrogen content. As expected, SLA, SLN, LAI, and A max varied across the supplie d resource gradient for all three species. Irrigation alone was

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10 sufficient in P. deltoides and P. occidentalis to increase SLA whereas SLA responded to both irrigation and fertilization in Q. pagoda A max reached peak rates for all species in the IRR+112 treatment. Finally, we wanted to determine the aboveground nutrient content and use efficiencies for these three species and understand the relationship between nitrogen, phosphorus, and potassium as it pertained to the resource gradient. We found that nutrient content, resorption efficiency and proficiency, leaf and canopy level nutrient use efficiency we re not necessarily influenced by increased resource availability. While many plants have adaptations to conserve nutrients when nutrient levels are low, the available resources supplied by an abandoned agricultural field appear to be sufficient as to not alter the mechanism for nutrient conservation Additionally, we found that maximum biomass production was not necessarily tied to maximum nutrient input. Understanding the interactions between the short rotation woody species and intensive practices will assist development dedicated energy plantations

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11 CHAPTER 1 BACKGROUND Introduction Land managers have realized the need to balance the increasing demand for wood products with environmental and recreational benefits of forests ( Steinbeck 1999, Stanosz 2000, Stanton et al., 2002, and Vogt et al., 2005 ). An increased need for hardwood pulp and short rotation woody crops (SRWC) for biofuel have led to innovative ideas to increase productivity on tree plantations while decreasing the harvesting pressures on natural forests (Kelty 2006) It has become apparent in recent years that management techniques such as fertilization and irrigation can increase biomass yields as much as four fold compared to traditional forest management practices. For example, Cobb et al. (2008) found significant biomass increases for sweetgum ( Liquidambar styraciflua L. ) and American sycamore ( Platanus occidentali s L. ) using a combined fertilization and irrigation system, called fertigation, in Georgia The economic and biological sustainability of a fertigation system depends on several factors such as species, fertilizer response, leaf area index (LAI), abov e and belowground carbon allocation, and insect or disease attacks. The knowledge to manage such a system for maximum growth efficiency ( GE), annual total tree biomass produced per unit leaf area) with the optimum level of resources (light, water, and nu trients) is just developing. For example, it is well known that productivity is closely correlated to leaf area (Waring 1983 Jose and Gillespie 1997 and Henderson and Jose 2005 ), and leaf area has been used for decades as a measure of forest productivit y. However, growth models incorporating leaf area have yet to be developed for many of the commercially important hardwood species.

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12 Tree leaf area regulates productivity through its influence on canopy light interception and resulting photosynthesis (Run ning et al. 1989 Will et al. 2005, Lhotka and Loewenstein 2009, and Motsinger et al. 2010 ). Variation in leaf area, light interception, and resulting productivity can be explained by site specific resource (light, water and nutrients) availability (Jose and Gillespie 1997 and Cobb et al 2008 ). However, the relationship between resource availability and resource use efficiency is seldom taken into account when making fertilizer or irrigation recommendations in intensively managed hardwood plantations. T he timing and quantity of fertilization and irrigation will play a crucial role in determining the leaf area index and canopy nutrient content of a stand, which in turn, will determine the canopy photosynthetic efficiency and the production potential. The re is a need to understand the temporal patterns of fertilizer and water requirements and resource use efficiencies of different hardwood species in order to estimate the proper rates and timing of application. Fertilizing and irrigating a forest stand of ten results in greater aboveground biomass production Coleman et al. 2006 and Cobb et al. 2008) However, how these practices influence belowground carbon allocation patterns is often overlooked. The belowground carbon allocation m ight also change in res ponse to variations in resource availability. According to Axelsson and Axelsson (1986) and Albaugh et al. (1998) reduced resource investment in fine roots is one of the most important mechanisms by which improved nutrient availability increases abovegrou nd biomass production. Hence, the proposed study was undertaken to examine the temporal variation in aboveground carbon allocation patterns in relation to leaf area development and associated nutrient and water use in three selected fast growing hardwood species

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13 cottonwood ( Populus deltoides Bartr. ex Marsh.), cherrybark oak ( Quercus pagoda Raf.), and American sycamore ( Platanus occidentalis L.) Literature Review Environmental Limits on Productivity: An Overview Primary productivity measurements are often described as the amount of carbon fixed minus respiration costs of maintenance and construction of new tissues (Pangle et al. 2009) Productivity can be measured by dry matter or biomass accumulation over time. Water and nutrient supply control can opy development and the relationship between above and belowground productivity (Loustau et al. 2001 and Binkley et al. 2010 ). Several techniques have been developed for quantifying aboveground productivity. Allometric biomass equations, measurements of physical processes, and eddy covariance have been common among them Allometric biomass equations require destructive measurements of individual components. Trees from several DBH (diameter at breast height) and height classes are harvested and weighed to develop components of the equations. Eddy covariance uses indirect measurements of climate and gas exchange within a forest stand to determine rates of productivity. Sensors at varying heights within the canopy collect the net uptake and release of CO 2 throughout the stand. Direct measurements of physical processes are nondestructive and are not subject to extreme extrapolation errors. Direct measurements of photosynthesis (A net P net or A max ) specific leaf area (SLA) leaf area index (LAI), water, and nutrient use can be combined to give an entire portrayal of primary productivity. While direct measurements are subject to potential bias, a more precise extrapolation can be obtained from direct measurements of individual trees. Direct measurements give a

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14 better understanding of the processes that can limit and alter primary productivity within a forest stand. Forest productivity and the rate of biomass produced per unit of carbon fixed are determined by leaf area. Likewise, assimilation rates gover n leaf initiation and expansion and thereby the rate of light interception. Light quality/quantity, CO 2 temperature, soil water, and nutrient availability can control photosynthesis and the resulting carbohydrate production on a unit area basis ( Foth 198 4, Kramer and Kozlowski 1979, Barnes et al. 1998, Kull and Niinemets 1998, Lambers et al. 1998, and Loustau et al. 2001 ). The product of leaf photosynthetic rate and total canopy leaf area can determine net canopy photosynthesis. Leaf area measurements a nd photosynthetic rates from selected sun and shade leaves will give net photosynthetic rates per unit leaf area. These measurements can be extrapolated to whole canopy measurements by estimating the total leaf area in the canopy of multiple trees. Supply ing required components that regulate assimilation can enhance productivity. Temperature, CO 2 and light quality/quantity are not easily manipulated. While light quantity can be increased with thinning after canopy closure (Lhotka and Loewenstein 2008 an d Motsinger et al. 2010) it is not always economically feasible. Light quality cannot reasonably be altered, while water and nutrients can. Fertilization experiments have shown how water and nutrients can control productivity of intensively managed tree plantations ( Dalla Tea and Jokela 1991, Kipp 1992, Albaugh et al. 1998, Leininger 2000, Loustau et al. 2001, Samuelson et al. 2001, Allen et al. 2002, Samuelson et al. 2007, and Cobb et al. 2008 ).

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15 Poor quality sites produce less dry matter than good sites because foliage cannot function as efficiently if one or more required resources are lacking. On poor quality sites, carbohydrate production may be shifted belowground in response to decreased water and nutrient supply. Deficiencies in water and nutrient s can slow photosynthesis and thus the creation of new tissues. For aboveground growth to occur belowground and respiration requirements must be met first When water supply is limited, photosynthesis is negatively affected as stomata close in response to water stress ( Kull and Kruijt, 1999 and Warren and Adams, 2001). Stomata closure decreases CO 2 uptake resulting in reduced carbohydrate production (Lambers et al. 1998). Water deficiencies will ultimately limit growth, as canopy expansion becomes seco ndary to increased root growth in response to decreased water supply. The result of prolonged water stress can be decreased aboveground productivity (Tschaplinski et al. 2006) and eventual plant death (Lambers et al. 1998). Either soil water holding capac ity or soil water supply must be increased to ensure adequate water supply for aboveground productivity. Increasing water availability can increase productivity of plants ( Kramer and Kozlowski 1979, Foth 1984, Barnes et al. 1998, Lambers et al. 1998 and Cobb et al. 2008 ). Barnes et al. (1998) reported that termination of shoot elongation occurred when soil water stress was severe for many hardwood species. Soil water replenishment though irrigation increases plant water balance, nutrient uptake Kreuzwi eser and Gessler 2010) growth rates and net primary production ( Birk 1997 Lambers et al. 1998 Tschaplinski et al., 2006, Karacic and Weih 2006, and Samuelson et al., 2007 ). Soil water also facilitates the uptake of nutrients that regulate photosynthesis stomatal

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16 closure, turgidity, and cell enlargement (Kramer and Kozlowski 1979). In a recent study by Samuelson et al. (2007), conducted to determine long term acclimation to irrigation and fertilization, irrigation was shown to increase aboveground bioma ss accrual. Their study suggested that while i rrigation and fertilization necessarily increased ab oveground biomass productivity, two and three fold, respectively, leaf area did not increase at the same rates suggesting the increased resources increased leaf level photosynthetic efficiency In a similar type study, Karacic and Weih (2006) found poplar clones increased two fold with the application of irrigation and fertilization with no significant differences in leaf area ration among treatments or clones. Cobb et al. (2008) found significa ntly greater leaf biomass with irrigation corresponding to an increase in aboveground biomass production for L. styraciflua and P. occidentalis Limited nutrient availability, especially nitrogen, can also decrease aboveground productivity ( Dalla Tea and J okela 1991 Zhang et al. 1997 and Jokela and Martin 2000). Jokela and Martin (2000) found that total aboveground biomass for seven year old loblolly pine ( Pinus taeda L.) and slash pine ( Pinus elliottii Englem.) grown in Florida was five and two fold hig her respectively for fertilized trees over control treatments. The trend of higher aboveground biomass for fertilized plots over control plots continued through age 16 with increases of nearly two fold for both species. Zhang et al. (1997) found that 25 year old fertilized P. taeda had both increased volume and increased DBH over control treatments the first year after fertilizer applications. They also found increases for needle length, fascicle number, and leaf area for the fertilized treatment. Dalla Tea and Jokela (1991) found increased productivity with fertilization for six year

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17 old P. taeda and P. elliottii They also found that aboveground biomass was increased over five fold for P. taeda and over two fold for P. elliottii with fertilization. In natural stands, nitrogen availability often depends on mineralization of nitrogen from decomposing litter by soil microorganisms. High rates of uptake and assimilation correspond to high rates of nitrogen mineralization. Without adequate nitrogen, fol iage cannot function efficiently in capturing light. Nitrogen is necessary for the production of chlorophyll and other enzymatic reactions that occur during photosynthesis. When soil nitrogen content and mineralization rates are low, foliar uptake maybe reduced resulting in lower canopy photosynthesis and biomass production. The greater the amount of foliage, the greater the amount of light captured. High rates of light capture increase carbohydrate production available to meet and exceed respiratory de mands. Fertilizers help to promote the growth and productivity of plants by adding essential nutrients. Nitrogen (N) phosphorus (P) and potassium (K) are the major limiting elements in both agricultural and forest productivity ( Lodhiyal and Lodhiyal 1997 and Johnson et al. 1998). Micronutrients such as iron, boron, manganese, zinc, copper, and molybdenum are also beneficial for plant growth (Lambers et al. 1998). Fertigation is the application of fertilizer introduced into an i rrigation system. Fertigation can ameliorate nutrient and water deficiencies. Nutrients frequently applied through fertigation include nitrogen, potassium, and phosphorus (Kipp 1992). Fertigation optimizes nutrient availability at the root zone. Direct application decreases nutrient leaching and runoff while increasing productivity. Fertigation applications show higher rates of productivity than broadcast fertilization with irrigation or irrigation alone (Kipp 1992).

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18 Paper and wood fiber industries and private landowners are beginning to implement fertigation on fast growing bottomland hardwood plantations (Leininger 2000). Several studies, ( Neilsen et al. 1997 Stiles 1997, Samuelson 1998, Weinert et al. 2000, and Samuelson et al. 2001 ) have tested the effectiveness of fertigation on aboveground biomass production. One response of increased nutrient availability was a shift in biomass allocation to aboveground and a decrease in belowground productivity ( Coyle and Coleman 2005) Because the nutrien ts are supplied directly at the base of the stem, root expansion required for nutrient and water uptake was decreased. Resource Availability and Ecophysiology Environmental variables such as water availability, which regulates leaf area, can influence fore st productivity (Gholz 1982). Aboveground n et primary productivity ( A NPP) shows a positive correlation with increased site water balance (Coyle and Coleman 2005) Because plant water balance controls stomatal conductance, the formation of hydrogen ion gr adients, and transporting photosynthate throughout the tree, measurements of water use efficiency can add to the understanding of plant productivity. Greater amounts of leaf area can affect the overall water balance by increasing transpiration rates but can also increase photosynthesis. Lockaby et al. (1997) found increased aboveground productivity when water stress was minimized by irrigation for cottonwood ( P opulus deltoides Bartr. Ex Marsh.) grown in the upper Coastal Plain of Alabama. Santana et al. (2000) found that greater aboveground productivity was strongly correlated to soil water availability for eucalypt plantations in Brazil. They also determined that nitrogen use efficiency (NUE ), the amount of biomass produced per unit of nutrient ) increa sed with increased water availability. In a similar study, Arp et

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19 al. (1998) found that when soil water was below optimal levels, NUE decreased as CO 2 uptake was reduced with stomatal closure. Because of decreased carbon assimilation, reduced growth rate s were found to be well correlated with decreased soil water. Santana et al. (2000) also reported that increased water availability increased leaf area, resulting in greater photosynthetic rates and net primary production. Jo se and Gillespie (1996) obse rved increases in SLA and LAI across an increasing moisture gradient in mixed hardwood stands located in the Midwest. A similar study by King et al. (1999) described incr eased leaf area with irrigated P. taeda Along with increased leaf area, Albaugh et al. (1998) and King et al. (1999) found decreased fine root production with irrigation. Albaugh et al. (1998) also found decreased leaf area during dry years when water availability was low. Wright et al. (2001) suggested that the cost of new leaf constr uction, on dry sites or in drought conditions, resulted in smaller leaves and reduced leaf area. Similarly, when water supply is limited canopy photosynthetic rates are presumably reduced due to decreased leaf area, reduced light captured per unit of leaf area, and rapid stomatal closure. A study by Samuelson (1998) found increased A net with irrigation for L. styraciflua and P. taeda which was attributed to decreased early stomatal closure. In times of drought, carbon allocation is shifted to root produc tion, increasing the root:shoot ratio (Loustau et al. 2001). The study by Arp et al. (1998) also suggests reduced water supply will decrease photosynthesis and aboveground production for a variety of species Kull and Kruit (1999) further support this th eory with the model they developed using data collected from European aspen ( Populus tremula

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20 L.) and filbert ( Corylus avellana L.) They estimated that both photosynthesis and positive carbon gain are slowed with water stress. Hardwood trees typically ha ve higher foliar nutrient concentrations than do coniferous species (Samuelson 1998) and are therefore more likely to grow on higher quality sites (Lockaby et al. 1997). Fertigation can increase soil nutrient availability required for nutrient demanding h ardwood trees (Samuelson 1998) allowing higher productivity levels in less than optimal environments. High soil nutrient levels have been shown to be correlated with A net and SLA for trees ( W ill et al. 2005) shrubs, C 3 and C 4 grasses (Wright et al. 2001). Jose and Gillespie (1996) suggested that canopy nutrients in a mixed mesic forest were well correlated with SLA. Knops and Reinhart (2000) found increased SLA and LAI with fertilization for C 3 and C 4 grasses in Minnesota. They also found increa sed foliar nitrogen levels and suggested higher rates of nitrogen cycling within their study site reduced the nitrogen cost for new leaf construction for some grass species. Allen et al. (2002) found that peak LAI for 15 year old P. taeda occurred with ir rigation and fertilization treatments. Despite planting on a sandy soil, t he LAI was over two fold higher for the combination of water and nutrient applications over the control treatments. Studies by Albaugh et al. (1998) and Warren and Adams (2001) fou nd that increased LAI was well correlated with increased nutrient and water availability as well as aboveground biomass production. A study by Jose and Gillespie (1996) suggested that leaves with lower SLA (scaled to the canopy level) from high in the cano py (domina nt and co domina nt species) would have higher photosynthetic rates than leaves growing in the shade. This idea was supported by the findings of Albaugh et al. (1996). They showed that

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21 increased photosynthetic rates and increased foliar nitrogen levels were positively correlated to increased A net Warren and Adams (2001) also support this finding. They found a strong positive linear relationship between increased light interception and A net Their study also showed increased nitrogen concentr ation per unit leaf area positively correlated with A net for maritime pine, ( Pinus pinaster Aiton) Reich et al. ( 1998b ) and Wright et al. (2001) also suggested that high rates of foliar nitrogen could be found in sun leaves owing to high photosynthetic rates. Anten et al. (1998) found increased photosynthetic nitrogen use efficiency with increased foliar nitrogen levels. Reich et al. ( 1998b ) sugge sted that leaf life span of several boreal tree species were shorter with high nutrient availability. Kaczmar e ck et al. ( unpublished data ) found a negative linear relationship with soil phosphorus availability and retranslocation and a positive relationsh ip with foliar concentrations for white oak ( Quercus alba L. ) in the Central Hardwood Region of the Midwest. They found mixed results for soil nitrogen availability with foliar concentration and retranslocation. Their findings suggest rates of mineraliza tion increases soil nutrient availability but may not decrease retranslocation due to high levels of competition for nitrogen in natural forests. LAI P roductivity Relationships Leaf production and the resulting leaf area are controlled by resource avai lability. In nutrient deficient soils, nutrients are allocated to root production for further nutrient capture. As resource allocation shifts to belowground, LAI is reduced. Limited leaf production results in reduced amounts of light capture and assimil ation rates resulting in less stem and wood production. Aboveground production is greater in fertile soils, or with fertilizer applications ( Coyle and Coleman 2005) Jokela and Martin (2000) found increased LAI and

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22 stemwood production for P. taeda and P. elliottii with fertilization. This increase was due to reduced resource allocation for root growth. The increase in resources at the leaf level results in greater amounts of leaf production and higher levels of light interception. High LAI values (high rates of intercepted light) indicate belowground resource needs are being met, allowing resources allocation to shift aboveground. Therefore, greater leaf area indicates a potential for greater amounts of light capture resulting in higher photosynthetic r ates and more carbon allocation for stem and wood production. Dalla Tea and Jokela (1991) also observed increased aboveground productivity with increasing LAI due to higher rates of light interception. They further suggested a strong linear relationship between growth and light interception. This relationship has been used to describe light use efficiency ( (LUE) the amount of carbon gain per unit of light intercepted by the canopy) Reduced available water and nutrients can decrease the slope of the lin ear relationship. Samuelson (1998) suggested that leaf area was more responsive to fertilization than A net Increased LAI may not translate into increased productivity above a threshold LAI. Jose and Gillespie (1997) found that although LAI increased acr oss a moisture gradient, foliage biomass remained relatively constant. They also found that the ratio between wood:leaf production was not significantly different across the moisture gradient for mixed hardwood stands in the Midwest. Sampson and Allen (1 995) suggested that increased self shading occurred with increased LAI. They suggest that productivity may not increase as leaf area increases unless all leaves are receiving adequate light to promote positive carbon gain. Wright et al. (2001) suggested SLA and LAI are at their highest when water and nutrient levels are optimal for a variety of trees,

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23 shrubs, and subshrubs in New South Wales. This study indicates that variability in resource use efficiency across an ecosystem will depend on the species w ithin the site. Much of the notable literature regarding tree productivity is centered on pine plantations It is well known that increasing aboveground biomass of pine plantations with fertilization and herbicide applications reduces harvest rotati ons and enhances profit. One area of research that needs more complete development is that of fast growing hardwood plantations. While there is some information available on this subject, it is limited in nature and not as extensive or well developed as that of pine plantations. Recently research has focused production of coppiced (Tharakan et al. 2003) and planted (Tharakan et al. 2005) willow ( Salix spp.) and poplar ( Populus ) clones in fertilized (Updegraff et al. 2004) and unfertilized, (Rytter and Stener 2005, Devine et al. 2010) studies to determine woody biomass production in SRWC systems Tharakan et al. (2003) suggested variability among Salix and Populus clones with respect to wo od quality and energy production for bioenergy sources. However, both Tharakan et al. (2003) and Devine et al (2010) suggest greater energy potential with multi clonal rather than single clonal tree crops. While previous research has suggested increased woody biomass production of some single clonal Populus plantations, Rytter and Stener (2005) suggest periodic thinning operations may further increase woody biomass production and thereby energy production. Upon investigating the economic feasibility of c o firing woody biomass with coal, Tharakan et al. (2005) suggested increased subsidies, tax breaks, and methods to maximize production may be needed to further interest in SRWC systems Understanding the requirements of resource additions and clonal selec tion for SRWC systems will be necessary for

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24 obtaining maximum biomass production for bioenergy and bioproducts purposes. As the amount of land available for hardwood harvesting decreases and the commercial need for hardwood for pulp and biomass/biofuel in creases, improving productivity of short rotation hardwood plantations become s more relevant. The following three chapters discuss the production ecology and ecophysiology of three commonly used short rotation hardwood plantation species in the U.S.

