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Seedling Recruitment as a Driver of Species Richness in the Understory of the Longleaf Pine Savanna

Permanent Link: http://ufdc.ufl.edu/UFE0022163/00001

Material Information

Title: Seedling Recruitment as a Driver of Species Richness in the Understory of the Longleaf Pine Savanna
Physical Description: 1 online resource (78 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: limitation, longleaf, microsite, pine, recruitment, savanna, seedling, wiregrass
Wildlife Ecology and Conservation -- Dissertations, Academic -- UF
Genre: Wildlife Ecology and Conservation thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: To understand the potential mechanistic controls on species richness in the threatened and diverse longleaf pine ecosystem, we used a long-term resource manipulation study across a natural soil moisture and productivity gradient to assess environmental controls on seedling recruitment. We used a factorial design to manipulate water and nitrogen over five years. High and low density experimentally seeded recruitment rates and natural recruitment rates were assessed over two growing seasons to examine the importance of microsite and seed limitations on seedling density and species richness. We also tested the possibility of facilitation of seedling recruitment by wiregrass (the dominant understory species), by measuring environmental variables at potential recruitment microsites that varied in proximity to wiregrass, and by manipulating shade over experimentally sown seeds. Water availability influenced species richness regardless of moisture gradient location, yet, seedling recruitment rates only varied by treatment for some functional groups. In addition, in xeric sites, higher seeding density resulted in higher recruitment only when water was added, suggesting that microsite limitations are more important than seed limitations in dry regions. Meanwhile, wiregrass presence modulated temperature and moisture availability at seedling recruitment microsites, but shade alone was not found to facilitate seedling recruitment regardless of water availability. These results indicate that water availability drives seedling recruitment by counteracting recruitment microsite limitations. Microsite availability may then influence species richness of the seedling community but the response seems to be guild specific. We suggest that episodic recruitment events based on water availability at recruitment microsites could drive high levels of species diversity, and that microsite moisture availability may be related to wiregrass abundance.
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.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Bruna, Emilio M.
Local: Co-adviser: Kirkman, Lelia Katherine.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022163:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022163/00001

Material Information

Title: Seedling Recruitment as a Driver of Species Richness in the Understory of the Longleaf Pine Savanna
Physical Description: 1 online resource (78 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: limitation, longleaf, microsite, pine, recruitment, savanna, seedling, wiregrass
Wildlife Ecology and Conservation -- Dissertations, Academic -- UF
Genre: Wildlife Ecology and Conservation thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: To understand the potential mechanistic controls on species richness in the threatened and diverse longleaf pine ecosystem, we used a long-term resource manipulation study across a natural soil moisture and productivity gradient to assess environmental controls on seedling recruitment. We used a factorial design to manipulate water and nitrogen over five years. High and low density experimentally seeded recruitment rates and natural recruitment rates were assessed over two growing seasons to examine the importance of microsite and seed limitations on seedling density and species richness. We also tested the possibility of facilitation of seedling recruitment by wiregrass (the dominant understory species), by measuring environmental variables at potential recruitment microsites that varied in proximity to wiregrass, and by manipulating shade over experimentally sown seeds. Water availability influenced species richness regardless of moisture gradient location, yet, seedling recruitment rates only varied by treatment for some functional groups. In addition, in xeric sites, higher seeding density resulted in higher recruitment only when water was added, suggesting that microsite limitations are more important than seed limitations in dry regions. Meanwhile, wiregrass presence modulated temperature and moisture availability at seedling recruitment microsites, but shade alone was not found to facilitate seedling recruitment regardless of water availability. These results indicate that water availability drives seedling recruitment by counteracting recruitment microsite limitations. Microsite availability may then influence species richness of the seedling community but the response seems to be guild specific. We suggest that episodic recruitment events based on water availability at recruitment microsites could drive high levels of species diversity, and that microsite moisture availability may be related to wiregrass abundance.
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.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Bruna, Emilio M.
Local: Co-adviser: Kirkman, Lelia Katherine.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022163:00001


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PAGE 1

SEEDLING RECRUITMENT AS A DRIVER OF SPECIES RICHNESS IN THE UNDERSTORY OF THE LONGLEAF PINE SAVANNA By GWENLLIAN D. IACONA A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008 1

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2008 Gwenllian D. Iacona 2

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To T.E.B Thanks for everything 3

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ACKNOWLEDGMENTS I am indebted to my advisors for taking on the difficult job of trying to teach me how to approach ecological questions from a scientific perspective. Kay Kirkman spent endless hours of field time teaching me how to identify plants even if there was only one tiny leaf present, and then even longer hours helping me shape my data into a coherent package and write it up in a reasonable fashion. Emilio Bruna navigated me through the hurdles of graduate school, provided stellar editing services, and was a tireless cheerleader at all times. Wendell Cropper provided helpful suggestions both in the planning and editing stages. Many other people at UF and Ichauway were also instrumental to this project. Meghan Brennan in IFAS Statistics helped me to navigate the jungle that is SAS and wrestle my data into shape. Melanie Kaeser in the plant lab provided logistical support throughout the project. Liz Cox at Ichauway could always find the most obscure references and books that I asked for, and Caprice, Monica, and Delores in the WEC office, and Cindy and Becky at Ichauway, handled all the logistics of my cosponsorship. I am forever grateful to Katie Stuble, Michelle Creech and Jennifer Falkey for not letting me starve while I was working on this project and being the best roommates ever. Scott Wiggers gave many hours of help in projects such as seed counting and fencing. Matt Trager and Ian Fiske introduced me to Krishna lunch, ultimate Frisbee, and other necessities of graduate school. I also want to thank my parents, Chip and Kathy Iacona who always encourage me to follow my dreams, my siblings John and Anna who help me reach them, and Minh who always answers the phone. Finally, I would like to thank Ma Yoga Shakti for always teaching by example that nothing is impossible if you keep on walking. This project was funded by the J.W. Jones Ecological Research Center and the U.F. Department of Wildlife Ecology and Conservation. 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 ABSTRACT .....................................................................................................................................9 INTRODUCTION .........................................................................................................................11 CHAPTER 1 EFFECTS OF RESOURCE AVAILABILITY ON PATTERNS OF UNDERSTORY SEEDLING RECRUITMENT IN A FIRE MAINTAINED SAVANNA.............................15 Introduction.............................................................................................................................15 Methods..................................................................................................................................18 Study Site.........................................................................................................................18 Analyses..........................................................................................................................21 Patterns of recruitment.............................................................................................22 Seed vs. microsite limitation....................................................................................23 Results.....................................................................................................................................23 Patterns of Recruitment...................................................................................................23 Seed vs. Microsite Limitation..........................................................................................25 Discussion...............................................................................................................................26 Relative Importance of Seed and Microsite Limitations.................................................26 Community Implications.................................................................................................26 Caveats............................................................................................................................27 Conclusion.......................................................................................................................28 2 WIREGRASS PRESENCE MODULATES RECRUITMENT MICROSITES....................41 Introduction.............................................................................................................................41 Methods..................................................................................................................................44 Study Site.........................................................................................................................44 Microhabitat Properties...................................................................................................45 Shade Cloth Treatments..................................................................................................46 Analyses..................................................................................................................................47 Microhabitat Properties...................................................................................................47 Shade Treatments............................................................................................................48 5

PAGE 6

Results.....................................................................................................................................48 Microhabitat Properties...................................................................................................48 Shade Treatments............................................................................................................49 Discussion...............................................................................................................................49 3 CONCLUSION.......................................................................................................................63 APPENDIX: SPECIES LIST.........................................................................................................67 WORKS CITED............................................................................................................................71 BIOGRAPHICAL SKETCH.........................................................................................................78 6

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LIST OF TABLES Table page 2-1 Results from analysis of mean seedling recruitment.........................................................36 2-2 Results from analysis of mean seedling recruitment by functional group.........................37 2-3 Results of split plot analysis of experimentally seeded recruitment at two levels of seed density........................................................................................................................39 2-4 Results of ChaoJaccard dissimilarity index calculations...................................................40 3-1 Results from mixed model analysis of variance of microsite measurement values..........60 3-2 Results of split-split plot analysis of seedling recruitment across two shade treatments...........................................................................................................................62 7