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25 CHA PTER 2 BIOMASS PRODUCTION P OTENTIAL OF THREE SHORT ROTATION WOODY CROP SPECIES UNDER V ARYING NITROGEN AND WATER AVAILABILITY Fast growing trees in short rotation woody crop (SRWC) systems may increasingly meet societal needs ranging from renewable energy to ecosystem services such as environmental mitigation and remediation (Rockwood et al. 2004 and Jose 2009) Trees have been identified as part of the bioenergy that investigated the feasibil ity of producing the estimated one billion dry tons of lignocellulosic biomass needed annually to meet the for a 30% replacement of the U.S. petroleum con sumption with biofuels by 2030. Demand for woody biomass for fuel or fiber combined with reduced land availability has forced land managers to increase productivity of SRWC systems. While some genetically improved pine species have shown superior growth r ates, (Jayawickrama 2001, South and Rakestraw 2002, and Xiao et al. 2003), production potential remains low without additional soil nutrient amendments. Fertilization has long been an answer to poor site quality and has become a readily employed silvicult ural tool for increasing productivity, especially in the southeastern U. S. (Allen 1987, Jokela et al. 1989, Zhang et al. 1997, and Scott et al. 2004). Much of the current research regarding fertilization rates and applications has focused on commercially important pine species such as loblolly pine ( Pinus taeda L.) and slash pine ( Pinus elliottii Englem.). Current research has focused primarily on growth response of pines to a single early application ; split application; and mid rotation application of various fertilizers (Jokela and Stearns Smith 1993, Jokela and Martin 2000, Will et al. 2002, Bekele et al. 2003, and Scott et al. 2004).

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26 Despite positive results reported from extensive fertilization research of pine plantations (Fox et al. 2007) few stu dies in the U.S. have focused on short rotation hardwood plantations with potential for bioenergy and biofuel applications. Recently, concerns about national energy security ha ve spurred incentives and policies encouraging renewable energy production ( Abt et al. 2010, Shiva n and Mehmood 2010, and Benjamin et al. 2009) New found interest in co firing wood in coal plants ha s led some states to enact a plant to generate electrical power with renewable energy sources (North Carolina General Assembly 2007). Much of the research regarding hardwood plantation production, specifically in the southeastern U.S., has focused mainly on the production response of hardwood coppice systems or to varying planting densities (Schlaegel 1981, Schlaegel and Wilson 1983, Sch laegel 1984a, b, c, Clatterbuck and Hodges 1987, Oliver et al. 1990, Steinbeck 1999, and Lockhart et al., 2003). More recent studies have concentrated on single fertilizer rates or the combined application of fertilizer with herbicide (Samuelson 1998 and Samuelson et al. 2001). Several studies have focused on the relationship between nutrient availability and mechanisms that influence biomass production in natural stands (Monteith 1972, Crow 1978, Nadelhoffer et al. 1983, Wang et al. 1995, Fassnacht and G ower 1997, Jose and Gillespie 1997, Reich et al. 1997, and Wright et al. 2001) and in plantations (Coleman et al. 1998, Samuelson 1998, and Green et al. 2001). For the best management of hardwood plantations as SRWC, it is not only useful to know productio n potential of the species, but also know the best combination and level of resources needed for the greatest return on investment. While it has been shown that increasing LAI increases light capture and is linked to improved

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27 ab oveground biomass productio n (Nadelhoffer et al. 1983, McCrady and Jokela 1996, Jose and Gillespie 1997, and Bolstad et al. 2001), the aboveground response in LAI and biomass is intrinsically linked to belowground resource availability (Rhodenbaugh and Pallardy 1993, Reich et al. 19 98a, Samuelson 1998, Chang 2003, and Albaugh et al. 2004). Many of the fast growing hardwood species that would be desirable for short rotation plantations such as sweetgum ( Liquidambar styraciflua L.; (Clatterbuck and Hodges 1987) green ash ( Fraxinus pe nnsylvanica Marsh.); (Kennedy 1988) nuttall oak ( Quercus nuttallii Palmer ); (Krinard and Kennedy 1981) are native to alluvial soils where water and where nutrients are readily accessible suggesting a greater demand for belowground resources may be require d to attain their maximum production potential. In the present study, we investigated the effect of water and nutrient availability, on survival, basal area (m 2 ha 1 ), volume (m 3 ha 1 ), standing biomass (Mg ha 1 ), aboveground net primary productivity (ANPP Mg ha 1 yr 1 where herbivory and litter from bark and branches were not calculated ), leaf area index (LAI m 2 m 2 ), and growth efficiency ( GE, Mg (ANPP) ha 1 yr 1 LAI 1 ), for three economically important hardwood species Populus deltoides Bartr. (c ottonwood) Quercus pagoda Raf. (cherrybark oak previously Quercus falcata var. pagodafolia Ell.) and Platanus occidentalis L. (sycamore), (nomenclature follows USDA, NRCS Plants Database 2009 ) Our objectives were to: 1) determine the level of resources needed for attaining the highest biomass production for each species, and 2) quantify the leaf area biomass relationship across the treatments We hypothesized that aboveground production would peak well below the maximum level of nutrient supply.

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28 Methods Study Site Our study was conducted in a fertigation trial established on an abandoned temperate with mild winters and hot, humid summers. Average rainfall is 1700mm with average minimum and maximum temperatures of 10 and 27 C respectively (NOAA, 2003) The soil is characterized as a well drained, Redbay sandy loam (a fine loamy, siliceous, thermic, Rhodic Paleudult) formed in thick beds of loamy marine deposits wi th an average water table depth of 1.8m (Lee and Jose 2003). Treatment plots of P. deltoides and P. occidentalis consisted of 40 trees plot 1 and Q. pagoda Q. pagoda contained 16 trees plot 1 ; (although the Q. pagoda plots were the smallest of the three species, and found for this study should be treated with caution, the results reflect data collected within the study). All treatment plots were planted at 2.13m X 3.35m spacing (1400 trees ha 1 ). The study design wa s a randomized complete b lock (RCB) with four replications of each treatment. Site preparation included disking and subsoiling to facilitate planting. Fertilization at the time of planting included broadcast application of diammonium phosphate, dolomitic lime, potash, and a micronutrient mixture. These treatments added elemental calcium, nitrogen, phosphorus magnesium, zinc, copper, and manganese ( 1009 50 56 1 26 3 3 and 2 kg ha 1 respectively Greg Leach, personal communication ). Soil pH was adju sted to 6.0, with 336 3 kg ha 1 of dolomitic lime, based on recommendations from a similar trial at North Carolina State University Research Cooperative (Coleman et al. 2004, Samuelson et al. 2004a, and Samuelson et al. 2004b). A combination of chemical ( s ulfometuron methyl and g lyphosate) and mechanical (mowing and manual

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29 pulling) treatments were used for weed control during the first and second growing seasons ( Greg Leach, personal communication ) Installation of the nutrient supply system and planting of trees occurred during spring 1995. The irrigation system operated for approximately two hours each day during the growing season (May Sep.) with nitrogen application occurring two to eight minutes each day (on average 390mm water, Greg Leach, personal communication) creating the nitrogen gradient across the treatments (Lee and Jose 2003; 2006). The f ive treatments created by the irrigation and fertigation system included a control (CON), irrigation only (IRR), and three nutrient supplements supplied th rough irrigation including 56, 112, and 224 kg N ha 1 yr 1 (referred to as IRR+56 IRR+112 and IRR+224 respectively). Data Collection Diameter at breast height ( DBH ) and height of all trees in each plot within each treatment were measured yearly. Basal area (m 2 ha 1 ), survival, and volume (m 3 ha 1 ) were calculated on a yearly basis, whereas standing biomass (Mg ha 1 ), ANPP (Mg ha 1 yr 1 excluding herbivory or litter of branches, bark, or fruits, as defined on page 9 ), LAI (m 2 m 2 calculated by multipl ying weight (g) and area (m2) of leaf litter collected in litter trays by SLA (m 2 g 1 ) of randomly selected canopy leaves, as defined on page 9 ), and total aboveground growth efficiency (Mg ha 1 yr 1 LAI 1 ANPP per year per LAI, as defined on page 9 ) wer e calculated for years six through eight Whole tree a llometric equations developed by Shelton et al. (1982) were used to calculate volume and aboveground woody biomass for P. deltoides Their equations for P. deltoides were developed from trees of compa rable age range, and soil type, grown in areas with similar longitude, latitude, and climate as this study. Standing woody biomass consisted of all woody components (bark, branches, and trunk/bole). Foliage biomass was

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30 determined by summing the weight of annual litter fall collected monthly (May to January) from five litter traps (0.5 m 2 ) for P. deltoides Biomass equations developed by Schlaegel and Kennedy (1986) were used to calculate volume and aboveground woody biomass for both Q. pagoda and P. occidentalis The original Schlaegel and Kennedy (1986) equations used diameter measured at approximately 15 cm above ground level. All Q. pagoda and P. occidentalis DBH data were corrected to reflect the dbh measurements of the equations at 15 cm height above ground level, by using regression equations developed from sampling 100 trees per species measured at the appropriate height (data not shown, R 2 = 0.97 and 0.93 respectively for Q. pagoda and P. occidentalis ). Foliage biomass was determined by summ ing the weight of annual litter fall collected from five and two litter traps (0.5 m 2 ), for P. occidentalis and Q. pagoda respectively in each plot. Projected LAI was calculated from the weight (g) and area (m 2 ) of the leaf litter trays and SLA (scaled to the canopy level m 2 g 1 ) for each species within each treatment Care was taken to ensure only leaf litter from the species within the plot was processed. If litter from other species fell or were blown into the tray, it was removed prior to collection Leaf litter was dried for 48 hours at 70 C and weighed to the nearest 0.01g. Statistical Analysis All the measured and calculated variables were compared among treatments using a repeated measure analysis of variance (ANOVA) (SAS Institute Inc. 2001) w ith among treatments were revealed, multiple pairwise comparisons of means were

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31 signif icance. Linear regression was used to analyze the relationships between LAI and ANPP and LAI and GE. Where no strong linear pattern emerged, appropriate curvilinear functions, using second order polynomial equations were fit to the data. It has been sug gested that as soil nitrogen levels increase, uptake of nitrogen can be limited by the availability of other nutrients (Aber et al. 1989). Additionally, b ecause of results from studies like Pastor and Bridgham (1999) and Bridgham et al. (1995) we hypothes ized that the highest rate of nitrogen application would be far greater t h an the trees could utilize As such c urvilinear functions were chosen a priori to ANOVA analysis and in accordance with our hypothesis that aboveground production variable responses were likely to plateau well below the maximum level of nutrients supplied by the treatments. Results Survival Among all three species, P. deltoides had the lowest survival rates throughout the study period (Fig 2 1 ), with the lowest rate occurring in the CON treatment. Survival for P. deltoides CON treatment decreased from 73 to 63 % from the beginning of the study through year eight. P. deltoides surv ival was greatest in the IRR treatment for all eight years ranging from 93 to 88 % P. occidentalis survival changed little until year four then decreased considerably in each treatment (Fig. 2 1 ). Like P. deltoides P. occidentalis survival was lowest i n the CON treatment and highest in the IRR treatment (Fig. 2 1 ). By year eight P. occidentalis survival was 66, 85, 77, 74, and 76 % (CON, IRR, IRR+56 IRR+112 and IRR+224 respectively). Q. pagoda survival rates were the highest of all three species during all eight years, with the lowest survival rate occurring

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32 in the IRR+56 treatment (83 % after year eight). Survival in the CON and IRR and IRR+112 and IRR+224 treatments for Q. pagoda reached 91 and 89 % respectively (Fig. 2 1 ). Basal Area and Volume Basal area for P. deltoides increased sharply through year five and then exhibited a slower rate of increase through year eight (Fig 2 2 ). Although the P. deltoides basal area in the CON treatment increased to 13.6 m 2 ha 1 in year five a slower r ate of increase occurred in year eight ( 17.5 m 2 ha 1 ) P. deltoides basal area for IRR, IRR+56 IRR+112 and IRR+224 reached a maximum at 24.6, 27.3, 25.7, and 25.6 m 2 ha 1 respectively). Basal area for Q. pagoda was minimal through year four, but incr eased substantially in all the treatments thereafter. Maximum basal area in year eight for Q. pagoda was 4.4, 7.4, 8.4, 9.8, and 12.9 m 2 ha 1 for CON, IRR, IRR+56 IRR+112 and IRR+224 respectively (Fig. 2 2 ). Basal area in the CON treatment was always lower than other treatments for all three species t hroughout the study. Similarly to P. deltoides P. occidentalis basal area increased sharply between years four and five, then more slowly through year eight. Year eight basal area for P. occidentalis wa s 11.2, 12.4, 13.8, 13.2, and 13.7 m 2 ha 1 for CON, IRR, IRR+56 IRR+112 and IRR+224 respectively (Fig. 2 2 ). Volume for each species (Table 2 1 ) during all three years followed the same trends as basal area. Volume was calculated as inside bark with equations developed by Shelton et al (1982) and Schlaegel and Kennedy (1986) for P. deltoides P. occidentalis and Q. pagoda respectively ). Maximum volume was obtained during year eight for all three species (2 19.6, 138.7, and 123.6 m 3 ha 1 respectively for P. deltoides

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33 P. occidentalis and Q. pagoda ). CON treatment volume remained consistently and significantly lower than either the IRR or the N treatments throughout the entire study. Foliar Biomass Foliar biomass production in the CON treatment was reduc ed for all species during the entire study period (Table 2 1 ) compared to the other treatments. Production of foliar biomass for P. deltoides peaked during years six and seven in the IRR+56 treatment and in the IRR t reatment during year eight (3.2, 3. 8, and 4. 7 Mg ha 1 respectively). Q. pagoda foliar production peaked during years six and seven in the IRR+224 treatment (2.6 and 3.1 Mg ha 1 respectively ) and in the IRR+112 treatment (3.6 Mg ha 1 ) du ring year eight, but was not significantly different from foliar production in the IRR or other fertigation treatments (Table 2 1 ). Peak foliar production for P. occidentalis was observed in either the IRR or IRR+224 treatments for years six, seven, and e ight (3.59, 4.86, and 5.48 Mg ha 1 respectively). Woody Biomass Woody biomass (bole, branches, and bark) for P. deltoides and P. occidentalis increased significantly between the CON and the IRR treatments during all three years (Table 2 1 ). However, the addition of N did not result in any further significantly different increases in woody biomass for P. deltoides Peak woody biomass for this species was found in the IRR+56 treatment for years six and seven (9.1 and 10.7 M g ha 1 respecti vely) and in the IRR treatment (12. 9 M g ha 1 ) in year eight. For Q. pagoda, the fertigation treatments increased woody biomass each year successively across the nutrient gradient (Table 2 1 ). Peak woody biomass for this species was found in the IRR+224 f or all three years (0. 9 1. 5 and 2. 4 Mg ha 1 for years six, seven and eight, respectively). Woody biomass for P. occidentalis showed significantly different woody

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34 biomass production between the IRR+56 and IRR+112 treatments with peak production occurring in the IRR+56 treatment for all three years (4. 2 4. 5 and 5.0 Mg ha 1 for years six, seven and eight, respectively). Total Standing Biomass As expected, throughout the study, total standing biomass (woody + foliage) was consistently and significantly low er in the CON treatment compared to the IRR and N treatments for all three species (). Total standing biomass for all three species in all three years showed the largest production values occurred during year eight (17. 6, 5.4, and 10. 4 Mg ha 1 Fig. 2 3 A, B, and C for P. deltoides Q. pagoda and P. occidentali s respectively). P. deltoides total standing biomass reached a plateau in the IRR+56 treatment during years six and seven (12. 4 and 14.5 Mg ha 1 respectively, Fig. 2 3 A) and in the I RR treatment in year eight (17. 6 Mg ha 1 ). Total standing biomass for Q. pagoda in the IRR+112 and IRR+224 treatments was significantly greater than the IRR and CON treatments in years six and seven (Table 2 1 ) During the eighth growing season, Q. pagoda total stan ding biomass reached a maximum (5.4 Mg ha 1 ) in the IRR+112 treatment (Fig. 2 3 B). P. occidentalis total standing biomass (Fig. 2 3 C) peaked in the IRR treatment during years six and eight (7. 4 and 10. 4 Mg ha 1 respectively) and in the IRR+224 treatment d uring year seven (9.0 Mg ha 1 ). However, the peaks in total standing biomass for years six and seven were not significantly greater across the IRR or N treatments. For year eight, P. occidentalis total standing biomass in the IRR treatment was only signi ficantly greater than the IRR+112 and IRR+224 treatments (Fig. 2 3 C).

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35 LAI No significant differences in one sided projected LAI for any of the three species were found between years seven and eight (Table 2 2 ). P. deltoides LAI was significantly lower in the CON treatment than in either the IRR or the N treatments in both years. However, during year eight, LAI in the IRR treatment was significantly higher than the N treatments. Q. pagoda LAI for year seven peaked in the hi ghest ( IRR+224 ) fertigation treatment ( 6.0 m 2 m 2 ) and during year eight, peaked in the IRR+56 treatment (6. 3 m 2 m 2 Table 2 2 ). During year seven, Q. pagoda LAI in the CON and IRR treatment were not significantly different and no significant increase in LAI was detected among the N treatments. In year eight, significant differences were found only for CON, IRR+56 and IRR+224 treatments (2. 5 6. 3 and 3.5 m 2 m 2 respectively). Peak LAI for P. occidentalis (9. 4 m 2 m 2 ) occurred in the IRR+224 treatment d uring year seven but was not significantly higher than LAI found in the IRR treatment. During year eight, no significant differences were found between the IRR and all of the N treatments with peak LAI occurring in the IRR+112 treatment (9. 7 m 2 m 2 Tabl e 2 2 ). ANPP The ANPP was significantly higher during year eight than year seven for all species and all treatments (Table 2 2 ). The CON treatment for all three species consistently had the lowest ANPP (2. 3 1. 5 3. 4 Mg ha 1 yr 1 in year seven and 3.1, 2.1, 2.8 Mg ha 1 yr 1 in year eight for P. deltoides Q. pagoda and P. occidentalis respectively, Table 2 2 ). For P. deltoides peak ANPP during year seven occurred in the IRR+56 treatment (5. 4 Mg ha 1 yr 1 ), but was not signific antly greater than the IRR or other N treatments. During year eight P. deltoides, ANPP in the IRR treatment was

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36 significantly higher than that observed in the N treatments (6. 7 vs. 5. 8 5. 5 and 5. 4 Mg ha 1 yr 1 respectively). During year seven, Q. pago da ANPP peaked in the IRR+224 treatment (3. 8 Mg ha 1 yr 1 ) and was significantly higher the IRR and IRR+56 treatments (2. 5 and 2. 6 Mg ha 1 yr 1 respectively). Q. pagoda ANPP during year eight peaked in the IRR+112 treatment but was not significantly greater than the IRR or N treatments. P. occidentalis ANPP peaked in the IRR+224 treatment (5. 2 Mg ha 1 yr 1 ) but was not significantly different from the IRR or other N treatments (Table 2 2 ). For year eight, P. occidentalis A NPP peaked in the IRR treatment (6.3 8 Mg ha 1 yr 1 ) and was only moderately but not significantly h igher than the N treatments (5. 8 5. 2 and 5. 4 Mg ha 1 yr 1 for IRR+56 IRR+112 and IRR+224 treatments respectively). Regression analysis indicated a signi ficant relationship between ANPP and LAI for all three species (Fig. 2 4 A, B, and C). For P. deltoides the relationship peaked near peak LAI values found in the IRR treatment (R 2 Q. pagoda and P. occidentalis ANPP peaked at LAI values well below the maximum calculated LAI values (Fig. 2 4 B and C, R 2 2 respectively) Growth Efficiency G rowth efficiency increased significantly between years seven and eight for P. deltoides and P. occidentalis (Table 2 2 ). However, no significant differences were found in growth efficiency across all treatments for both species within each year (Table 2 2 ). For P. deltoides GE ranged from 0. 7 Mg ha 1 yr 1 LAI 1 ( IRR+224 ) to 0. 9 Mg ha 1 yr 1 LAI 1 ( IRR+56 ) fo r year seven. During year eight, GE increased for eac h treatment and ranged from 0.9 Mg ha 1 yr 1 LAI 1 (IRR and IRR+224 ) to 1.1 Mg ha 1 yr 1 LAI 1 (CON) but showed no significant differences among all treatments. For, P. occidentalis GE

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37 ranged from 0.5 M g ha 1 yr 1 to 0.6 Mg ha 1 yr 1 (IRR and CON) in year seven and from 0.5 Mg ha 1 yr 1 to 0.71 Mg ha 1 yr 1 (CON and IRR) in year eight, indicating a reduction in GE in both the CON and IRR+112 treatments between the two years likely due to increased mortality and crown breakage Q. pagoda growth efficiency was lower in the N treatments compared to the CON and IRR treatments in year seven. However, during year eight, Q. pagoda GE was significantly higher in IRR+224 treatment (1. 1 Mg ha 1 yr 1 LAI 1 ). GE decreased with increasing LAI for all three species (Fig 2 5 ). P. deltoides growth efficiency decreased linearly with increasing LAI and showed a weak ly significant relationship between the two variables (R 2 0.20, p = 0.04). B oth Q. pagoda and P. occidentalis exhibited a curvilinear relationship with a more highly correlated association between the two variables (R 2 = 0.53, p = 0.001 and R 2 = 0.47, p = 0.006 respectively) For example, maximum growth efficiency for Q. pagoda and P. occidentalis w as observed at average LAI values of 4 m 2 m 2 and 9 m 2 m 2 respectively whereas individual species LAI reached a maximum near 8 m 2 m 2 and 12 m 2 m 2 It is likely that at the highest LAI values self shading occurred resulting in less efficient light capture and therefore reduced biomass accumulation. Discussion B asal area and survival for all three species suggests that stand development followed an e xpected, but distinct pattern for each species. Although P. deltoides mortality was the highest among all three species (Fig. 2 1 ) basal area was much greater than that of Q. pagoda or P. occidentalis (Fig. 2 2 ). Sulfometuron methyl was applied to inhib it herbaceous and woody competition. It has been suggested that s ulfometuron methyl presence in water and soil can range from 14 to 60 days and five

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38 to 33 days respectively (Buckwalter et al. 1996). We do not feel that mortality rates in this study were increased by the herbicide application, and possible presence in the soil and water, given that the highest mortality rates found in this study were in the CON treatment which did not have IRR or fertigation treatments. A study conducted by Netzer (1995) suggests that hybrid p oplar response to s ulfometuron methyl application ranged from slight decreases in growth to foliar damage. Additionally survival rates remained high throughout the study for Q. pagoda which had been subjected to the same herbicide applications as P. deltoides and P. occidentalis (Fig. 2 1 ). P. occidentalis mortality remained low during early stand development. Nevertheless, as basal area increased (Fig. 2 2 ) mortality remained relatively steady through year five. Lastly, because year eight woody biomass produc ed for P. deltoides in the CON treatment was comparable to P. occidentalis (Table 2 1 ) in the IRR and fertigation treatments we feel no deleterious effects of sulfometuron methyl on any of the three species. Although canopy differentiation between crown classes was not measured, observations within the stands suggested the canopy was stratifying into dominate, co dominate, intermediate, and over topped individuals. Mortality rates seen in P. deltoides towar d the end of the study were likely caused by death of intermediate and overtopped trees. Following a rapid increase in basal area through age five, basal area growth for both P. deltoides and P. occidentalis slowed and appeared to reach a plateau (Fig. 2 2 ). Despite reduced mortality for Q. pagoda basal area remained low suggesting a slower growth pattern, which was confirmed by the low ANPP values observed in our study (Table 2 2 ). The ANPP values suggest available growing space was more limiting for

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39 P. deltoides and P. occidentalis than for Q. pagoda despite the increase in Q. pagoda standing biomass each year and across successive treatments (Fig. 2 3 ). In a study designed to determine growth rates of various clonal Populus species, Devine et al. ( 2010) found that mortality was higher for more closely spaced individuals, but failure of an irrigation system may have confounded their results. Although, a study by Jokela et al. (2004) suggested that silvicultural treatments such as fertilization did not necessarily alter size density relationships and competition related mortality, growing space is likely to become limited much earlier in stand development when fertilization is used. This can be substantiated by higher mortality rates in the N treatm ents versus IRR treatments for all three species (Fig. 2 1 ). High mortality rates in the CON treatments for P. deltoides and P. occidentalis might be explained by inadequate water and nutrient supplies (Henderson and Jose, 2005). The below normal rainfal l (Fig. 2 6 ) experienced during this study likely altered mortality more so in the CON plots than in the IRR or N treatments. The IRR and N treatments likely supplemented yearly precipitation levels providing sufficient water resources to decrease mortali ty levels for P. deltoides and P. occidentalis which would be expected to require greater resources due to increased biomass production. However, DeBell and Whitesell (1988) suggested that self thinning and therefore mortality occurred at higher rates of planting densities for Eucalyptus saligna SM. (Sydney bluegum) regardless of stand age, site quality, or soil fertility level s. Their study further suggest s spacing standards to reduce mortality for this species given selected target diameters for harvesting. To expand on this idea, Goelz and Meadows (1997) suggested specific relationships between planting densities and mortality rates depending on stand

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40 man agement, length of rotation, production goals, and thinning schedules for various southern bottomland hardwood plantation systems. A study by Lockaby et al. (1997) found that IRR was not as beneficial for increasing P. deltoides biomass, as was the combina tion of IRR and fertilization treatments. In our study, P. deltoides height increased more than one meter and DBH increased more than 15 % with IRR and fertilization over the CON treatment (data not shown). In years six and seven of our study, P. deltoid es woody biomass (Table 2 1 ) in the IRR+56 treatment was slightly less than three fold of that found in the CON treatment. By year eight, woody biomass was greatest in the IRR treatment, slightly greater than threefold, but no significant differences were found between the IRR and N treatments. Production for natural stands, of similarly aged quaking aspen ( Populus tremuloides ) in northern Wisconsin, peaked at 25.0 Mg ha 1 (Ruark and Bockheim, 1988), which is comparable to the maximum found in one plot of the IRR treatment (24.3 Mg ha 1 individual data points not shown) in our study. In another study, involving quaking aspen, production varied from 0. 8 to 4.2 Mg ha 1 for second growth natural stand of varying ages (Crow, 1978). Although stem density was greater in the Crow (1978) study versus our study, production values for P. deltoides in the CON treatments (2. 8 to 7.2 Mg ha 1 ) were on the upper range of values reported by Crow (1978). In British Columbia, production for slightly younger stands of qua king aspen on mildly fertile soils growing in mixed stand of black and white spruce, reported by Wang et al. (1995), were similar to those we calculated during year six for the IRR treatment (7. 1 to 16. 6 Mg ha 1 ) in our study

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41 Lockaby et al. (1997) also f ound 80 % increase in DBH and height in response to the combination of irrigation and fertilization over the control treatment for P. occidentalis Our year six and seven woody biomass for P. occidentalis indicated a 50 and 60 % increase, respectively, in the IRR+56 treatment. While standing biomass for P. occidentalis was similar to Saucier et al. (1972) decreased survival (85 to 77% year seven and eight respectively) likely kept production from reaching its potential. Addi tionally, biomass was lost for this species between years seven and eight when top dieback occurred in several trees further reducing production potential. The loss of height can account for the small differences found among treatments, as allometric equ ations used in this study to determine standing crop, integrate height into the calculations for biomass (Schlaegel and Kennedy 1986). While we did not thoroughly investigate the cause of the dieback, Burns and Honkala (1990) indicate plantation grown P. occidentalis are susceptible to a host of diseases and insect infestations such as Xylella fasitiosa, which may increase susceptibility to other opportunistic infestations. X. fasitiosa causes xylem vessels to become blocked stopping water transport throu gh branches to leaves. This bacterium was considered the likely cause for the top dieback in P. occidentalis in our study. Additionally, Fiddler et al. (1989) suggested that for some species such as Pinus ponderosa Laws. (ponderosa pine) thinning of dens ely stocked stands not only reduces mortality rates, but likely reduces the rate of infestation from insects and pathogens. Unlike P. deltoides and P. occidentalis Q. pagoda production increased linearly with increased resource availability (Fig. 2 1 B). The different production responses between P. deltoides P. occidentalis and Q. pagoda can be explained in part by the

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42 silvical characteristics of the three species. P. deltoides and P. occidentalis are fast growing early successional species, both are described as intolerant of competition, are typically found in low lying areas, and often dominate floodplains because of profuse seed crops (Burns and Honkala, 1990). Q. pagoda is a much slower growi ng longer lived species and while it can be found in fertile bottomlands, it is generally found on the drier areas or ridges along floodplains, and is slightly more tolerant of competition than P. deltoides or P. occidentalis (Burns and Honkala, 1990). A ccording to our original hypothesis, peak biomass production for all three species was expected along the resource gradient at a point below the highest level of supplied nutrients. Standing biomass (Fig. 2 3 ) and ANPP (Table 2 2 ) of both P. deltoides and P. occidentalis reached peak biomass values in the IRR treatment during the last year of the study period. Additional N did not result in increased production for either of these species. Q. pagoda standing biomass (Fig. 2 3 ), on the other hand, showed positive response to both IRR and across successive N treatments. In general, both standing biomass and ANPP for Q. pagoda increased with increasing resources. Maximum standing biomass was observed in the IRR+224 treatment and maximum ANPP in the IRR+112 and IRR+224 treatments. Other measured variables for Q. pagoda such as basal area and volume followed similar trends (Fig. 2 2 and Table 2 1 respectively). ANPP and LAI were significantly correlated for all three species (Fig. 2 4 R 2 = 0.69, 0.77, and 0.75, p < 0.0001 for P. deltoides Q. pagoda and P. occidentalis respectively). The observed relationships are similar to findings from other studies from both natural stands and plantations (Fassnacht and Gower, 1997, Albaugh et al., 1998

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43 and Bolstad et al., 2001). The curvilinear relationship for all three species indicates that maximum ANPP is often reached at an optimum LAI and not at maximum LAI. This relationship also suggests that additional resources, beyond those affecting optimum LAI will like ly not result in increased aboveground biomass production. Other studies have also shown a curvilinear relationship between ANPP and LAI (Waring 1983, Jose and Gille spie 1997, and Fang et al. 1998 ). Increased LAI beyond this threshold can result in self shading and decreased canopy photosynthetic efficiency (Henderson and Jose, 2005). This was also revealed while examining the relationship between growth efficiency and LAI (Fig. 2 5 ). A negative linear relationship was found for P. deltoides and curvilinear relationships were observed for Q. pagoda and P. occidentalis with increasing LAI. This sugges ts that increased light capture allo wed by increased foliar biomass + does not necessarily translate into higher carbon allocation aboveground. A companion study by Henderson and Jose (2005) also revealed canopy photosynthetic thresholds suggesting no further increase in aboveground biomass production was observed for the same species. The lack of response of LAI and ANPP to increasing levels of N is perhaps an indication of sufficient mineralized N in the soil. For P. deltoides and P. occidentalis IRR alone was sufficient to increase LAI and ANPP to the peak values. In fact, P. deltoides ANPP and LAI exhibited significant decreases with the add ition of N. ANPP and LAI of Q. pagoda also decreased i n the IRR+224 treatment compa red to the other N treatments. P. deltoides and P. occidentalis ANPP followed the same trend as standing biomass with ANPP reaching a plateau in the IRR treatment. The ANP P values in our

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44 study are within the range reported in other studies (Table 2 3 ). For example, a study by Singh (1998) estimated average ANPP, for a 10 year old P. deltoides plantation, ranged from 1. 5 to 4. 9 Mg ha 1 yr 1 with a much higher planting densi ty (Table 2 3 ) than our study. In our study, we found ANPP rates for the same species to be much higher at 6. 8 Mg ha 1 yr 1 with a stocking density of 1400 trees ha 1 A study by Netzer et al. (2002) of hybrid Poplar clones report ed ANPP values ranging from 3.5 to 9. 5 Mg ha 1 yr 1 for eight year old plantations, planted at approximately 25 % greater density than our study. Ruark and Bockheim (1988) reported ANPP values of 5.9 Mg ha 1 yr 1 for eight year old P. tremuloides grown in the north central USA that are comparable to our findings. ANPP found in other studies (see Table 2 3 ; Bowersox and Ward 1976, Dawson et al. 1976, and Hopmans et al. 1990) were similar to our findings but with greater stocking densities. In a similar study, Strong and Hansen (1993) reported ANPP values for Populus clone NE 41, ranged from 4.9 to 6.3 Mg ha 1 yr 1 for an eight year old plantation. A few studies (see Table 2 3 ; Hopmans et al. 1990, Strong and Hansen 1993, and De Bell and Harrington 1997) reported higher ANPP rat es with greater stocking densities. P. occidentalis had ANPP values similar to those found in plantations or natural stand of similarly aged hardwoods (Table 2 3 ). For example, Saucier et al. (1972) found ANPP for four year old P. occidentalis ranging fr om 1.8 to 4.0 Mg ha 1 yr 1 In our study, maximum ANPP of eight year old P. occidentalis reached a maximum of 6.31 Mg ha 1 yr 1 Several studies found comparable (see Table 2 3 ; Coyle and Coleman 2005) or lower (see Table 2 3 ; Krinard and Kennedy 1981, T uskan and de la Cruz 1982 and Francis 1984, ) rates of ANPP for younger P. occidentalis planted at a lower rate of

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45 stocking density. Other studies with greater stocking densities found lower (see Table 2 3 ; Wood et al, 1976, Wittwer et al. 1978, Dickman e t al. 1985, Tschaplinski et al. 1991, and Steinbeck 1999), comparable (see Table 2 3 ; Wittwer 1980 and Tang and Land 1996), or higher (see Table 2 3 ; Steinbeck et al. 1972 and van Miegroet et al. 1994) rates of ANPP. These comparisons suggest the applicat ion of IRR and N treatments can increase site nutrient availability resulting in increased biomass with lower stocking density. Fewer trees ha 1 would suggest increased growing space and a delay in competition both above and below ground. ANPP values for Q. pagoda also followed the same trend found for standing biomass, with peak values observed in either the IRR+112 (year seven) or the IRR+224 (year eight) treatments. Although no comparable studies on Q. pagoda ANPP were found in the lite rature, values observed in this study were within the range reported for oak dominated natural stands. A study by Fassnacht and Gower (1997) reported ANPP values ranging from 3.3 to 4.0 Mg ha 1 yr 1 for natural stands dominated by Quercus species in the u pper Midwest. Additionally, Jose and Gillespie (1997) found ANPP for mature mixed hardwood stands that ranged from approximately 1.0 to 6.0 Mg ha 1 yr 1 Few studies have looked at simultaneous application of irrigation with fertilizer for growing SRWC on agricultural fields. While fertigation techniques are relatively infrequent, the resource gradient allows an opportunity to study resource requirements and mechanisms necessary for maximum aboveground biomass production. Given the data collected during our study and the current literature available for these three species, several generalizations regarding the use of P. deltoides Q. pagoda and P. occidentalis in short rotation plantations can be made. Our data indicate that plantation

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46 establishment fo r P. deltoides may not have to be restricted to bottomland sites for maximum production. The significant production response of P. deltoides to the IRR treatment and the lack of significant differences for the N treatments suggest that IRR alone may be su fficient to obtain maximum growth on good quality sites such as agricultural fields. P. deltoides may benefit from low levels of fertilization on low quality sites. Q. pagoda prove benefi cial for accelerating production in plantations. Its slow growth rate, compared to P. deltoides and P. occidentalis would otherwise preclude it from short rotation operations. Despite increased production in response to the IRR treatment, standing bioma ss for P. occidentalis was considerably lower than P. deltoides Disease, crown breakage, and mortality may have confounded the production response of P. occidentalis to cultural treatments. Overall, our results suggest that ANPP is highly correlated wit h LAI; however, the relationship is species specific given the differing growth rates among the three species Maximum ANPP was reached well below the maximum LAI for Q. pagoda and P. occidentalis P. deltoides ANPP was highest at the maximum LAI, which was achieved with IRR alone. These results suggest that species specific cultural practices that produce the optimum LAI and maximum ANPP need to be identified before fertigation techniques can be widely adopted for increasing biomass production potenti al of SRWC. The high biomass production potential of SRWC such as those tested in our study will play a significant role in helping to meet renewable energy standards.