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LIST OF FIGURES Figure page 2-1 Number of seedling recruits displayed a trend towards responding to resource availability..........................................................................................................................30 2-2 Number of seedlings per functional group varied across resource treatment after two years of recruitment...........................................................................................................31 2-3 Number of species per ring varied across treatments after two years of recruitment........32 2-4 Frequency distribution of the number of plots that each species was observed in............33 2-5 Seed density influenced the number of seedling recruits...................................................34 2-6 More cumulative species per plot were observed in potential recruits (seed rain seedlings) then in actual recruits (field seedlings).............................................................35 3-1 Environmental variables within potential recruitment microsites.....................................54 3-2 Shading influenced recruitment.........................................................................................59 4-1 Conceptual model..............................................................................................................66 8

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SEEDLING RECRUITMENT AS A DRIVER OF SPECIES RICHNESS IN THE UNDERSTORY OF THE LONGLEAF PINE SAVANNA By Gwenllian D. Iacona May 2008 Chair: Emilio Bruna Cochair: L. Katherine Kirkman Major: Wildlife Ecology and Conservation To understand the potential mechanistic controls on species richness in the threatened and diverse longleaf pine ecosystem, we used a long-term resource manipulation study across a natural soil moisture and productivity gradient to assess environmental controls on seedling recruitment. We used a factorial design to manipulate water and nitrogen over five years. High and low density experimentally seeded recruitment rates and natural recruitment rates were assessed over two growing seasons to examine the importance of microsite and seed limitations on seedling density and species richness. We also tested the possibility of facilitation of seedling recruitment by wiregrass (the dominant understory species), by measuring environmental variables at potential recruitment microsites that varied in proximity to wiregrass, and by manipulating shade over experimentally sown seeds. Water availability influenced species richness regardless of moisture gradient location, yet, seedling recruitment rates only varied by treatment for some functional groups. In addition, in xeric sites, higher seeding density resulted in higher recruitment only when water was added, suggesting that microsite limitations are more important than seed limitations in dry regions. Meanwhile, wiregrass presence modulated temperature and moisture availability at seedling 9

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recruitment microsites, but shade alone was not found to facilitate seedling recruitment regardless of water availability. These results indicate that water availability drives seedling recruitment by counteracting recruitment microsite limitations. Microsite availability may then influence species richness of the seedling community but the response seems to be guild specific. We suggest that episodic recruitment events based on water availability at recruitment microsites could drive high levels of species diversity, and that microsite moisture availability may be related to wiregrass abundance. 10

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CHAPTER 1 INTRODUCTION Understanding the mechanisms that influence local species diversity is a central area of research in ecology. Species diversity is an important indicator of ecosystem integrity that is often used in identifying priority areas for conservation (Margules and Usher 1981), and it may also influence ecosystem processes and stability (Tilman et al. 1996). Because seedling recruitment is a critical bottleneck for many plant species (Eriksson and Ehrlen 1992), the mechanistic drivers of seedling community composition are of interest because of their influence on local plant diversity (Grubb 1977). Of particular interest are the mechanisms by which limitations to seedling recruitment interact to influence species diversity in environmentally stressful, yet species rich ecosystems. Seedling recruitment success is dependent on overcoming multiple limitations. The first requirement for a recruitment event is the presence of viable seed. Seed limitations commonly constrain the potential for regeneration in a system by a simple lack of seed (Eriksson and Ehrlen 1992, Turnbull et al. 2000). The availability of seed is dependent on plant fecundity (Clark and Ji 1995), successful dispersal (Satterthwaite 2007), and the avoidance of pathogens or predation (Wright 2002). In addition to being present, the seed must reach an appropriate recruitment microsite for a successful recruit to appear. Microsite limitations regulate the distribution of seedlings within a community even when seed supply is not limiting (Grubb 1977, Moore and Elmendorf 2006). Factors such as water availability (MacMahon and Schimpf 1981, Holmgren et al. 1997), nutrient supply (Shumway 2000), and environmental stress (Setterfield 2002) all regulate the suitability of a microsite for seedling recruitment. Finally, post-germination establishment limitations can produce seedling mortality through effects such as competition for 11