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47 Table 2 1 Volume, woody, and foliar biomass for all three species for P. deltoides Q. pagoda and P. occidentalis and all five treatments control (CON), irrigation (IRR), 56 kg N ha 1 yr 1 ( IRR+56 ), 112 kg N ha 1 yr 1 ( IRR+112 ), and 224 kg N ha 1 yr 1 ( IRR+224 ). Upper case letters indicate significant differences between year and l ower case letters indicate significant differences among treatments within a year for ea ch species. Volume (m 3 ha 1 ) Woody Biomass (Mg ha 1 ) Foliar Biomass (Mg ha 1 ) P. deltoides Q. p agoda P. occidentalis P. deltoides Q. p agoda P. occidentalis P. deltoides Q. p agoda P. occidentalis Year 6 CON 90.63 Ab 12.34 Ad 65.26 Ac 3.25 Ab 0.08 Ad 2.81 Ac 1.61 Ac 1.36 Ac 2.66 Ad IRR 142.04 Aa 26.03 Ac 85.81 Ab 8.53 Aa 0.36 Ac 3.77 Aab 2.95 Aab 1.86 Abc 3.59 Aa IRR+56 163.30 Aa 33.10 Abc 97.85 Aa 9.14 Aa 0.50 Abc 4.18 Aa 3.21 Aa 1.95 Ab 2.82 Acd IRR+112 153.56 Aa 37.85 Ab 87.25 Ab 8.25 Aa 0.59 Ab 3.62 Ab 2.57 Aab 2.18 Aab 3.14 Abc IRR+224 150.29 Aa 52.97 Aa 96.95 Aa 9.09 Aa 0.88 Aa 3.90 Aab 2.36 Ab 2.62 Aa 3.29 Aab Year 7 CON 98.94 Bb 18.01 Bd 67.93 Bd 3.64 Bb 0.14 Bc 2.78 Bc 1.86 Bb 1.40 Ac 3.42 Bc IRR 155.19 Ba 40.07 Bc 96.82 Bbc 9.51 Ba 0.68 Bb 4.05 Bab 3.51 Ba 2.12 Ab 4.44 Bab IRR+56 188.32 Ba 50.66 Bbc 111.89 Ba 10.73 Ba 0.89 Bb 4.49 Ba 3.78 Ba 2.15 Ab 4.17 Bb IRR+112 167.70 Ba 55.75 Bb 95.65 Bc 9.13 Ba 0.99 Bb 3.65 Bb 3.46 Ba 2.57 Aab 4.36 Bab IRR+224 168.75 Ba 80.92 Ba 108.60 Bab 10.28 Ba 1.48 Ba 4.15 Bab 3.29 Ba 3.14 Aa 4.86 Ba Year 8 CON 111.72 Cb 30.98 Cc 82.18 Cc 4.22 Cb 0.51 Cc 2.42 Cc 2.52 Cb 1.77 Bb 3.16 Cc IRR 199.26 Ca 65.98 Cb 123.82 Cb 12.87 Ca 1.28 Cb 4.88 Ca 4.68 Ca 2.80 Bab 5.48 Ca IRR+56 219.61 Ca 77.11 Cb 142.02 Ca 12.60 Ca 1.41 Cb 4.97 Ca 3.89 Ca 3.09 Bab 5.22 Cab IRR+112 194.44 Ca 89.80 Cb 124.82 Cb 10.63 Ca 1.78 Cab 4.12 Cb 3.91 Ca 3.64 Ba 4.64 Cb IRR+224 195.67 Ca 123.56 Ca 138.71 Cab 11.69 Ca 2.38 Ca 4.55 Cab 3.99 Ca 2.84 Bab 4.92 Cab

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48 Table 2 2 ANPP, LAI, and growth efficiency (GE) for P. deltoides ), Q. pagoda and P. occidentalis and all five treatments control (CON), irrigation (IRR), 56 kg N ha 1 yr 1 ( IRR+56 ), 112 kg N ha 1 yr 1 ( IRR+112 ), and kg N ha 1 yr 1 ( IRR+224 ). significant differences among ANPP (Mg ha 1 yr 1 ) LAI (m 2 m 2 ) GE (Mg ha 1 yr 1 LAI 1 ) Year 7 P. deltoides Q. pagoda P. occidentalis P. deltoides Q. pagoda P. occidentalis P. deltoides Q. pagoda P. occidentalis CON 2.25 Ab 1.47 Ac 3.39 Ab 2.85 Ab 1.50 Ab 5.48 Ac 0.79 Aa 0.98 Aa 0.62 Aa IRR 4.48 Aa 2.44 Ab 4.73 Aa 6.37 Aa 3.05 Ab 9.08 Aab 0.70 Aa 0.80 Aa 0.53 Aa IRR+56 5.37 Aa 2.54 Ab 4.48 Aa 6.26 Aa 5.48 Aa 8.01 Ab 0.86 Aa 0.46 Ab 0.56 Aa IRR+112 4.34 Aa 2.97 Aab 4.39 Aa 5.62 Aa 5.58 Aa 7.49 Ab 0.77 Aa 0.53 Ab 0.59 Aa IRR+224 4.46 Aa 3.75 Aa 5.11 Aa 6.62 Aa 5.93 Aa 9.39 Aa 0.67 Aa 0.63 Aab 0.55 Aa Year 8 CON 3.10 Bc 2.14 Bb 2.81 Bc 2.76 Ac 2.46 Ac 5.46 Ab 1.12 Ba 0.87 Abc 0.51 Ba IRR 6.66 Ba 3.40 Bab 6.31 Ba 7.41 Ab 4.09 Aab 8.87 Aa 0.90 Ba 0.83 Aabc 0.71 Ba IRR+56 5.76 Bb 3.62 Bab 5.71 Bab 5.84 Aa 6.26 Ab 9.18 Aa 0.99 Ba 0.58 Ac 0.62 Ba IRR+112 5.41 Bb 4.43 Ba 5.11 Bb 5.94 Aa 4.71 Aab 9.68 Aa 0.91 Ba 0.94 Aab 0.53 Ba IRR+224 5.39 Bb 3.74 Bab 5.31 Bb 5.98 Aa 3.51 Aa 9.38 Aa 0.90 Ba 1.07 Aa 0.57 Ba

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49 Table 2 3 Aboveground net primary productivity ( ANPP ) of plantations and naturally occurring stands of P. deltoides Q. pagoda and P. occidentalis from published literature. Species Location ANPP (Mg ha 1 yr 1 ) Stocking (trees ha 1 ) Age (years) Reference P. deltoides Florida USA 6. 7 1400 8 This study P. deltoides Pennsylvania, USA 5.1 >21,739 4 Bowersox and Ward (1976) Populus clone ( Tristis Wisconsin, USA 2.5 31 338 1 Dawson et al. (1976) P. deltoides § Washington, USA 16 20 000 3 De Bell and Harrington (1997) P. deltoides Wodonga, Australia 11.7 620 4 Hopmans et al. (1990) P. deltoides Wodonga, Australia 6.8 6 620 4 Hopmans et al. (1990) P. deltoides Banthra, India 4. 9 1 666 10 Singh (1998) Populus clone NE 41 Wisconsin, USA 12.8 10,000 16 Strong and Hansen (1993) Q. pagoda Florida, USA 4.4 1 400 8 This study P. occidentalis Florida, USA 6.3 1 400 8 This study P. occidentalis South Carolina, USA 6.3 1 333 3 Coyle and Coleman (2005) P. occidentalis Georgia, USA 4.6 3 472 4 Dickmann et al. (1985) P. occidentalis Arkansas, USA 2.5 883 5 Francis (1984) P. occidentalis Mississippi, USA 2.4 1 076 5 Krinard and Kennedy (1981) P. occidentalis Georgia, USA 9.2 26 898 4 Steinbeck et al. (1972) P. occidentalis Georgia, USA 5.8 3 363 4 Steinbeck (1999) P. occidentalis Mississippi, USA 6.8 2 252 3 Tang and Land (1996) P. occidentalis Tennessee, USA 4.0 4 000 1 Tschaplinski et al. (1991) P. occidentalis Mississippi, USA 4.3 1 200 5 Tuskan and de la Cruz (1982) P. occidentalis Tennessee, USA 14.5 3 333 3 van Miegroet et al. (1994) P. occidentalis Kentucky, USA 4.1 37 037 3 Wood et al. (1976) P. occidentalis Kentucky, USA 3.4 5 978 5 Wittwer et al. (1978) P. occidentalis Kentucky, USA 6.5 6 050 5 Wittwer et al. (1980) §calculated average from de Bell and Harrington (1997).

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50 Figure 2 1 Percent survival for, P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) within each treatment

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51 Figure 2 2 Annual basal area accretion (m 2 ha 1 ) species, P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) within each treatment for each

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52 Figure 2 3 Total (woody + foliar) standing biomass (Mg ha 1 ) for years six, seven, and eight for P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) Capital letters indicate significant differences between years (p 0.05). Lower case letters indicate significant differences among treatments within a year (p 0.05).

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53 Figure 2 4 Relationship between ANPP (M g ha 1 yr 1 ) and LAI for P. deltoides (A) Q. pagoda (B) and P. occidentalis (C)

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54 Figure 2 5 Growth Efficiency (Mg ha 1 yr 1 LAI 1 ) and mean LAI (per species per treatment) for P. deltoides (A) Q. pagoda (B) and P. occidentalis (C)

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55 Figure 2 6 Yearly rainfall averages during the study 1995 (year 0 ) through 2003 (year 8) Dark bars indicate the annual rainfall and light bars indicate the addition of moisture from the fertigation system. H istoric annual rainfall average s near 1700 mm represented by the solid line

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56 CHAPTER 3 PRODUCTION PHYSIOLOG Y OF THREE FAST GROW ING HARDWOOD SPECIES UNDER VARYING NITROG EN AND WATER AVAILAB ILITY It is well known that a suite of canopy level mechanisms influence potential aboveground production and that e ach mechanism can influence the functionality of others. The highly correlated relationships between leaf area (LA), net photosynthesis ( P net P max A net or A max ), and foliar nitrogen (N) content can b e characterized mechanistically, either singly or in combination, to describe limitations placed on aboveground biomass accumulation. These mechanisms have been studied extensively, (Wang et al. 1991, Ellsworth and Reich 1992, Jose and Gillespie, 1997, Jo kela and Martin 2000, and Samuelson et al. 2001) and found to be the main constraints operating within tree canopies, influencing carbon fixation and allocation patterns (Tan and Hogan 1997, Albaugh et al., 1998, and King et al. 1999). The amount of LA ava ilable for light capture is controlled by both resource availability and growing space (Rhodenbaugh and Pallardy 1993, Samuelson 1998, Reich et al. 1998a Chang 2003, and Allen et al. 2004). Studies have shown a positive correlation between leaf area and water availability in both plantations and natural stands (Jose and Gillespie 1997, Albaugh 1998, and Stape et al. 2004). Decreased water availability can also restrict P net on a leaf area and weight basis, which often translates into reduced aboveground biomass (Pereire et al. 1992, Davis et al. 1999, and Samuelson 2000). It is well known that leaf gas exchange, and therefore intake of CO 2 decreases as water availability decreases (Rhodenbaugh and Pallardy 1993, Reich et al. 1999, and Gunderson et al. 20 02). However, if LA can be increased by increasing water availability, even without simultaneous increases in photosynthetic rates, the net result is an overall increase in the rate of canopy P net (Binkley et al. 2004). For

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57 instance, a study of Eucalyptu s spp. plantations conducted by Stape (2002) found that irrigation increased LA and thereby light capture resulting in a two fold increase in woody biomass increment over the control treatment while P net changed very little. Conversely, in a study of 4 y ear old sweetgum ( Liquidambar styraciflua L.), P opulus deltoides Bartr. and P latanus o ccidentalis L. Lockaby et al. (1997) found that irrigation alone did not translate into increased woody biomass. While specific site characteristics, species, and stag e of tree development can make comparisons between studies difficult, the above examples and other studies suggest that water may be limiting potential gains in aboveground biomass. Regardless of the amount of LA displayed, low photosynthetic rates, especi ally for slower growing species, may only be sufficient to fulfill energy demands of respiration and cell maintenance, which will inherently restrict potential aboveground growth (Lambers et al. 1998). When nutrients such as nitrogen are limiting for phot osynthetic processes, feedback mechanisms can function to increase belowground biomass enhancing resource capture (Lambers et al. 1998). Studies have shown that increased foliar N content is positively correlated with P net and subsequent increases in abov eground biomass (Monteith 1972, Coleman et al. 1998, Samuelson 1998, Green et al. 2001, and Wright et al. 2001). However, a study by King et al. ( 1999) indicated an increase in belowground biomass for 8 year old fertilized and irrigated loblolly pine ( P in us taeda L. ) over the control treatment, indicating for some species, shifts in biomass partitioning may be constrained by ontogeny as well as environmental conditions. Samuelson et al. (2001) found that the combination of irrigation and fertilization res ulted in increased growth, but not increased net photosynthesis or

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58 production efficiency for loblolly pine, suggesting belowground biomass may have increased with irrigation and fertilization. Given the above findings, further research is needed to unders tand the link between canopy processes and the interactions between leaf area, P net and foliar N concentration in regulating aboveground biomass production. Our study was designed to investigate the canopy processes regulating aboveground biomass production across a broad soil resource gradient. W e wanted to determine how photosynthesis, SLA (specific leaf area cm 2 g 1 or m 2 kg 1 ) SLN (g N m 2 ), and project ed LAI (m 2 m 2 ) would respond to increas ed resources. We hypothesized that by increasing water and nitrogen availability we would see a corresponding increase in SLA and SLN, which, in turn, would increase A max on a leaf area basis. Increased A max in com bination with an increase in LAI would result in greater aboveground biomass production along the increasing resource gradient. Methods and Materials Study Site Our study was conducted in a fertigation trial established on an abandoned agricultural field ( temperate with mild winters and hot, humid summers. Average rainfall is 1700mm, with average minimum and maximum temperatures of 10 and 27 C respectively (NOAA, 2003). The soil is cha racterized as a well drained, Redbay sandy loam (a fine loamy, siliceous, thermic, Rhodic Paleudult) formed in thick beds of loamy marine deposits with an average water table depth of 1.8m (Lee and Jose 2003). Treatment plots of P. deltoides (cottonwood) a nd P. occidentalis (sycamore) consisted of 40 trees plot 1 and Q uercus pagoda Raf., (cherrybark oak). Q. pagoda

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59 contained 16 trees plot 1 ; (although the Q. pagoda plots were the smallest of the three species, and found for this study should be treated with caution, the results reflect data collected within the study). All treatment plots were planted at 2.13m X 3.35m spacing (1400 trees ha 1 ). The study design wa s a randomized complete block (RCB) with four replications of each treatment. Site preparation included disking and subsoiling to facilitate planting. Fertilization at the time of planting included broadcast application of diammonium phosphate, dolomitic lime, potash, and a micronutrient mixture. These treatments added elemental calcium, nitrogen, phosphorus, magnesium, zinc, copper, and manganese (1009, 50, 56, 126, 3, 3, and 2 kg ha 1 respectively, Greg Leach, personal communication ). Soil pH was adju sted to 6.0, with 3363 kg ha 1 of dolomitic lime, based on recommendations from a similar trial at North Carolina State University Research Cooperative (Coleman et al. 2004, Samuelson et al. 2004a, and Samuelson et al. 2004b) and rem a ined near that pH lev el throughout the study (Lee and Jose 2003). Herbaceous weed control was attained with combinations of chemical (sulfometuron methyl and glyphosate) and mechanical (mowing and manual pulling) treatments during the first and second growing seasons. Instal lation of the nutrient supply system and planting of trees occurred during spring 1995. The irrigation system operated for approximately two hours each day (on average 390mm water Greg Leach, personal communication ) during the growing season (May Sep.) wit h nitrogen application occurring two to eight minutes each day creating the nitrogen gradient across the treatments (Lee and Jose 2003; 2006). The study design was a Randomized Block Design with four replications of each treatment. Five treatments were e stablished including control (CON), irrigated only

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60 (IRR), and three n itrogen supplements, (56, 112, and 224 kg N ha 1 yr 1 referred to as IRR+56, IRR+112, and IRR+224 ), supplied through irrigation. Treatment plots of P. deltoides ( Populus deltoides Bartr .) and P. occidentalis ( Platanus occidentalis L.) consisted of 40 trees per plot and Q. pagoda ( Quercus falcata var. pagodafolia ) had only 16 trees per plot due to space constraints. All treatment plots were planted at 2.13m X 3.35m spacing (1400 trees per ha). Data Collection Light saturated photosynthesis, A max ( mol m 2 s 1 ), was measured with a Li Cor 6400 portable infrared gas analyzer (Li Cor, Lincoln, NE, USA) during mid summer and peak foliar production, (June 26 through July 2, 2002; 6 year old trees). Relative humidity, temperature, and CO 2 were maintained at ambient levels with irradiance kept constant at 2000 mol m 2 s 1 by a red/blue LED light source. Samples were collected between the hours of 0800 and 1500. Measurements were made on fiv e randomly chosen sun (upper one third of the canopy and exterior position), and shade (lower one third of the canopy), leaves on two representative trees within each plot for each species and each treatment. All leaves sampled for photosynthesis were lab eled, stored on dry ice in the field and then refrigerated at the lab for not more than three days before analysis. Leaf area (cm 2 ) was determined by passing each leaf through a Li Cor LI 3100 Leaf Area Meter and then weighed to the nearest 0.01g. SLA wa s calculated by dividing the foliar area by weight. The foliar samples were dried at 70 C for 48 hours, ground to a fine powder and analyzed for total nitrogen (Kjeldahl), at the University of Florida Analytical Research Laboratory. Specific leaf nitrog en (SLN g N m

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61 2 ) was calculated using specific leaf weight (SLW g m 2 ) and leaf nitrogen concentration (mg N g 1 leaf weight). Canopy photosynthesis was calculated by slightly modifying the protocol of Herrick and Thomas (2003). Rather than multiplying th e mean sun and shade leaf A max by total leaf area, we multiplied mean sun and shade A max by LAI. This is different from calculating the whole canopy photosynthesis of individual trees; however, it gives a measure of canopy photosynthesis per square meter of ground area. For biomass calculations, diameter at breast height ( DBH ) and height of all trees in each plot within each treatment were measured yearly. S tanding biomass (Mg ha 1 ), ANPP (Mg ha 1 yr 1 excluding herbivory or litter of branches, bark, or fruits, as defined on page 9), LAI (m 2 m 2 calculated by multiplying weight (g) and area (m2) of leaf litter collected in litter trays by SLA (m 2 g 1 ) of randomly selected canopy leaves, as defined on page 9), were calculated for year six Whole tree a l lometric equations developed by Shelton et al. (1982) were used to calculate volume and aboveground woody biomass for P. deltoides Their equations for P. deltoides were developed from trees of comparable age range, and soil type, grown in areas with simi lar longitude, latitude, and climate as this study. Standing woody biomass consisted of all woody components (bark, branches, and trunk/bole). Foliage biomass was determined by summing the weight of annual litter fall collected monthly (May to January) f rom five litter traps (0.5 m 2 ) for P. deltoides Biomass equations developed by Schlaegel and Kennedy (1986) were used to calculate volume and aboveground woody biomass for both Q. pagoda and P. occidentalis The original Schlaegel and Kennedy (1986) equa tions used diameter

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62 measured at approximately 15 cm above ground level. All Q. pagoda and P. occidentalis DBH data were corrected to reflect the dbh measurements of the equations at 15 cm height above ground level, by using regression equations developed from sampling 100 trees per species measured at the appropriate height (data not shown, R 2 = 0.97 and 0.93 respectively for Q. pagoda and P. occidentalis ). Foliage biomass was determined by summing the weight of annual litter fall collected from five and two litter traps (0.5 m 2 ), for P. occidentalis and Q. pagoda respectively in each plot. Projected LAI was calculated from the weight (g) and area (m 2 ) of the leaf litter trays and SLA (scaled to the canopy level m 2 g 1 ) for each species within each treatment. Care was taken to ensure only leaf litter from the species within the plot was processed. If litter from other species fell or were blown into the tray, it was removed prior to collection. Leaf litter was dried for 48 hours at 70 C and weighed to the nearest 0.01g. Statistical Analysis All the measured and calculated variables were compared among treatments using analysis of variance (ANOVA) for a randomized block design (SAS Institute Inc. 1999) for each speci es. If significant differences ( = 0.05) among treatments were multiple range test for mean separation. Linear regression was used to analyze the relationships between A max an d SLN. It has been suggested that as soil nitrogen levels increase, uptake of nitrogen can be limited by the availability of other nutrients (Aber et al. 1989). Additionally, b ecause of results from studies like Pastor and Bridgham (1999) and Bridgham et al. (1995) we hypothesized that the highest rate of nitrogen application

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63 would be far greater than the trees could utilize. As such, curvilinear functions were chosen a priori to ANOVA analysis and in accordance with our hypothesis that physiological res ponses and the relationships with growth parameters would likely to plateau well below the maximum level of nutrients supplied by the treatments, specifically, the relationship between canopy A max and ANPP. Results A max A max reached peak rates for all thr ee species in the IRR+112 treatment for both sun a nd shade leaves (Fig. 3 1 ). Overall, P. deltoides had the highest rate of photosynthesis for sun leaves ( 34.0 mol m 2 s 1 Fig 3 1 A) with Q. pagoda and P. occidentalis having slightly lower photosynthetic rates (31.6 and 28. 6 mol m 2 s 1 respectively, Fig 3 1 B and 3 1 C). The same trend was observed for photosynthetic rates of shade leaves. The lowest photosynthetic rates for shade leaves were observed in the CON tre atment for all three species, except for P. deltoides where CON and IRR only treatment had similar levels of A max SLA Shade leaves had higher SLA than sun leaves for all species in all the treatments as expected (Table 3 1 ). The response of sun or sh ade leaf SLA to individual treatments, however, differed significantly. For example, P. deltoides SLA, for sun leaves, was greatest in the IRR only treatment (19. 1 m 2 kg 1 scaled to canopy level from cm 2 g 1 ). Although the N treatments increased SLA of sun leaves compared to CON the values were significantly lower than the SLA of the IRR only treatment. P. occidentalis followed similar trends with both IRR and fertilization having a major influence on sun and shade leaf SLA. Sun leaf SLA of Q. pagoda however, did not

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64 show any response to the IRR only treatment. Rather, a significant increase (32%) was observed only in the IRR+56 treatment compared to the CON For shade leaves, SLA was higher for the three N treatments compared to the CON and IRR on ly treatments. SLN The trend for sun and shade leaf SLN, for all three species, was similar with shade leaves consistently exhibiting lower SLN irresp ective of the treatments (Fig. 3 2 ). P. deltoides SLN for sun and shade leaves (1.7 and 0. 7 g N m 2 res pectively) peaked in the IRR+112 treatment, with no significant differences found among the CON IRR IRR+ 56, or IRR+224 treatments. However, SLN for Q. pagoda and P. occidentalis (1.9 and 1. 1 g N m 2 for Q. pagoda and 1.5 and 0. 9 g N m 2 for P. occident alis sun and shade leaves, respectively) attained a plateau in the IRR+56 treatment. Apparently, addition of N beyond 56 kg ha 1 did not result in any appreciable increase in SLN in both sun and shade leaves of both species suggesting N demand may have be en met by the residual fertility within this abandoned agriculture field SLN vs. A max A max was positively correlated to SLN in all species and for both sun and shade leaves (Fig 3 3 ). The linear relationships clearly indicate that nitrogen limitation to photosynthesis was occurring in the CON treatment, but N fertilization helped to alleviate this with an increase in SLN. For example, an increase in sun leaf SLN from 0.5 to 1.5 g N m 2 in P. deltoides resulted in 84.4% increase in A max Similar increases were observed in both Q. pagoda and P. occidentalis LAI Both P. deltoides and P. occidentalis LAI reached a plateau in the IRR only treatment, with no significant differences foun d between the IRR and fertilization

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65 treatments (Fig 3 4 ). In other words, addition of N did not result in any increase in LAI in these species. However, Q. pagoda LAI responded to both IRR and fertilization. The N treatments resulted in significantly h igher LAI in Q. pagoda compared to the CON and IRR treatments. Addition of N beyond 56 kg, however, did not result in any further increase in LAI. ANPP and Canopy A max ANPP for both P. deltoides and P. occidentalis was significantly higher in the IRR and fertigation treatments compared to the CON However, a plateau in ANPP was reached in the IRR only treatments for P. deltoides and P. occidentalis respectively (Table 3 2 ). The intermediate and highest levels of fertilization yielded the highest ANPP for Q. pagoda with no significant differences among them. CON treatment had the lowest canopy A max for all three species and showed an increasing trend with increasing resources (Fig 3 5 ). Canopy level photosynthesis and ANPP exhibited significant curvilinear relationships for all three species. As is evident from Fig 3 5 ANPP reached a plateau at about 60% of the maximum canopy A max for all three species. Discussion One of the mechanisms studied extensively, to explain production differences in forest stands, is the relationship between foliar biomass or area and light capture (Dalla Tea and Jokela 1991, Pereira et al. 1992, Samuelson 1998, King et al. 1999, Stape et al. 2004 and Wilson and Maguire 2009 ). Increased leaf area with increased reso urce availability, either natural or created with irrigation and fertilization, has been shown to increase light capture and thereby production (Chang 2003, Jose et al. 200 3 and Allen et al. 2004). The underlying physiological basis for this observation is an increase in

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66 leaf level and/or canopy level photosynthesis in response to resources. Increased resource availability, especially nitrogen, has been shown to increase leaf level P net in a number of forest species (Coleman et al. 1998, Wright et al. 20 01, Jose et al. 200 3 and van Kuijk and Anten 2009 ). The functional interpretation of this relationship is that by increasing nitrogen availability, production of photosynthetic enzymes and proteins will increase, allowing a higher rate of photosynthesi s to occur (Evans 1989, Reich et al. 1991, and Warren and Adams 2001). For our study, the significant differences for all three species between the CON and fertigation treatments for leaf level A max and LAI corroborate these findings while revealing the r estrictions placed on aboveground production when one or more resources are limiting. While the IRR treatment alone had a significant impact on Q. pagoda and P. occidentalis A max over the CON treatments, the addition of N through the irrigation system fu rther increased A max for all species. Several studies have shown similar photosynthetic responses to increased resource availability, such as N, (Ellsworth and Reich 1992, Sullivan et al. 1996, Gardiner et al. 2001 Binkley et al. 2009 and Liberloo et al. 2009 ) as well as to the simultaneous applications of water and N (Samuelson et al. 1998, Samuelson 2000, Samuelson et. al 2001, Jose et al. 2003 Binkley et al 2009, and van Kuijk and Anten 2009 ). All species in our study showed significant gains in A max with the addition of N for both sun and shade leaves through the IRR+112 treatment. However, the sharp increase observed in A max declined significantly with the highest rate ( IRR+224 ) of fertilization (Fig 3 1 ). This decline was associated with a corre sponding decline in SLN for all three species. Significant positive correlations

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67 were also observed between SLN and leaf A max for both sun and shade leaves in all the species (Fig 3 3 ). This is not surprising considering the widely recognized correlatio ns between foliar nitrogen and RuBP carboxylase activity (Evans 1986) and foliar nitrogen and chlorophyll content (Evans 1987). The biochemical basis for these correlations are explained in a review by Evans (1989) and hence not described here. The factor s contributing to reduced SLN and subsequent reduction in A max in the highest N treatment are not obvious. As explained earlier, A max will be limited by N when available N for enzyme and protein (mainly for RuBP carboxylase) synthesis is limited (Evans, 1 989; Reich et al., 1991; Coleman et al. 1998 and Warren and Adams 2001). SLN for the IRR+224 treatment was, in fact, similar to the SLN of the CON treatment for both sun and shade leaves (Fig 3 2 ). A dilution effect might be considered the cause for red uced N content on a leaf area basis in the IRR+224 treatment, if SLA and LAI were greatest in these treatments. SLA, in general, was similar or lower in the IRR+224 treatment compared to the IRR or IRR+56 and IRR+112 treatments. Peak LAI for P. deltoides and P. occidentalis reached in the IRR only treatment and was not significantly different from the LAI found in the N treatments. Peak LAI for Q. pagoda reached in the IRR+56 treatment with no differences among the N treatments. These findings suggest t hat a dilution effect was minimal for Q. pagoda and P. occidentalis and did not exist for P. deltoides Knecht and Goransson (2004) suggested that the rate of uptake and use of any nutrient might be down regulated if other essential nutrients were not available in sufficient supply to meet the demand. That is, a specific ratio is necessary to sustain a given growth rate regardless of any excessive amount of any nutrient. All nutrients

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68 must be maintained in a specific ratio. It is possible that repeat ed fertilization for six years at the rate of 224 kg N ha 1 has changed the soil chemistry and created nutrient imbalance in the soil and foliage. Some changes in soil properties have already been reported from the study site. For instance, in a companion study, Lee and Jose (2003) observed a reduction in soil C O 2 efflux and microbial carbon along the same increasing resource gradient. In another study, these authors reported that estimated NO 3 N leaching at a depth of 0.9 m was 97% of the applied N in the IRR+224 treatment whereas it was only 70 and 72% in the IRR+56 and IRR+112 treatments, respectively (Lee and Jose, 200 5 ). This, perhaps, is an indication of N saturation in the soil, which could result in nutrient imbalance. ANPP reached a plateau in the IRR only treatments for P. deltoides and P. occidentalis and additional N did not result in any appreciable increase in production. Q. pagoda ANPP was significantly higher in the IRR only treatment compared to the CON but fertilization at IRR+ 112 and IRR+224 further increased production. Allen et al. (2004) observed a similar response for four year old sweetgum ( Liquidambar styraciflua L.) and P. occidentalis plantations in Georgia, USA. They observed that irrigation alone was sufficient for increasing volume production in P. occidentalis whereas 85 rather than 114 kg N ha 1 yielded the best volume production in sweetgum. Although the physiological basis for this phenomenon was not explored in detail, they observed strong correlations between light interception and volume growth. Light interception (which is a function of canopy LAI) is an indicator of the carbon fixation potential; however, canopy A max may better explain the interactive effects of LAI and foliar A max For example, the relationship between canopy A max and ANPP showed that

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69 productivity, for all species, reached a plateau well below the maximum estimated canopy A max The optimum level of canopy A max that yielded the maximum ANPP was in the IRR only treatment for P. deltoides and P. occidentalis and in t he IRR+112 treatment for Q. pagoda This clearly indicates that an increase in LAI alone with no additional increase in foliar A max was sufficient for P. deltoides and P. occidentalis to result in optimum canopy A max and provide the maximum production pot ential. However, the combined effect of LAI and foliar A max in controlling ANPP was evident for Q. pagoda Despite having higher LAI in all the N treatments (Fig 3 4 ), increased LAI alone was not sufficient, but an increase in foliar A max was also neces sary to yield optimum canopy A max and maximum ANPP in this species. The positive impact of irrigation and fertilization on productivity has been well documented. However, as suggested by Allen et al. (2004), the reasons for increased growth response are less evident. Several factors related to canopy dynamics have been identified by others (Albaugh et al. 1998, Samuelson 1998, 2000) and were explored in detail in our study. As expected, SLA, SLN, LAI, A max and ANPP varied across the supplied soil resou rce gradient for all three species. Irrigation alone was sufficient in P. deltoides and P. occidentalis to increase SLA whereas SLA responded to both IRR and fertilization in Q. pagoda A corresponding increase in LAI, similar to that of SLA, was also ob served for all three species. A max reached peak rates for all species in the IRR+112 treatment for both sun and shade leaves and showed strong positive correlations with SLN across the gradient. ANPP exhibited a curvilinear relationship with canopy A max with peak production occurring well below the maximum estimated canopy A max An increase in LAI alone was sufficient to achieve maximum ANPP in

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70 both P. deltoides and P. occidentalis However, an increase in both LAI and foliar A max resulted in the maximum ANPP in Q. pagoda Although we hypothesized that increased foliar A max in combination with an increase in LAI would be necessary to increase ANPP along the increasing resource gradient, our results indicate that an increase in LAI alone can increase canopy A max and lead to increased productivity. An increase in foliar A max may or may not be necessary to yield the highest ANPP depending on the species. Further research is needed to determine the relationship between N and other po tentially limiting nutrients so that the role of nutrient imbalance can be better understood. Given the appeal for managing short rotation plantations with fertigation, this study should give land managers a solid scientific basis for developing efficient fertigation strategies.

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71 Table 3 1 Year six s pecific leaf area (SLA) for P. deltoides Q. pagoda and P. occidentalis sun and shade leaves along a soil resource gradient. Capital letters indicate significant differences between sun and shade leave s and lower case letters indicate significant differences among treatments Sun SLA (m 2 kg 1 ) Shade SLA (m 2 kg 1 ) P. deltoides Q. pagoda P. occidentalis P. deltoides Q. pagoda P. occidentalis Treatment CON 11.96 Aa 12.98 Aa 12.68 Aa 19.26 Ba 18.81 Ba 21.65 Ba IRR 19.08 Ac 13.30 Aa 16.90 Ac 24.74 Bb 21.99 Bb 28.87 Bb IRR+56 15.70 Ab 17.63 Ab 15.34 Abc 20.25 Ba 25.89 Bc 27.93 Bb IRR+112 14.48 Ab 14.23 Aa 14.06 Aab 19.47 Ba 23.52 Bc 22.63 Ba IRR+224 14.61 Ab 15.72 Aab 16.91 Ac 24.39 Bb 24.77 Bc 26.94 Bb

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72 Table 3 2 Year six ANPP of P. deltoides Q. pagoda and P. occidentalis along a soil resource gradient. Lower case letters indicate significant differences among treatments ANPP ( Mg ha 1 yr 1 ) P. deltoides Q. pagoda P. occidentalis Treatment CON 2.25 b 1.47 c 3.39 b IRR 4.48 a 2.44 b 4.73 a IRR+56 5.37 a 2.54 b 4.48 a IRR+112 4.34 a 2.97 ab 4.39 a IRR+224 4.46 a 3.75 a 5.11 a

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73 Figure 3 1 Average light saturated photosynthesis for 6 year old P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) leaves. Capital letters indicate significant differences between sun and shade leaves (p 0.05). Lower case letters indicate significant diffe rences among treatments (p 0.05).

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74 Figure 3 2 Average specific leaf nitrogen for 6 year old P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) leaves. Capital letters indicate significant differences among sun and shade leaves (p 0.05). Lower case letters indicate significant differences among treatments (p 0.05)

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75 Figure 3 3 The relationship between specific leaf nitrogen and light saturated photosynthesis for 6 year old P. deltoides (A) Q. pagoda (B) and P. occid entalis (C) sun and shade leaves.

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76 Figure 3 4 Average leaf area index for 6 year old P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) leaves. Lower case letters indicate significant differences among treatments (p 0.05).

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77 Figure 3 5 The relationship between canopy photosynthesis and aboveground net primary productivity ( ANPP ), for 6 year old P. deltoides (A) Q. pagoda (B) and P. occidentalis (C)

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78 CHAPTER 4 NUTRIENT USE EFFICIE NCY OF THREE FAST GR OWING HARDWOOD SPECIES ACROSS A RESOURCE GR ADIENT Changes in attitudes about energy production shifted interest from traditional energy sources and techniques toward renewable resources in recent years (Dickmann 2006). One target of the focus on renewable energy is fas t growing hardwood species with the concentration placed on species that could be harvested on rotations ranging from as little as six (Tuskan 1998) or up to 15 years (Dickmann 2006). The concept of short rotation woody crop (SRWC) supply systems were fir st formalized nearly 50 years ago (Tuskan 1998). In some areas of the United States, forest management practices that had previously focused on extensive management for fiber production have shifted to intensive management for biomass and biofuel producti on using SRWC systems (Geyer and Melichar 1986 Coyle and Coleman 2005, and Augusto et al. 2009 ). Techniques for increasing production potential of SRWC such as high stocking rates hybrid selection/development, and intensive stand management have bec o me industry standards (Dickmann 2006). However, our knowledge about fertilizer uptake patterns and use by these fast growing species is limited at best. Tree production can be limited by a number of factors such as light (Wang et al. 1991, Ellsworth and Re ich 1992, Jose and Gillespie 1997, Jokela and Martin 2000, and Henderson and Jose 2005), water (Lockaby et al. 1997, Albaugh et al. 1998, King et al. 1999, Allen et al. 2002, and Albaugh et al. 2004), and growing space (Cochran et al. 1991, Schubert et al. 2004, Lockhart et al. 2006, Clark et al. 2008, and Curtis 2008) or enhanced by practices such as fertilization (Singh 1998, Will et al. 2002, Bekele et al 2003, Allen et al 2004, and Samuelson et al. 2004a), or irrigation (Allen et al 2005, Stape et al. 2 008, Zalesny et al. 2007, and Zalesny et al. 2008). Most frequently,

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79 biomass accumulation and stand development are restricted to inherent resource availability within a site or by community composition (Wang et al. 1995, Wang et al. 1996, Smith et al. 19 98, Vogel and Gower 1998, Blanco et al. 2006, Schilling and Lockaby 2006, and Yan et al. 2006). As a counter to nutrient losses, plants have mechanisms to minimize nutrient losses such as nutrient resorption or retranslocation (Vitousek 1982, Berendse and Aerts 1987, Aerts and Berendse 1988, Aerts 1996, Aerts 1997, and Wright and Westoby 2001). Although it would seem somewhat intuitive, the nature of and driving force behind nutrient availability, uptake, and resorption are not well understood as is indi cated by inconsistent findings between studies. Some studies indicate nutrient limitation should lead to higher rates of resorption efficiency and proficiency (actual nutrient level within leaf litter; a reflection of soil resource) and that low rates of resorption could contribute to nutrient limitations, reduced biomass production, and survival (Boerner 1984, Killingbeck 1984, Killingbeck 1986, Killingbeck 1993, and Killingbeck 1996). Other studies suggest higher leaf level nutrient status (Lathja 1987) or resource availability (Xu and Timmer 1999) is linked to higher or lower (Aerts and de Caluwe 1994, Vitousek 1998) resorption ratios or may have no effect on the ratios (Chapin and Kedrowski 1983, Birk and Vitousek 1986, Aerts 1996, Wright and Westoby 2 003, and Yuan and Chen 2010). However, it appears that reaction to and indications of nutrient use can vary in response to site fertility (Bloom et al. 1985, Wright and Westoby 2003), water availability (Boerner 1985, del Arco et al. 1991, Escudero et al. 1992, Wright and Westoby 2003), soil chemistry (del Arco et al. 1991, Bridgham et al. 1995, Choi et al. 2005, and Campo et al. 2007) as well as between members of the

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80 same species (Birk and Vitousek 1986, Aerts and de Caluwe 1994, and Bungart and Httl 20 04). Nutrient resorption proficiency (NRP), has been described as a way to measure the success of nutrient conservation and to reflect environmental constraints of site conditions ( Killingbeck 1996), particularly for nitrogen (N) and phosphorus (P) NRP can be described as the realized resorption or the quantity of nutrient remaining in senesced tissue after retranslocation NRP trends between species (deciduous versus evergreens) across varying levels of environmental limitations (nutrients and water) a ppear to influence processes such as nutrient uptake and productivity (Killingbeck 1996). Killingbeck (1996) further pointed out that the variation found within his results could be attributed to forest stand conditions as well as the relationship between inter nutrient dependence (i.e. N and P). Given the intensified demand for SRWC worldwide and interest in increasing wood biomass production, determining ways to enhance yield is paramount for plantation development (Chang 2003, Bungart and Httl 2004, Coyle and Coleman 2005, DesRochers et al. 2006). Intensive culture of hardwoods is often accompanied by site preparation, competiti on control, genetically improved planting stock, and selection of fast growing species to increase the production potential (Fang et al. 1999, Chang 2001, Samuelson et al. 2001, Bungart and Httl 2004, Lee and Jose 2005, and DesRochers et al. 2006). By fa r the most advantageous of the silvicultural methods used to increase production is fertilization (Allen 1987). Fertilizer application, while generally one of the least expensive silvicultural tools, can become more costly than necessary if application ra tes exceed nutrient uptake or demand of the trees. When coupled with irrigation, fertilization has the capability to increase production on infertile