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resources (Stevens et al. 2004), selective herbivory (Kuijper et al. 2005), or environmental effects (Lloret et al. 2005). Species richness of the seedling recruits is also dependent on seed supply (Eriksson and Ehrlen 1992), but inherent properties of microsites and the randomness involved in their availability are also important (Hurtt and Pacala 1995). Diversity of the regional seed supply is a prerequisite to community level species richness (Franzen and Eriksson 2001), however, temporal variation in seed supply also can influence the diversity of the seedling community (Wright et al. 2005). Meanwhile, stochasticity in recruitment microsite properties and availability is often thought to be the primary driver of species richness at a local scale (Grubb 1977). There is significant evidence that heterogeneity of resource availability produces a gradient of recruitment microsites that result in species diversity as individual species compete for their optimum niches (Silvertown et al. 1999, Lundholm and Larson 2003a, Gundale et al. 2006, Oster et al. 2007). However, in highly diverse systems, niche heterogeneity does not seem to adequately explain patterns of species richness and evidence from field experiments and theoretical models suggest that a combination of microsite limitation and the chance involved in propagule availability results in species diversity (Sale 1977, Hurtt and Pacala 1995, Hubbell et al. 1999, Brokaw and Busing 2000, Busing and Brokaw 2002). Plant-to-plant interactions often regulate the availability of suitable recruitment microsites. In many ecosystems, competitive exclusion decreases the availability of appropriate microsites as system productivity increases through mechanisms such as reduced light availability (Goldberg and Werner 1983), litter buildup (Sydes and Grime 1981), or below ground competition for resources (Rajaniemi et al. 2003). However, in arid or environmentally stressful systems, the presence of an adult plant often facilitates the production of recruitment microsites 12

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and promotes seedling success by ameliorating resource deficiencies (Holmgren et al. 1997, Shumway 2000), providing protection from environmental extremes (Hacker and Bertness 1995, Greenlee and Callaway 1996), or counteracting physical disturbance (Kuiters and Slim 2003, Callaway et al. 2005). In turn, environmental gradients can regulate the magnitude and direction of plant-to-plant interaction and thus feedback on species richness (Bruno et al. 2003). In general, competitive effects tend to dominate at low levels of stress while facilitative interactions are more apparent at higher stress levels (Callaway and Walker 1997). These shifts in the direction of interaction across stress gradients can be due to variation in water availability, (Hacker and Bertness 1995, Greenlee and Callaway 1996), temperature (Kikvidze et al. 2006), or grazing pressure (Smit et al. 2007). Ecosystems with exceptionally high species richness provide an opportunity to investigate mechanisms that control species richness. The longleaf pine savanna is of particular interest, because it extends across a wide environmental gradient with corresponding resource and temperature stress. This ecosystem was once the dominant vegetation type in the southeastern US but has been reduced to about 3% of its previous range (Ware et al. 1993). It is characterized by frequent fires and seasonally dry conditions. A ubiquitous wiregrass (Aristida stricta.) herbaceous layer provides fuel for the fires and is inhabited by an extremely species rich community of forbs and legumes (Peet and Allard 1993). Some of the highest species diversity rates in North America have been documented in this understory, with the regions of greatest diversity coinciding with areas of maximum plant primary productivity and soil water availability in the system (Kirkman et al. 2001). Meanwhile, the species that are present in this 13

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ecosystem are highly resistant to drought and fire stress when mature, but the slow regenerative capacity of these habitats suggests seedling recruitment limitations (Kirkman et al. 2004). This relationship between understory species diversity and system productivity in the longleaf pine savanna may best be explained by variability in seedling recruitment that is driven by resource limitations interacting with facilitative effects. The extent of seed and microsite limitations within this system are not known. Meanwhile the periodic water stress in the system could promote facilitative interactions. The objective of this study is to examine how environmental controls on seedling recruitment drive patterns of species diversity in the understory of the longleaf pine savanna. The second chapter presents a study of the influence of resource availability and seed rain on patterns of seedling recruitment and species diversity across a natural moisture gradient. In the third chapter, we discuss the potential for facilitation of seedling recruitment by wiregrass and provide results that show differences in microsites below versus between wiregrass clumps. Finally, the fourth chapter presents a conceptual model illustrating the episodic nature of seedling recruitment in the longleaf pine ecosystem, and draws conclusions about the implications of these results from a theoretical perspective. 14