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81 sites, in areas where rainfall is limited, or on soils that lack necessary water holding capacity (Axelss on and Axelsson 1986., Lockaby et al. 1997, King et al. 1999, Bekele et al. 2003, and Coyle and Coleman 2005). Many studies have indicated that growth response to differing fertilization rates for economically important tree species are species specific a nd/or vary with site resource levels (Wienand and Stock 1995, Jokela et al. 2004, Prietzel et al. 2004, Sword Sayer et al. 2004, Ladanai et al. 200 6 Saarsalmi et al. 2006, and Moscatelli et al. 2008). However, questions remain regarding the extent that p roduction could be enhanced by increasing resource availability and at what levels additional resources become luxury ? In the present study, we investigated the effect of water and nutrient availability, on nutrient content (kg ha 1 ), resorption efficie ncy ( % ), resorption proficiency ( g nutrient kg litter/senesced tissue ), and leaf and canopy level nutrient use efficiency of nitrogen (N), phosphorus (P), and potassium (K) for Populus deltoides Bartr. (cottonwood), Quercus pagoda Raf. ( cherrybark oak, pr eviously Quercus falcata var. pagodafolia Ell.) and Platanus occidentalis L. (sycamore), (nomenclature follows USDA, NRCS Plants Database 2009). Our objectives were to: 1) determine the aboveground nutrient content for each nutrient and species across a n itrogen/water gradient ; s pecifically, what rates of fertilization are actually captured and utilized by the canopy to influence production? 2) quantify the nutrient resorption efficiency and proficiency of N, P, and K for all three species; d oes an increa se in foliar nutrient content result in increased biomass production? and 3) determine the nutrient use efficiency on a leaf and canopy level basis for the three nutrients and species. I s nutrient use efficiency decreased in similar magnitudes as the application of fertilization? Are the amounts of fertilizer that

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82 are taken up reflected in the magnitude of resorption? We hypothesized that nutrient levels, budget, efficiencies, and ratios would peak well below the maximum level of nitrogen supplied. Methods Study Site Our study was conducted in a fertigation trial established on an abandoned temperate with mild winters and hot, humid summers. Average rainfall is 1700mm, with average minimum and maximum temperatures of 10 and 27 C respectively (NOAA, 2003). The soil is characterized as a well drained, Redbay sandy loam (a fine loamy, siliceous, thermic, Rhodic Paleudult) formed in thick beds of loamy marine dep osits with an average water table depth of 1.8m (Lee and Jose 2003). Soil variables calculated after this study ended include pH (5.8 down from the original pH of 6.0 ), cation exchange capacity (4 CEC meq/100 g ), and soil nutrient levels of phosphorus (3 4 and 55), potassium (92 and 122), calcium (599), and magnesium (179 and 146) (kg ha 1 ). To our knowledge, nitrogen levels were not measured prior to the study, but were expected to be relatively high given the site was an abandoned agricultural field. N itrogen levels measured within the control plots during a companion study indicated total inorganic nitrogen ranged from near 2.5 to 4.5 kg ha 1 (Lee and Jose 2006). Treatment plots of P. deltoides and P. occidentalis consisted of 40 trees plot 1 and Q. pagoda Q. pagoda contained 16 trees plot 1 ; (although the Q. pagoda plots were the smallest of the three species, and found for this study should be treated with caution, the results reflect data collected within the study). All treatment plots were plant ed at 2.13m X 3.35m spacing (1400 trees ha 1 ). The study design was a

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83 randomized complete block (RCB) with four replications of each treatment. Site preparation included disking and subsoiling to facilitate planting. Fertilization at the time of plantin g included broadcast application of diammonium phosphate, dolomitic lime, potash, and a micronutrient mixture. These treatments added elemental calcium, nitrogen, phosphorus, magnesium, zinc, copper, and manganese (1009, 50, 56, 126, 3, 3, and 2 kg ha 1 r espectively, Greg Leach, personal communication ). Soil pH was adjusted to 6.0, with 3363 kg ha 1 of dolomitic lime, based on recommendations from a similar trial at North Carolina State University Research Cooperative (Coleman et al. 2004, Samuelson et al 2004a, and Samuelson et al. 2004b). Herbaceous weed control was attained with combinations of chemical (sulfometuron methyl and glyphosate) and mechanical (mowing and manual pulling) treatments during the first and second growing seasons. Installation o f the nutrient supply system and planting of trees occurred during spring 1995. The irrigation system operated for approximately two hours each day (on average 390mm water Greg Leach, personal communication ) during the growing season (May Sep.) with nitrogen application occurring two to eight minutes each day creating the nitrogen gradient across the treatments (Lee and Jose 2003; 2006). Five treatments were established including control (CON), irrigation only (IRR), and three nutrient supplemen ts supplied through irrigation including 56, 112, and 224 kg N ha 1 yr 1 (referred to as IRR+56, IRR+112, and IRR+224, respectively). Each treatment was applied to each plot of P. deltoides Q. pagoda P. occidentalis For each species, each plot consiste d of 40 trees for P. deltoides and P. deltoides and Q. pagoda contained 16 trees per plot due to space constraints ; (although the Q. pagoda

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84 plots were the smallest of the three species, and found for this study should be treated with caution, the results reflect data collected within the study). All treatment plots were planted at 2.13m X 3.35m spacing (1400 trees ha 1 ). The study design was a r andomized b lock d esign with four replications of each treatment for each species. Data Collection Leaf sampl es were collected from upper one third (sun leaves) and lower one third (shade leaves) of the canopy on a monthly basis during the eighth growing season. Samples were collected within each plot for each species, bagged, labeled for identification purposes and placed in a cooler for transport. Leaf area (cm 2 ) was determined by passing each leaf through a Li Cor LI 3300 Leaf Area Meter and then weighed to the nearest 0.01g. Specific leaf weight (SLW) was determined by dividing the foliar weight by area. Samples of bark, branches, and wood were collected in mid growing season. Ten trees per treatment, per each species were randomly selected for woody component (bark, branch, and bole) nutrient analysis and combined, to obtain a treatment level sample for nutrient content for each tissue type. Bark sample removal was completed by surficially scraping, cutting, breaking, or peeling samples from the trees of each species. Collection of branches from the same randomly selected trees, were gathered by pruning newly formed branches from the lower and upper third of the canopy for each species. Collection of bole material consisted of coring each randomly selected tree at DBH (diameter at breast height ~ 1.5m above ground level) with an increment borer. Foliar and woody samples were dried at 70 C for 48 hours, ground to a fine powder and analyzed for total nitrogen (N) (Kjeldahl), phosphorus (P) (EPA

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85 Method 200.7 ICP (Inductively Coupled Plasma) Spectrophotometer ), and potassium (K) at the University of Florida Analyti cal Research Laboratory (ARL). For biomass calculations used with nutrient data, d iameter at breast height ( DBH ) and height of all trees in each plot within each treatment were measured yearly. Standing biomass (Mg ha 1 ), ANPP (Mg ha 1 yr 1 excluding herbivory or litter of branches, bark, or fruits, as defined on page 9), LAI (m 2 m 2 calculated by multiplying weight (g) and area (m2) of leaf litter collected in litter trays by SLA (m 2 g 1 ) of randomly selected canopy leaves, as defined o n page 9), for year eight Whole tree a llometric equations developed by Shelton et al. (1982) were used to calculate volume and aboveground woody biomass for P. deltoides Their equations for P. deltoides were developed from trees of comparable age range and soil type, grown in areas with similar longitude, latitude, and climate as this study. Standing woody biomass consisted of all woody components (bark, branches, and trunk/bole). Foliage biomass was determined by summing the weight of annual litter fall collected monthly (May to January) from five litter traps (0.5 m 2 ) for P. deltoides Biomass equations developed by Schlaegel and Kennedy (1986) were used to calculate volume and aboveground woody biomass for both Q. pagoda and P. occidentalis The o riginal Schlaegel and Kennedy (1986) equations used diameter measured at approximately 15 cm above ground level. All Q. pagoda and P. occidentalis DBH data were corrected to reflect the dbh measurements of the equations at 15 cm height above ground level, by using regression equations developed from sampling 100 trees per species measured at the appropriate height (data not shown, R 2 = 0.97 and 0.93 respectively for Q. pagoda and P. occidentalis ). Foliage biomass was

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86 determined by summing the weight of an nual litter fall collected from five and two litter traps (0.5 m 2 ), for P. occidentalis and Q. pagoda respectively in each plot. To determine nutrient use on a leaf and canopy level, projected LAI, was calculated from the weight (g) and area (m 2 ) of the le af litter trays and SLA (scaled to the canopy level m 2 g 1 ) for each species within each treatment. Care was taken to ensure only leaf litter from the species within the plot was processed. If litter from other species fell or were blown into the tray, it was removed prior to collection. Leaf litter was dried for 48 hours at 70 C and weighed to the nearest 0.01g. Nutrient content of each species for each aboveground component was calculated for N, P, and K using the equations used for the purpose of biomass production estimation developed by Shelton et al. (1 982) and Schlaegel and Kennedy (1986) and for the calculation of the nutrient concentrations for woody and fo liar components (Equation 1). The RE was calculated by determining the difference between peak nutrient concentration of green leaves and those fo und in fresh leaf litter (Equation 2). Leaf level nutrient use efficiency LNUE was calculated using leaf level nutrient content and leaf litter resorption rates (Equation 3). Canopy nutrient use efficiency (CNUE) was calculated using aboveground biomass produced in year eight divided by the peak production (peak foliar production was determined from monthly leaf litter collection), and nutrient content of green leaves for each species in each treatment (Equation 4). Resorption proficiency was reported as the nutrient content in senesced leaves (g N kg 1 litter i.e. realized resorption ) (1) Nutrient content (kg ha 1 ) = kg ha 1 ( biomass ) (kg kg 1 nutrient ) (2) Resorption (%) = (foliar (live) foliar (litter) / foliar (live) )*100

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87 (3) Leaf nutrient us e efficiency (g g 1 ) = 1 / (( g g 1 )*(1 resorption)) (4) Canopy nutrient use efficiency (Mg kg 1 ) = Mg / (kg foliage ( kg kg 1 nutrient )) Analysis All the measured and calculated variables were compared among treatments using analysis of variance (ANOVA) (SAS Institute Inc. 2001) with treatment assigned among treatments were reve multiple mean test for mean separation and determining significance. Linear regression was used to analyze the relationships between ANPP and N:P. It has been suggested that as soi l nitrogen levels increase, uptake of nitrogen can be limited by the availability of other nutrients (Aber et al. 1989). Furthermore, b ecause of results from studies like Pastor and Bridgham (1999) and Bridgham et al. (1995) we hypothesized that the highe st rate of nitrogen application would be far greater than the trees could utilize. As such, curvilinear functions were chosen a priori to ANOVA analysis and in accordance with our hypothesis that nutrient use variable responses were likely to plateau well below the maximum level of N supplied by the treatments. Results Nutrient Content The N content of aboveground components (bole, branch, bark and foliage 4.0 17. 1 1.8 and 217.7 kg ha 1 respectively ) and the total N (240.5 kg ha 1 ) of the combined aboveground biomass in P. deltoides were significantly lower in the control (CON) treatment compared to that of the IRR and IRR + Fertilizer treatments (IRR+56,

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88 IRR+112, and IRR+224). The IRR and all the IRR + Fertilizer (IRR+56, IRR+112, and IRR+224) treatments had similar total N content (502.9, 415.8, 422.1, and 439.0 kg ha 1 respectively, Table 4 1 ). In other words, N content for year eight reached its highest at a level below the maximum N application (502.9 kg ha 1 in the IRR treatment) The overall trend for each component (branch, bark, foliage) or total tree was to reach the highest N content in the IRR treatment with significant differences found among the CON and all IRR treatments The only exception to this trend was for the bole content, which reached its peak at the IRR+56 treatment (10.5 kg ha 1, Table 4 1 ) which was not significantly different from the other IRR or IRR + Fertilizer treatments. Branch, foliar, and total tree P nutrient budget for P. delto ides exhibited similar trends as N by reaching its peak in the IRR treatment ( 11.9, 37.6, and 50. 6 kg ha 1 respectively ) with significant differences found among treatments for each component Bole and bark P content reached their peaks in the IRR+224 w hich was significantly different from all other treatments, and IRR+122 treatments (1.4 and 0.3 kg ha 1 respectively Table 4 1 ) Significant differences were found among the Con and all other IRR treatmen t s P. deltoides branch, bark and total tree com ponents for K reached its maximum content in the IRR treatment (70. 6 1. 2 and 353.0 kg ha 1 respectively Table 4 1 ) and were significantly different from the CON treatment Maximum K content for the bole and foliar components were found in the IRR+56 and IRR+224 treatments (29. 9 and 267. 8 kg ha 1 respectively ) with significant differences found among treatments. Maximum N content for bole, branch and bark for Q. pagoda (6. 3 8. 9 and 4.8 kg ha 1 respectively) was found in the IRR+224 treatment and significant differences were

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89 found among treatments for these components (Table 4 1 ). Maximum N for the foliar and total tree occurred in the IRR+112 treatment (367.1 and 382.2 kg ha 1 respectively) with significant differences found only among the c ontrol and IRR+112 treatments For P, the bole and bark components were greatest in the IRR+224 treatment (0.3 and 0. 2 kg ha 1 respectively) while branch and foliar components reached the highest levels in the IRR+56 and IRR+112 treatments (0. 8 and 22.7 kg ha 1 respectively ) Significant differences for the bole component were found among the CON and IRR+224 treatments and among treatments for the branch component. The total tree peak P was found in the IRR+122 treatment, and was likely influenced by t he foliar P content level (23.7 kg ha 1 ) although no significant differences were found among treatments For Q. pagoda the highest K for bole branch and bark in the IRR+ 224 treatment (5.9, 6.6, and 1.7 kg ha 1 respectively). Foliar and total tree pe ak K nutrient content was found in the IRR+112 treatment with significant differences found among the CON and IRR+112 treatments (151.6 and 162. 1 kg ha 1 respectively) P. occidentalis had its highest N and K contents in the IRR treatment for branch, bark foliar, and total tree components (8. 9 4. 7 536.6 and 559.1 kg N ha 1 and 6. 6 1.3, 257.5 and 275.7 kg K ha 1 respectively) with significant differences found among treatments Both N and K bole content were greatest in the IRR+112 treatment (11.9 and 12.3 kg N ha 1 and kg K ha 1 respectively) with significant differences found among treatments Maximum P content for P. occidentalis occurred in the IRR+56, IRR, CON, and IRR treatments for bole, branch, bark, foliar and total content (2.2, 1 .6, 0.4, 41.7, and 45.6 kg P ha 1 respectively) with significant differences found among treatments

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90 Nutrient Use, Resorption Efficiency and Proficiency No significant differences were found for RE or LNUE for any of the three species across all treatment s for N, P, or K (Table 4 2 RE rang ed from 65.1 CON to 57.0 % IRR+112, 57.6 CON to 52.8 % IRR+112 and 85. 2 CON to 72. 2 % IRR+56, for N, P, and K, and LNUE ranged from 136.3 CON to 106.6 g g 1 IRR+112, 1231.7 CON to 1103.2 g g 1 IRR+56, and 732.4 CON to 331.2 g g 1 IRR+224 for N, P, and K, respectively ). CNUE for all three species and nutrients (Table 4 3 ) followed irregular patterns. Only N and K for P. deltoides exhibited significant differences among treatments N and K pea ked in the IRR+56 ( 5.1 and 5. 6 Mg kg 1 respectively) and IRR treatments respectively. For both nutrients, CNUE was lowest in the IRR+112 treatment (3. 3 and 3.8 g g 1 respectively) For RP no significant differences were found for any of the three speci es across all treat ments for N, P, or K (Fig. 4 1 ). N:P N:P ratios were based on foliar levels of N and P. The only significant difference found for N:P among treatments for all three species occurred for P. occidentalis (Fig. 4 2 ). N:P was lowest in the CON treatment (~11) but was not significantly different from the IRR, IRR+56, or IRR+112 treatments (~12, 14, and 14) There was a significant difference between N:P for CON and IRR+224 (~14.5) N:P and ANPP No significant relationship was found for N:P and aboveground net primary productivity (ANPP) for P. deltoides (Fig. 4 3 ). The trend for Q. pagoda and P. occidentalis for N:P and ANPP (Fig. 4 3 ) was a significant (p > 0.05) curvilinear relationship with the peak occurring at or near the N:P ratio of 17 and 14 respectively. For both species, when N:P increased past these points, ANPP tended to decrease.

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91 Discussion We wanted to determine how resources were utilized for biomass production with respect to varying levels of irrigation, nitrogen, or the combined application of irrigation and nitrogen application (fertigation, IRR+56, IRR+112, and IRR+224). The differences between N uptake and utilization, reflected in the N content of the combined aboveground parts, for P. deltoides and P. occidentalis w as likely influenced by the greater biomass production of these two species than was seen in Q. pagoda and was more highly influenced by the irrigation treatment for P. deltoides and P. occidentalis than for Q. pagoda (Table 4 1 ) Despite these differenc es, N content in Q. pagoda were greater at higher N application rates. Our hypothesis of nutrient levels peaking well below the maximum rate of N application was true for two of the three species. For the combined aboveground parts N, P, and K nutrient co ntent were highly affected by the large foliar fraction for all three species (Table 4 1 ). Water availability necessary for nutrient uptake (Lambers et al. 1998) regulates foliar production (Jose and Gillespie 1996 and 1997) and therefore the amount of wo ody biomass that can be produced (Henderson and Jose 2010). Soils for this area are sandy and well drained; suggesting, for this combination of species and soil parameters, low water storage capacity and therefore water availability may be as limiting for growth and production as N for these early successional species. In fact, the last five years of this study (1999 2003), combined irrigation application and annual rainfall totals were either below or consistent with historic rainfall averages for this s ite (Henderson and Jose 2010) Lockaby et al. ( 1997 ) suggested that cultural treatments could exacerbate moisture needs of early successional species in well drained soils. P. deltoides and P. occidentalis reached their maximum N budget in the IRR treatm ent. Given the inherent

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92 fertility of an abandoned agriculture field, these species may have had their nutrient requirements met by past land management techniques. From our analysis, it appears that not only did the fertigation treatments not significant ly alter nutrient uptake or biomass production in year eight (Henderson and Jose 2010) but also the N treatments may have increased water requirements, which may not have been met by the fertigation treatments. This finding was substantiated by the lack of significant differences between the treatments. In a companion study, Lee and Jose ( 2005 ) found that after seven years of fertigation treatments, between 46 60 kg N ha 1 yr 1 was lost in groundwater on an annual basis in the IRR+56 treatment They found that between 65 and 96 % of the nitrate applied in the P. deltoides treatments was leached from the site and suggested that N application rates above the IRR+56 treatment could not be utilized for increased growth exceeding the biological and non bio logical N retention capacity of the system (Lee and Jose 2005). These findings suggest that nutrient availability in old agricultural fields may be sufficient for maximum production, and additional N application would be underutilized and likely lost. De pending on the desired length of rotation for short rotation woody crops (SRWC), it could be suggested from this study that by year eight, any advantage of N application would not be realized in additional uptake or biomass production (Henderson and Jose 2 010). At this time, it could also be suggested that additional N application may not be necessary for additional biomass production, or that thinning should occur to relieve below ground competition and release the most desirable (domina nt or co domina n t) trees within the stands.