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CHAPTER 2 EFFECTS OF RESOURCE AVAILABILITY ON PATTERNS OF UNDERSTORY SEEDLING RECRUITMENT IN A FIRE MAINTAINED SAVANNA Introduction Plant community diversity is influenced by the mechanistic drivers of seedling recruitment (Grubb 1977). In particular, patterns of seedling recruitment are often a result of recruitment limitations. Seed limitations determine the availability of a viable propagule, and are influenced by factors such as regional community composition, plant fecundity, and herbivory (Eriksson and Ehrlen 1992). However, even if viable seeds are present, seedling recruitment cannot occur unless there is an appropriate microsite for regeneration that can support the germination and establishment of a seedling by supplying all the necessary resources (Eriksson and Ehrlen 1992, Caspersen and Saprunoff 2005). The presence of appropriate microsites is often a stochastic process that is dependent on disturbance (Harper et al. 1965), system productivity (Stevens et al. 2004), or plant to plant interactions (Valientebanuet and Ezcurra 1991), but physical features such as topography and climate are also important (Pollock et al. 1998). Both seed and microsite limitations are dependent on environmental controls but few studies have experimentally manipulated both microsite availability and seed supply across environmental gradients (Poulsen et al. 2007). Recruitment limitations are often examined with regard to population dynamics, but they can also play a significant role in driving community composition. Niche heterogeneity theory declares that physical and environmental variability results in recruitment microsites that are uniquely situated to each species competitive strategy (Grubb 1977). In turn, deterministic recruitment events occur where the best competitor that is present at an available microsite occupies the site regardless of the presence of other seeds or seedlings (Turnbull et al. 2000). Evidence that spatial heterogeneity of resources can influence species diversity at a site includes 15

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variability in nitrogen mineralization (Smith and Huston 1989, Gundale et al. 2006), soil properties such as cation availability and texture (Freestone and Inouye 2006), or water supply (Silvertown et al. 1999, Lundholm and Larson 2003b) influencing variability in plant communities. An alternative explanation for the promotion of species diversity at the seedling establishment stage is the idea that recruitment operates as a lottery. For some extremely diverse ecosystems, the apparent niche overlap between co-occurring species precludes niche heterogeneity as the most plausible explanation for the observed patterns of diversity. In these cases, the random chance that is involved in occupying recruitment sites is considered to provide a lottery like basis for the maintenance of species diversity (Sale 1977). In this case species richness is dependent on chance and the composition of the regional seed pool. However, several assumptions must be met before this strategy can explain high levels of diversity. The first requirement is that the availability of microhabitats for recruitment must be the limiting factor that determines propagule success. The second assumption is that the chance of a propagule reaching an available microsite must not be closely related to the density of adults in the community (i.e., not seed limited). Examples of lottery based recruitment mechanisms are found primarily in species rich tropical forests (Hubbell et al. 1999, Brokaw and Busing 2000). The processes of niche heterogeneity and lottery recruitment as drivers of species recruitment are not mutually exclusive, however. Models of competitive exclusion based microsite occupancy that act in concert with lottery based seed availability result in the highest levels of species diversity (Hurtt and Pacala 1995, Hubbell et al. 1999). The longleaf pine savanna is an ideal model ecosystem for examining the relationship between recruitment limitations and species richness in an exceptionally diverse ecosystem. The 16

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grass-dominated understory of the longleaf pine ecosystem harbors the highest small scale species richness levels in North America (Peet and Allard 1993, Kirkman et al. 2001). Meanwhile, considerable environmental stress is characteristic of the system. Deep sandy soils, in concert with high summer temperatures, often result in water limitations in some regions, while nitrogen availability is very low across the range of the system (Wilson et al. 1999). In addition, disturbance by fire is ubiquitous with return intervals as often as one to three years (Robins and Myers 1992). Regeneration processes and the mechanisms maintaining high species richness are not well understood. The dominantly perennial grasses, forbs, and legumes, are thought to be resilient to frequent disturbance and resource limitation and may be long lived once established (Clewell 1989). However, these same disturbances and resource limitations have the potential to limit recruitment and potentially result in episodic recruitment events throughout the system. Within the longleaf pine ecosystem, a positive linear relationship between soil moisture and species diversity suggests that soil moisture may have the potential to regulate species richness by affecting seedling recruitment rates (Kirkman et al. 2001). In addition, the positive relationship between productivity and species diversity in this system suggests that plant to plant interactions may promote seedling regeneration, potentially through enhancing microsite availability (Kirkman et al. 2001). Based on these patterns of species diversity, we hypothesized that the availability of appropriate recruitment microsites drives species richness in the longleaf pine savanna via a lottery based recruitment strategy. In this resource limited system, the suitability of a recruitment microsite is likely synonymous with the provision of critical resources to the seedling for germination and establishment. Under these criteria, we consider microsite limitations to apply if a viable seed is unable to germinate and survive until assessment at a given location. If only 17

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microsites are limiting in this system, we would expect to see more seedling recruits in treatments where resource availability was greater, if seed supply was constant across the treatments. However, if seed limitations were in effect, an increase in seed supply would result in more seedlings regardless of resource availability. Finally, if both seed supply and microsite availability were limiting, we might expect to see more seedlings when seed was added, but resource availability could affect the magnitude of the response. For this study, we determined how the availability of two limiting resources influences patterns of seedling recruitment across a naturally occurring productivity gradient. Specifically we asked: (1) Does resource availability regulate seedling recruitment rates across the natural moisture gradient (microsite limitation)? (2) How does seed supply regulate patterns of recruitment (seed limitation)? and, (3) How does seedling species richness relate to seedling recruitment rate (evidence of lottery based recruitment patterns)? Methods Study Site Ichauway (31 13 N, 84 29 W) is an 11,300 ha property of the Joseph W. Jones Ecological Research Center, consisting of 7,500 ha of mature longleaf-pine (Pinus palustris) savanna. A historically undisturbed wiregrass (Aristida stricta)-dominated ground cover has been maintained under a 2-5 year fire return interval for at least the past 90 years and is characterized by notably high levels of fine scale species richness (Drew et al. 1998). This site is located in the Lower Coastal Plain and Flatwoods (LCPF) section of southwest Georgia, USA (McNab and Avers 1994). Average yearly temperatures range from 5-17 C in the winter to 21-34C in the summer and about 131 cm of precipitation is evenly distributed throughout the year (Goebel et al. 1997). A naturally occurring moisture gradient representative of the wide ecological amplitude of the longleaf pine system occurs on Ichauway. Across its range, the 18

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longleaf pine-wiregrass savanna occupies a wide moisture gradient that extends from extremely mesic locations with saturated soils and a perched water table to extremely xeric locations along deep sand ridges. At Ichauway, seasonally saturated Aquic Arenic Paleudult soils characterize the mesic regions while Typic Quartzipsamment soils are found across the xeric regions (Goebel et al. 1997). This study is part of an on-going resource manipulation study established in 2002. The overall long-term experiment examines the addition of nitrogen and water in a 2 x 2 factorial design conducted at mesic and xeric ends of the natural soil moisture gradient represented at Ichauway (mean volumetric soil moisture 8 0.68 in xeric, and 15 1.04 in mesic). For the two soil moisture extremes, 16 plots (50m X 50m) were established and four factor-level combinations were randomly assigned (water only, N only, water + N, control) with four replications of each treatment. Water addition of about 825 mm of irrigation per year (~65% increase in yearly precipitation) maintained the plots at close to 40% field capacity. All irrigation water was treated with reverse osmosis to minimize cation accumulation in the soil. Nitrogen fertilizer (ammonium nitrate, 34-0-0) was applied by hand three times a year at a rate of 50kg ha -1 yr -1 (natural N mineralization:10-20 kg -1 ha -1 yr -1 ). The percent of the yearly total fertilizer that was distributed at each application varied to mirror natural nitrogen mineralization rates in the system with 23% applied in January, 60% added in May, and 17% in September. All sites were burned every two years as part of the prescribed fire management regime at Ichauway. We formed experimental seedling recruitment microsites by installing PVC rings (n = 13, 30 cm diameter x 10 cm deep) in each plot in 2003. These rings allowed us to ensure that observed seedlings were recruits from seed rather than vegetative resprouts. To install the PVC, we first hammered a 30 cm steel ring form into the soil to sever the plant roots and then extracted 19