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93 The distributions and amount of N, P, and K (Table 4 1 ) within each tissue component for these three species are reflective of both the range in biomass produced between the treatments and the nutrient availability supplied by eac h treatment. Other studies have found similar nutrient content values on an area basis, for the same set of tissue components to those found in the CON and IRR treatments. For these studies, direct comparisons of nutrient content values are marginal at best as species, nutrients studied, site conditions, and treatments were dissimilar. A few studies have investigated the effects of thinning (Blanco et al. 200 6 ), mixed species stands (Vogel and Gower 1998 and Wang et al. 2000), or multiple aged trees (Mi ller et al. 1993), chronosequence studies of single species (Wang et al. 1995 and 1996), or the effect of elevated CO 2 on nutrient contents (Calfapietra et al. 2007) but did not entail analysis of nutrient budgets ac ross a fertilization gradient. I n a thin ning study of unfertilized 32 year old stand of Pinus sylvestris L., conducted by Blanco et al. 2006 they found N total content values ten times higher than were found in this study (4193 5641 kg ha 1 versus our 240 502 kg ha 1 for P. deltoides ). A study conducted by Wang et al. (1996) consisting of a mixed Betula papyrifera Marsh and Abies lasiocarpa (Hook) Nutt., total tree N content for 75 year old B. papyrifera was similar to the values found in the IRR+224 treatment in this study (431 kg ha 1 versus 439 kg ha 1 found in our study). The values of P and K reported by these authors were higher and lower, respectively, than were found in our study (65 and 217 kg ha 1 versus 22.6 50.6 and 168.1 353 kg ha 1 of P and K. respectively). However, t he findings from the Wang et al. (1996) study were based on soils without any amendments. Vogel and Gower ( 1998 ) found much lower total N values in a mixed stand of Pinus banksiana

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94 Lamb. and Alnus crispus (Ait.) Pursh. than were found for P. deltoides in our study but were similar to those found for Q. pagoda in the CON treatment (170 versus 168 kg ha 1 found in our study ). The conditions for their study consisted of a much shorter growing season and degraded soils making direct links between the two studies only superficially comparable. In a study designed to determine NUE for Eucalyptus spp., Safou Matondo et al. (2005) found similar total N, P and K content for a similarly aged plantation that had been fertilized at the time of planting. Their f indings suggest that species or clones selected for superior growth produce high quantities of biomass with low levels of nutrient availability. If the species selected for this study had been hybrid or clonal varieties, it is likely much greater amounts of biomass could have been produced. Lambers et al ( 1998 ) suggests that at least on a short term basis, the application of one nutrient can force additional uptake of other nutrients. The question could then be asked, can the application of one specifi c nutrient (N) not only alter the rates of uptake of other nutrients (P and K), but would these effects be long term so that increased induced nutrient contents are reflected in the content of bole, branches, and bark components? For our study, when compa red to CON, it appears that increased levels of N application increased the P content of all components of P. deltoides and Q. pagoda for all treatments. P. occidentalis had similar results with the exception of P content for branches in the IRR+112 and IRR+224 treatments (Table 4 1 ). For K, when comparing the CON to all other treatments, all three species had increased K content with increased N application (Table 4 1 ). Our hypothesis of aboveground nutrient

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95 content for N, P, and K peaking well below t he maximum input of N can only be partially supported. In general, studies have found that plants growing in nutrient poor habitats have mechanisms to conserve and recycle nutrients more efficiently than those found in nutrient rich environments (Aerts 1 996, Feller et al. 1999, May et al. 2005). No significant differences were found for RE in our study (Table 4 2 ) and the relationships in our findings were not strong enough to support our hypothesis of RE peaking below the maximum level of N application. The RE levels we found for N, P, and K for all three species were similar to other studies for N Pugnaire and Chap in ( 1993 ) found RE levels ranging from just over 60 % and up to slightly greater than 80 % P. occidentalis had the highest rates of RE ranging from 66 % in the IRR+224 treatment to 74 % in the CON treatment (Table 4 2 ) while P. deltoides ( 57 to 65 % ) and Q. pagoda (60 to 62 % ) RE were similar more similar to the lower ranges Pugnaire and Chapin (1993) found for several chaparral species grown in nutrient poor soils. Eckstein et al. ( 1999 ) Drenovsky and Richards ( 200 6 ) May et al. ( 2005 ) Cai and Bongers ( 2007 ) and Calfapietra et al. ( 2007) reported similar RE values. Feller et al. ( 1999 ) found P RE values for P fertilized Rhizophora mangle (red mangrove) trees (approximately 48 to 55 % ) similar to P. deltoides (53 to 58 % ) and P. occidentalis (51 to 57 % ). These values agree with Aerts and Chapin ( 2000 ) for deciduous species and Kozovits et al. ( 2007) for two savanna tree species Qualea parviflora and Schefflera macrocarpa for P RE Hagen Thorn et al. ( 2006 ) and Chatain et al. (2009) found K RE values for Quercus robur L., (English oak) and Nothofagus species (approximately 38 % and 40 % respectively ) tha t we re similar for Q. pagoda in this study (37 to 45 % ). B lanco et al.

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96 ( 2009 ) found K RE values that were more similar (upwards of 80 % ) to those found for P. deltoides (72 to 85 % ) and P. occidentalis (73 to 85 % ). The three species in this study could be described as being moderately efficient at resorption (Killingbeck 1993). When these findings are considered singularly, a slight de crease in nutrient resorption might seem unimportant and would suggest that the nutrient levels in an abandoned agricultural field would be sufficient to allow biomass production for SRWC. However, when compared to the IRR and fertigation treatments for a ll three species, significant biomass production differences were found (Henderson and Jose 2010). Together these findings indicate that while RE was n o t significantly altered by the application of N, which would suggest ample nutrient availability, for P deltoides RE for all three nutrients was very similar between the CON and IRR+224 treatments suggesting something other than N supply may have been controlling RE for this species. This relationship was not reflected in the RE patterns for the other two species, with the exception of N for Q. pagoda indicating a species specific mechanism for P. deltoides RE. If the sink strength (Nambiar and Fife 1991) of the woody biomass produced were the constraint for nutrient resorption, then the trends of RE sho uld mirror the trends we found for biomass. Although the relationship appears to be minor, the additional biomass in the IRR fertigation treatments did not appear to be the cause of similar RE values across treatments. Nutrient resorption proficiency (NRP ) can be used as a measure to judge the level by which species reduce nutrients in their senescing leaves (Killingbeck 1996). To this end, NRP can be utilized as an index of soil fertility, site ability to supply adequate nutrients in proper ratios for bi omass production, and determine potential and realized

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97 resorption (Killingbeck 1996 and Drenovsky and Richards 2006). The values we found for N in the leaf litter, for all three species agree with the findings of other authors (Yuan and Li 2007 for N 6 g kg 1 ). Most studies report NRP either as a percentage or on an area basis. However, due to lack of leaf area data for the litter we report NRP on the dry weight basis similar to the above studies. Although the lack of significant findings suggests our results cannot support the hypothesis of NRP peaking well below the maximum level or N application, we did find that the highest N, P, and K NRP were at levels below the highest rate of N application (Fig. 4 1 ). While no studies could be found that reported P and K NRP on a dry weight basis, we suggest that because the N P and K NRP values for all three species were so similar between the species, no one species appeared to minimize nutrient loss for these specific nutrients. Percent resorption for all three nutrients and all three species exceeded the >1.0% Killingbeck ( 1996 ) used to describe incomplete resorption (data not shown). It appears that adequate balance of all three nutrients were available su ch that the trees were not attempting to conserve any one specific nutrient. Our LNUE (Table 4 2 ) values agree with the findings of other studies (Tateno and Kawaguchi 2002 (70 to 130 g N g 1 leaf litter) However, LNUE does not necessarily correspond t o patterns found for CNUE (Table 4 3 ). LNUE appeared to be more closely related to regulating nutrient balance, as supported by the lack of significance for resorption, while CNUE appear to be more highly influenced by the amount foliar biomass needed to support the woody biomass accrued, although sink strength would not appear to be the driving factor. For this study, it could be suggested that the decomposition and nutrient release from leaf litter was N dependent, such that

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98 because of the apparent incr eased rates of P and K found with increased N application (within various woody components) were needed to retain nutrient balance. Both foliar and woody components influenced and were integral in the calculation of CNUE. With these findings, we cannot f ully support our hypothesis of CNUE peaking well below the maximum input of N, as P. occidentalis P and K CNUE peaked in the IRR+224 treatment, but not significantly. Our findings for the N:P (Fig. 4 2 ) support that as more N was applied through the ferti gation system, more P was taken up. All three species, although not significant for P. deltoides or Q. pagoda show slightly increased N:P with increased N application (11 13, 13 16, 11 14 for P. deltoides Q. pagoda and P. occidentalis CON vs. IRR+224). N:P ratios have been used to identify nutrient limitations that limit plant growth indicating either N or P deficient growing conditions. Several authors have suggested ranges of N:P that indicate nutrient deficiencies (11 18 Graciano et al 2006 ) Meuleman 1996, and Aerts and Chapin 2000, and <10 or >10 Lambers et al 1998), although a few studies have indicated rations a high as 27 (Vogt et al 1986). Knecht and Goransson ( 200 4 ) suggest that plants require nutrients in optimal ratios, but that these ratios may not be constant across species depending on which nutrients are limiting for growth. They also suggest that nutrients may be taken up in excess of the levels required fo r growth. Further, Song et al. ( 2010 ) found that moderate application of N increased the P concentrations in leaves and roots of Bauhinia faberi seedlings with the addition of water, but also noted that high levels of N application decreased growth. In a nother study, Graciano et al ( 2006 ) found that the

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99 addition of P increased the absorption of N in young Eucalyptus grandis When comparing the relationship between N and P and N and K nutrient content for the wood component in our study, regression analy sis indicates strong relationships for all three species (R 2 0.51, 0.87, 0.78, 0.95, 0.45, and 0.96 for P. deltoides Q. pagoda and P. occidentalis respectively, data not shown). Relationships for the other components would be expected to be similar as the nutrient content for N, P, and K in the wood component was the lowest for all of the components investigated. Our findings would support the need for plants to maintain nutrient balance. In further support of these findings, when ANPP was plotted ag ainst N:P (Fig 4 3 ) particular trends become apparent. At the lower bounds of the N:P for P. deltoides (CON and IRR), production was lowest suggesting N may be limiting biomass production. At the point where N and P would appear to be in the correct rat io, the largest ANPP gains were detected. Significant trends were apparent for Q. pagoda and P. occidentalis For both species, the lowest rates of ANPP were in the range of N:P that would suggest N limitation. As the ratio reached the range of balanced N:P suggesting P was becoming more limiting for growth. At this point in the correlation, N application for both species was at IRR+112 or 224 N ha 1 yr 1 further supporting the hypothesis of a plateau ing response to N application. Bungart and Huttl ( 2004 ) report both biomass production and N:P for Poplar clones. Although their data suggests greater biomass production may have been related to clonal differ ences, the N:P between plots of varying hybrids also indicates a compensatory mechanism of nutrient uptake and balance to biomass production for this

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100 species. Lockaby and Conner ( 1999 ) also found that within an optimum range of N:P (approximately 12), gre ater leaf biomass was produced. Like our study, other authors have found that these relationships are likely species specific (Lockaby and Conner 1999, Aerts and Chapin 2000, Drenovsky and Richards 200 6 and Specht and Turner 2006) and are potentially tie d to genotype It could be suggested that periodic testing of N:P in SRWC would assist fertilization management to obtain maximum biomass by circumventing nutrient imbalance We found that aboveground nutrient content nutrient resorption efficiency and proficiency, and leaf and canopy level nutrient use efficiency are not necessarily influenced by increased nitrogen availability. Although nutrient contents and levels tracked over several growing seasons, might indic ate differing levels of nutrient uptake, use, storage, and remobilization, we believe our findings are representative of this entire study length as nutrient application was consistent across years. While many plants have adaptations to conserve nutrients when nutrient levels are low, the available resources supplied by an abandoned agricultural field appear to be sufficient as to not alter the mechanism for nutrient conservati on Additionally, we found that maximum biomass production was not necessarily tied to maximum nutrient input. Production as well as nutrient requirements are species specific and may include a compensatory mechanism providing sufficient resources available from the site, to deter nutrient imbalance. These findings could suggest th at if N and P are supplied simultaneously, regular inspection of the N:P should occur throughout a rotation to ensure nutrient uptake remained balanced for maximum biomass production for SRWC species.

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101 Table 4 1 Average nutrient and standard deviation for nitrogen, phosphorus, and potassium content of bole, main branches, bark foliage, and total tree for P. deltoides Q. pagoda and P. occidentalis during year eight (2003) of the study. Letters indicate significant differences among treatments P. deltoides Nitrogen Bole kg ha 1 Branch kg ha 1 Bark kg ha 1 Foliar kg ha 1 Total CON 4.0 (1. 5 ) a 17. 1 (6.2) a 1.8 (0.6 ) a 217.7 (44.1) a 240.5 (50. 1 ) a IRR 9. 2 (2.0) b 56.1 (17.5) b 4.8 (1. 4 ) b 432.8 (115. 4 ) b 502.9 (135. 8 ) b IRR+56 10. 5 (3.0) b 46.5 (13.2) b 4.4 (1.2) b 354.4 (45.6) b 415.8 (62. 2) b IRR+112 9.1 (2. 2 ) b 42. 8 (9.9) b 4.2 (0. 9 ) b 36 6.0 (26.5) b 422. 1 ( 36.1) b IRR+224 10.0 ( 2. 0 ) b 48.7 (9. 5 ) b 3. 8 (0. 7 ) b 376.5 (34.0) b 439.0 (37.6) b Phosphorus CON 0.5 (0. 2 ) a 3.6 (13.3) a 0.1 (0.0) a 18. 4 (3.7) a 22.6 (5.0) a IRR 0.8 (0. 2 ) a 11. 9 (3.7) c 0. 3 (0. 1 ) b 37. 6 (10.0) b 50.6 (13.9) b IRR+56 0.8 (0.2) a 8. 4 (2. 4 ) b c 0.2 (0. 1 ) b 26. 8 (3.4) ab 36.2 (6.0) ab IRR+112 0.7 (0. 2 ) a 7. 6 (1. 8 ) ab 0.3 (0. 1 ) b 27.9 (2.0) ab 36. 6 (3.6) ab IRR+224 1.4 (0. 3 ) b 9.4 (1.8) bc 0. 3 (0. 1 ) b 28. 2 (2.5) ab 39. 3 (3. 6 ) b Potassium CON 8. 3 (3. 1 ) a 20.0 (7. 3 ) a 0. 4 (0.1) a 139.5 (28. 3 ) a 168.1 (37. 1 ) a IRR 19. 5 (4.2) b 70. 6 (22.0 ) b 1. 2 (0.3 ) b 261.9 (69.8 ) b 353.0 (95.5 ) b IRR+56 29. 9 (8. 6) c 60. 7 (17.3 ) b 1.1 (0.3 ) b 220.6 (28.4 ) b 312.2 (53.6 ) b IRR+112 17.4 (4.1) ab 61.8 (14.4 ) b 1.0 (0.2 ) b 236.5 (17.1 ) b 316.8 (32.4 ) b IRR+224 23.9 (4. 7 ) bc 57.7 (11.2 ) b 1.1 (0.2 ) b 267.8 (24.2 ) b 350.6 (30.9 ) b Q. pagoda Nitrogen CON 1.7 (0.5 ) a 2.9 (0.9 ) a 1.6 (0.5) a 153.0 (124. 4 ) a 159. 3 (126.2) a IRR 2.9 (1.0 ) a 5.4 (1.5 ) ab 3.0 (1.0 ) ab 266.1 (102.8 ) ab 277.4 (105.6) ab IRR+56 2. 9 (0.9 ) a 5.0 (1.5 ) ab 2.9 (0. 9) ab 308.8 (58.6 ) ab 319.7 (61.5 ) ab IRR+112 4.6 (0.8 ) b 6.4 (1.4 ) b 4.1 (0.7) bc 367.1 (33. 7) b 382.2 (31.8 ) b IRR+224 6.3 (0.4 ) c 8.9 (0.6 ) c 4.8 (0.3) c 294.0 (68.1 ) ab 314.0 (69.0 ) ab

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102 Table 4 1 Continued Q. pagoda Phosphorus Bole kg ha 1 Branch kg ha 1 Bark kg ha 1 Foliar kg ha 1 Total CON 0.2 (0. 1 ) a 0. 5 (0.1 ) a 0.1 (0.0) a 11.0 ( 9.0 ) a 11.7 (9.2 ) a IRR 0.3 (0.1 ) ab 0.7 (0.2 ) a 0.1 (0.0 ) a 16.6 (6.4 ) a 17. 7 (6.7 ) a IRR+56 0.2 (0.1 ) ab 0.8 (0.2 ) a 0.1 (0.0 ) ab 19.0 (3.6 ) a 20. 2 (3.9 ) a IRR+112 0.2 (0.0 ) ab 0.6 (0.1 ) a 0.2 (0.0 ) bc 22.7 (2.1 ) a 23.7 (2.0 ) a IRR+224 0.3 (0.0) b 0.7 (0. 1 ) a 0. 2 (0.0) c 18.1 (4.2 ) a 19.3 (4. 3 ) a Potassium CON 2.3 (0.8 ) a 2. 5 (0.7) a 0.3 (0.1 ) a 64.9 (52.8 ) a 70.0 (54.3 ) a IRR 3.1 (1.1 ) ab 3.6 (1.0 ) ab 0. 5 (0.2 ) a 100.4 (38.8 ) b 107. 5 (40.6 ) ab IRR+56 3.6 (1.1 ) ab 4.2 (1.3 ) ab 0.9 (0.3 ) b 104.5 (19.8 ) b 113.2 (22.2 ) ab IRR+112 4.3 (0.7 ) bc 4. 9 (1.0 ) bc 1.3 (0.2) bc 151.6 (13.9) b 162.1 (12.6) b IRR+224 5.9 (0. 4 ) c 6.6 (0. 5 ) c 1.7 (0.1) c 122.0 (28.2) b 136.2 (28.9) ab P. occidentalis Nitrogen CON 6.2 (0.8 ) a 4. 3 (0.6 ) a 2. 4 (0.3 ) a 278.6 (41.9 ) a 291.4 (41.1 ) a IRR 9.0 (0.2 ) b 8.9 (0.8 ) c 4.7 (0.2 ) c 536.6 (73.2 ) b 559.1 (74.1 ) b IRR+56 9.1 (0.6 ) b 8.6 (0.5 ) c 4.6 (0.3 ) c 511.5 (31.8 ) b 534.0 (31.9 ) b IRR+112 11.9 (0.3 ) c 6.7 (0.2 ) b 3.7 (0.1 ) b 470.9 (14.1 ) b 493.1 (13.9 ) b IRR+224 9.9 (1.3 ) b 8.5 ( 1.0 ) c 4.2 (0.5) bc 482.0 (47.5) b 504.6 (49.1) b Phosphorus CON 1.0 (0.1 ) a 0. 3 (0.1 ) a 0. 4 (0.0 ) a 24. 8 (3.7 ) a 26. 9 (3.6 ) a IRR 2.1 (0.1 ) b 1.6 (0.2 ) b 0. 0 (0.0 ) c 41.7 (5. 7 ) c 45.6 (5.8 ) c IRR+56 2.2 ( 0. 2 ) b 1.5 (0.1 ) b 0. 0 (0.0 ) c 39.7 (2. 5) bc 43.7 (2.5) c IRR+112 2.2 (0.1 ) b 0.9 (0.0 ) a 0. 0 (0.0 ) b 34.6 (1.0 ) bc 38.0 1.0 ) bc IRR+224 1.3 (0.2 ) b 0.9 (0.1 ) a 0. 3 (0.0 ) b 33. 1 (3. 3 ) c 35.5 (3.4) b

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103 Table 4 1 Continued P. occidentalis Potassium Bole kg ha 1 Branch kg ha 1 Bark kg ha 1 Foliar kg ha 1 Total CON 6.7 (0.9) a 3. 6 (0.5) a 0.3 (0.0) a 146.0 (22.0) a 156. 7 (21.3) a IRR 10.3 (0.2) b 6.6 (0.6) c 1.3 (0.1) c 257.5 (35.1) b 275. 7 (35.8) b IRR+56 10.5 (0.7) c 6.4 (0.4) c 1.3 (0.1) c 245.5 (15.1) b 263. 7 (15.5) b IRR+112 12.3 (0.3) c 4.2 (0.2) a 1.3 (0.0) c 226.0 (6.8) b 243. 7 (6.6) b IRR+224 10.6 (1.4) c 5.1 (0.6) b 1.0 (0.1) b 213.0 (21.0) b 229.2 (22.1) b

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104 Table 4 2 Average % n utrient resorption efficiency ( RE%) and leaf level nutrient use efficiency (LNUE g g 1 ) for nitrogen, phosphorus, and potassium with standard deviation for P. deltoides Q. pagoda and P. occidentalis during year eight (2003) of the study. Letters indicate significant differences among treatments RE LNUE P. deltoides N P K N P K CON 65 ( 9 )a 5 8 (11) a 85 (8)a 136 (13)a 1250 (7 6 )a 732 (18 2 )a IRR 6 2 (4 )a 56 (6)a 81 (14 )a 116 (20)a 123 2 (299)a 41 8 (34 5 )a IRR+56 60 (3 )a 56 ( 7 )a 72 (8 )a 112 (19)a 1103 (8 8 )a 363 (158 )a IRR+112 5 7 (2)a 53 (7)a 72 (9) a 107 (7 )a 111 8 (74 )a 361 (118)a IRR+224 63 ( 5 )a 57 (7 )a 83 ( 8 ) a 111 (15)a 1037 (182)a 331 (18 3 )a Q. pagoda CON 6 2 (4)a 40 (6 )a 6 4 (22 )a 140 ( 7 )a 1107 (13 1 )a 27 4 ( 80 )a IRR 60 (5)a 39 (6 )a 57 (11 )a 114 (12)a 1092 (84 )a 265 (3 1 )a IRR+56 60 (7 )a 37 (2 )a 54 (1 8 )a 114 (12)a 1031 (72)a 2 70 (125)a IRR+112 62 ( 7 )a 37 ( 5 )a 51 (8 )a 113 (2 1 )a 1049 (84 )a 21 4 (6 3 )a IRR+224 61 ( 7 )a 4 5 ( 16 )a 52 (18 )a 111 (2 3 )a 1035 (9 5 )a 218 (7 2 )a P. occidentalis CON 74 (4)a 5 7 (1 3 )a 85 ( 3 )a 17 6 (3 7 )a 944 (11 2 )a 476 (7 6 )a IRR 72 (6 )a 5 1 ( 3 ) a 83 (6 )a 14 4 (7)a 88 6 (4 2 )a 404 (7 9 )a IRR+56 70 ( 5 )a 50 (4 )a 83 (5 )a 132 (26)a 866 (8 1 )a 37 3 (9 2 )a IRR+112 70 (7 )a 5 2 (10)a 82 (4 )a 126 (22)a 863 ( 97 )a 37 6 (10 7 )a IRR+224 66 ( 3 )a 50 (4 )a 73 (11 )a 134 (7) a 934 (12 7 )a 40 1 ( 100 )a

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10 5 Table 4 3 Canopy nutrient use efficiency (CNUE) of unit woody biomass (Mg) per unit nitrogen, phosphorus, and potassium (kg) for P. deltoides Q. pagoda and P. occidentalis for year eight (2003) of the study for each treatment. Letters in dicate significant differences among treatments CNUE P. deltoides N (Mg/kg) P (Mg/kg) K (Mg/kg) CON 3. 5 (0.3) a 40. 5 (3 .3 )a 4.8 (1.2 )ab IRR 4.9 (0. 3 ) b 55.6 (9.4 )a 6.3 ( 1.0 ) a IRR+56 5.1 (0.6 ) b 63.6 (9.9 )a 5.6 (1.0)ab IRR+112 3. 3 (0.5 ) a 43.5 (8.9 )a 3.8 (0. 7 ) b IRR+224 4. 1 (1. 2 )ab 59.8 (28.4 )a 4.4 (3. 5 )ab Q. pagoda CON 2. 1 (0.3) a 26. 3 (1.9 )a 4. 2 (0.4 ) a IRR 1. 5 (0. 2 ) a 23.3 (4.4 )a 4.0 (1.3 ) a IRR+56 1.3 (0.4 ) a 23. 2 (5.1 )a 3.9 (0.7) a IRR+112 1. 3 (0.4 ) a 20.9 (5.1 )a 3.2 (0. 7) a IRR+224 1. 5 (0.7 ) a 23.3 (3.3)a 2.9 (0. 6 ) a P. occidentalis CON 1.9 (0.5 ) a 23.1 (7.5 )a 3.0 (0. 7 ) a IRR 1.8 (0.8 ) a 22.1 (7.6 )a 3.1 (1. 8) a IRR+56 1.6 (0.4 ) a 20.4 (2.5 )a 2.6 (0.6) a IRR+112 1.3 (0.3 ) a 19.6 (3.9 )a 2.6 (0.7) a IRR+224 1.8 (0.1) a 23.2 (3. 7 )a 3.4 (1.8) a

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106 Figure 4 1 Average and standard error of nutrient resorption proficiency (g nutrient kg 1 dry weight) of litterfall nitrogen (A), phosphorus (B) and potassium (C) for P. deltoides (square), Q. pagoda (triangle), and P. occidentalis (circle).

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107 Figure 4 2 N:P foliar ratios for P. deltoides Q. pagoda and P. occidentalis for each treatment during year eight (2003) of the study. Letters above the treatments indicate significant differences.

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108 Figure 4 3 Biomass production (Mg ha 1 yr 1 ) and foliar N:P for year eight (2003) of the study for P. deltoides (A) Q. pagoda (B) and P. occidentalis (C) for all treatments (circle = CON, sq uare = IRR, triangle = IRR + 56 kg N ha 1 yr 1 diamond = IRR + 112 kg N ha 1 yr 1 and X = IRR + 224 kg N ha 1 yr 1 ).

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109 CHAPTER 5 SUMMARY OF CONCLUSIO NS The overall objective of this study was to examine the production ecology of three economically important hardwood species, Populus deltoides Quercus pagoda and Platanus occidentalis to varying levels of resource application. Specifically, we wanted to determine how biomass production was altered, physiologically supported, and how nutrient uptake and storage within various biomass components were influenced by the resource applications of irrigation and the combined treatment of irrigation and nitrogen application (fertigation) at 56, 112, and 224 kg N ha 1 yr 1 The first objective was to determine the amount of woody and foliar biomass produced for the three species. Allometric equations developed by Shelton et al. 1982 were used for P. deltoides and equations developed by Schlaegel and Kennedy 1986 were used for Q. pagod a and P. occidentalis These equations were used for woody biomass production and litter traps were used to determine actual foliar biomass (Chapter 2). Next, to determine how much light the canopy was capturing and the extent of which the treatments wer e influencing physiological mechanisms, photosynthesis data was collected from sun and shade leaves for all three species. Leaves were collected and measured, weighed, dried, and analyzed for nitrogen content (Chapter 3). Finally, we wanted to determine the aboveground N, P, and K content and use efficiencies for these three species and understand the relationship between nitrogen, phosphorus, and potassium as it pertained to the resource gradient. To determine the amount of nitrogen being utilized in va rious above ground tissues, versus the amount applied on an annual basis, samples were taken from the bole, branches, bark, and foliage for all three species and analyzed for nitrogen, phosphorus, and potassium (Chapter 4).

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110 Few studies have looked at sim ultaneous application of irrigation with fertilizer for growing SRWC on agricultural fields. While fertigation techniques are relatively infrequent, the resource gradient allows an opportunity to study resource requirements and mechanisms necessary for ma ximum aboveground biomass production. Given the data collected during our study and the current literature available for these three species, several generalizations regarding the use of P. deltoides Q. pagoda and P. occidentalis in short rotation plant ations can be made. Our data indicate that plantation establishment for P. deltoides may not have to be restricted to bottomland sites for maximum production. The significant production response of P. deltoides to the IRR treatment and the lack of signif icant differences for the N treatments suggest that IRR alone may be sufficient to obtain maximum growth on good quality sites such as agricultural fields with similar nutrient levels as this study P. deltoides may benefit from low levels of fertilizatio n on low quality sites. Q. pagoda treatments applied in this study may prove beneficial for accelerating production in plantations. Its slow growth rate, compared to P. deltoides and P. occidentalis would otherwise preclude it from short rotation operations. Despite increased production in response to the IRR treatment, standing biomass for P. occidentalis was considerably lower than P. deltoides Disease, crown breakage, and mortality may have confounded the production re sponse of P. occidentalis to cultural treatments. Overall, our results suggest that ANPP is highly correlated with LAI; however, the relationship is species specific. Maximum ANPP was reached well below the maximum LAI for Q. pagoda and P. occidentalis P. deltoides ANPP was highest at the maximum LAI, which was achieved with IRR alone.

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111 These results suggest that species specific cultural practices that produce the optimum LAI and maximum ANPP need to be identified before fertigation techniques can be widely adopted for increasing biomass production potential of SRWC. The high biomass production potential of SRWC such as those tested in our study will play a significant role in helping to meet renewable energy standards. The positive impact of irr igation and fertilization on productivity has been well documented. However, as has been suggested by other researcher, the reasons for increased growth response are less evident. Several factors related to canopy dynamics have been identified by others and were explored in detail in our study. As expected, SLA, SLN, LAI, A max and ANPP varied across the supplied soil resource gradient for all three species. Irrigation alone was sufficient in P. deltoides and P. occidentalis to increase SLA whereas SLA r esponded to both irrigation and fertilization in Q. pagoda A corresponding increase in LAI, similar to that of SLA, was also observed for all three species. A max reached peak rates for all species in the IRR+112 treatment for both sun and shade leaves a nd showed strong positive correlations with SLN across the gradient. ANPP exhibited a curvilinear relationship with canopy A max with peak production occurring well below the maximum estimated canopy A max An increase in LAI alone was sufficient to achiev e maximum ANPP in both P. deltoides and P. occidentalis However, an increase in both LAI and foliar A max resulted in the maximum ANPP in Q. pagoda Although we hypothesized that increased foliar A max in combination with an increase in LAI would be neces sary to increase ANPP along the increasing resource gradient, our results indicate that an increase in LAI alone can increase canopy A max and lead to increased productivity. An increase in foliar A max may

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112 or may not be necessary to yield the highest ANPP depending on the species. Further research was needed to determine the relationship between N and other potentially limiting nutrients so that the role of nutrient imbalance can be better understood. Given the appeal for managing short rotation plantation s with fertigation, this study should give land managers a solid scientific basis for developing efficient fertigation strategies. We found that nutrient content nutrient resorption efficiency and proficiency, and leaf and canopy level nutrient use effic iency are not necessarily influenced by increased resource availability. Although nutrient contents and levels tracked over several growing seasons might indicate differing levels of nutrient uptake, use, storage, and remobilization, we feel our findings are representative of this entire study length as nutrient application was consistent across years. While many plants have adaptations to conserve nutrients when nutrient levels are low, the available resources supplied by an abandoned agricultural field appear to be sufficient as to not alter the mechanism for nutrient conservatism. Additionally, we found that maximum biomass production was not necessarily tied to maximum nutrient input. Production as well as nutrient requirements are species specific a nd may include a compensatory mechanism providing sufficient resources are available from the site, to deter nutrient imbalance. These findings could suggest that if N and P are supplied simultaneously, regular inspection of the N:P should occur throughou t a rotation to ensure nutrient uptake remained balanced for maximum biomass production for SRWC species. The concept of SRWC supply systems are inherently tied to changes in attitudes about energy production. F ast growing hardwood species are among a sui te of options considered for biopower and liquid fuels based on biomass By studying the production

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113 ecology of three fast growing hardwood species, we were able to make some generalizations about the applicability of using P. deltoides Q. pagoda and P. occidentalis for SRWC supply systems. Our results indicate species specific responses to biomass production, the physiological mechanisms that control production, and the relationships between nutrient use and accumulating biomass. Understanding the inte ractions between the SRWC species and intensive practices such as fertilization will assist future development of biomass/biofuel practices using fast growing hardwood species for SRWC systems. Future directions for this type of study should include hybrid or clonal varieties selected for their potential in biomass accrual, particularly if the objective is for biomass/biofuel purposes. Additionally, should co firing of woody biomass with coal be a viable alternative for the production of electricity, tonna ge of woody biomass needed for sustainable production will likely require intensive management in natural forests and dedicated energy plantations Given the response of these three species to N application, a study with varying levels of other essential nutrients (such as P and K) should be considered to determine if biomass production could be further increased with multiple levels of nutrients.

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114 LIST OF REFERENCES Aber, J.D., Nadelhoffer, K.J., Steudler, P., and Melillo, J.M. 1989. Nitrogen saturation in northern forest ecosystems. BioSci. 39(6):378 386. Abt, R.C., Abt, K.L., Cubbage, F.W., and Henderson, J.D. Effect of policy based bioenergy demand on southern timber markets: A case study of North Carolina Biomass and Bioenergy (2010), doi:10.1016/j.biombioe.2010.05.007. Aerts, R. 1997. Nitrogen partitioning between resorption and decomposition pathways: a trade off between nitrogen use efficiency and litter decomposability. Oikos 80(3)603 606. Aer ts, R. 1996. Nutrient resorption from senescing leaves of perennials: are there general patterns? J. Ecol. 84(4):597 608. Aerts, R. and Berendse, F. 1988. The effect of increased nutrient availability on vegetation dynamics in wet heathlands. Ve getatio 76(1 2):63 69. Aerts, R. and de Caluwe, H. 1994. Nitrogen use efficiency of Carex species in Relation to nitrogen supply. Ecology 75(8):2362 2372. Aerts, R and Chapin III, F.S. 2000. The mineral nutrition of wild plants revisited. A re evaluation of processes and patterns. Adv. Ecol. Res. 30:1 67. Albaugh, T.J., Allen, H.L., Dougherty, P.M., and Johnsen, K.H. 2004. Long term responses of loblolly pine to optimal nutrient and wat er resource availability. For. Ecol. and Manage. 192:3 19. Albaugh, T. J., Allen, H. L., Dougherty, P. M. Kress, L. W. and King J. S. 1998. Leaf area and above and belowground growt h responses of loblolly pine to Nutrient and water additions. For. Sci., 44(2):317 328. Allen, C .B., Will, R .E., McGravey, R .C., Coyle, D .R., and Coleman, M D. 2005. Radiation use efficiency and gas exchange responses to water and nutrient availability in irrigated and fertilized st ands of sweetgum and P. occidentalis Tre e Phys 25 :191 200. Allen, C. B., Will, R.E. Sarigumba, T. Jacobson, M.A. Daniels R.F. and Kennerly S.A. 2004. Relationships between canopy dynamics and stem volume production of four species receiving irrigation and fertilization. Connor, K.F. Ed. Proceedings of the 12 th biennial southern silviculture research conference. Gen. Tech. Rep. SRS 71. Asheville, NC, US. Dept. of Ag. For. Serv. South. Res. Sta. 594p.

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115 Allen, H.L. 1987. Forest Fertilizers. J. For. 85(2):37 46. Allen, H. L., Albaugh, T. J. and Johnsen K. 2002. Water and nutrient effects on l oblolly pine production and stand developme nt on a sandhill site. General Technical Report SRS 48, pg. 594 595. Asheville, NC. Department of Agriculture, Forest Service, Southern Research Station. Anten, N.P., Werger, M. J. and Medina E. 1998. Nitrogen distribution and lead area indices in relation to photosynthetic nitrogen use efficiency in savanna grass. Plant Ecol., 138:63 75. Arp, W. J., Van Mierlo, J.E. Berendse, F. and Snijders W. 1998 Interactions between elevated CO 2 concentration, nitrogen, and water: Effects on growth and water of six perennial plant species. Plant, Cell and Environ., 21(1):1 10. Augusto, L., Bakker, M.R., de Lavaissiere, C., Meille, L., and Saur, E. 2009. E stimation of nutrient content of woody plants using allometric relationships: quantifying the difference between concentration values from the literature and actual. For. 82(4):463 477. Axelsson, E. and Axelsson B. 1986. Changes in carbon allocati on patterns in spruce and pine trees following irrigation and fertilization. Tree Physiol., 2:189 204. Barnes, B. V., Zak, D. R. Denton, S. R. and Spurr S.H. 1998. Forest Ecology. John Wiley & Sons, Inc., New York, NY. 774p. Bekele, A., Hudnall, W.H., and Tiarks, A.E. 2003. Respo nse of densely stocked loblolly p ine ( Pinus taeda L.) to applied nitrogen and p hosphorus. South. J. App. For. 27(3):180 189. Benjamin, J ., Lilieholm, R.J., and Damery, D. 2009. Chall enges and opportunities for t he northeastern forest bioindustry. J. For. 107(3):125 131. Berendse, F. and Aerts, R. 1987. Nitrogen use efficiency: a biologically meaningful definition? Func. Ecol. 1(3):293 296. Binkley, D. Stape, J.L., Bauerle, W.L., and Ryan, M.G. 2010. Explain ing growth of individual trees: Light interception and efficiency of light use by Eucalyptus at four sites in Brazil. For. Ecol. Manage. 259:1704 1713. Binkley, D., Stape J.L. and Ryan, M.G. 2004. Thinking about efficiency of resource use in forests For. Ecol. Manage. 193:5 16. Birk, E M. 1997. Fertilizer use in the management of pine and eucalypt plantations in Australia: a review of past and current practices. N. Z. Jour. For. Sci., 24:289 320.

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135 BIOGRAPHICAL SKETCH Dawn Elizabeth Henderson was born in southern Illinois. It was there among the natural beauty of Illinois that an appreciation for the outdoors developed. After exploring several different vocations, she found wo nder, excitement, and challenge in furthering her education. While attending St. Charles Community College she discovered her enthusiasm for science. After receiving her Associate of Arts, she began her work on a Bachelor of Science at Southern Illinois University at Edwardsv ille, Illinois. During the course of her b studies, she found a passion for floodplain forests and research in general. It was that drive to more fully experience and understand her natural surroundings that compe lled her to receive her Master of Science in Science at Southern Illinois University. After accepting a Research Assistantship with Shibu Jose at University of Florida her interests came to include forest ecology and physiology. She obtained her PhD in 2010. She now works for one of