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the form without disturbing the soil. The PVC rings were inserted into the pre-cut trench so that the top was flush with the ground. In 2005, we killed the existing vegetation within each ring by two treatments of glyphosate applied in summer and fall. All sites were prescription burned in February 2004 and 2006. To examine the effect of resource availability on the seedling community composition, we censused all seedling recruits in five PVC rings per plot in May and November, 2006 and 2007. The seedlings were classified into functional groups (graminoid, forb, legume) and identified to species. In cases where seedling identification was not possible because of small size and death before maturity, we recorded the functional group only. We examined the relative importance of resource amendment (microsite limitation) and seed supply (seed limitation) on seedling recruitment success by planting seed of common understory species for two consecutive years. Seeds were sown at high or low density (250 or 50 seeds) with one of three species per ring (3 species x 2 density = 6 PVC rings per plot). We chose species representative of three major functional groups found in the understory of the longleaf pine savanna: Desmodium ciliare (DECI) is a common legume, Rudbekia hirta (RUHI) is a widespread forb, and Sorghastrum secundum (SOSE) is a common grass species. The percent viability of each species was determined in 2007 before planting using petri dish germination or tetrazolium tests (RUHI: 75% viable, SOSE: 40% viable, DECI: 90% viable). In 2006, seeds were sown in April. Because of limited germination success of RUHI in 2006, we re-seeded all species in early March the following year. An 8 cm tall ring of aluminum window screen was fitted around each PVC ring to reduce loss of seeds due to wind. We counted all seedlings in May and November 2006 and 2007. 20

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To gauge the magnitude of seed supply in the system, and because seed availability could be a function of resource availability, we sampled the seed rain and seed bank in each plot. Seed rain was collected monthly in nine funnel traps per plot from August 2006 to August 2007. We constructed seed traps from a 100mm diameter plastic funnel fitted inside a 100mm PVC tube that was recessed to ground level so that the mouth of the funnel was level with the soil surface. Any matter that fell into a funnel was caught in an attached micromesh bag. The traps were grouped into arrays of three, with each array surrounding three of the natural recruitment PVC rings per plot (3 funnels x 3 rings = 9 traps per plot). The contents of the three funnels at each array from a two month period were combined and sifted into a pot containing potting mix (0.07 m 3 Miracle Grow Potting Mix: 22.7 kg sand). For an estimate of seed bank composition, we obtained a 30 cm x 50 cm soil core during the installation of one seed trap per plot. Soil samples were stored at 4 C for four months then sieved to remove large debris and rhizomes. One liter of soil from each soil core sample was spread on top of potting mix in a tray. All experimental pots and trays were maintained in a climate controlled greenhouse and any seedling recruits were removed biweekly upon identification. Analyses A mixed model analysis was used to determine differences in mean seedling recruitment parameters between resource treatments (PROC MIXED, SAS Institute Inc. Version 9.1). In all analyses, the descriptor variables (resource treatment and plot) remained constant but the response variable (mean value per plot) varied depending on question. We modeled resource treatment (water only (W), N only (N), water + N (B), control (C)) as a fixed factor, and plot (n = 32) as a random factor. We also constructed pre-planned contrasts to examine the independent effects of water and fertilizer, if appropriate. Separate analyses were performed for xeric and mesic locations and each collection period (May 2006 (1), November 2006 (2), May 2007 (3), 21

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November 2007 (4)). The model was fit using a gaussian distribution despite left skewed response variables because examination of the model residuals indicated that the robustness of the model to non-normal distributions allowed for adequate model fit. Statistical significance was noted at p 0.05. Patterns of recruitment We examined the mean number of seedlings per plot and the mean species richness per plot to identify differences in seedling recruitment rate and species distribution across resource treatments (response variable = mean of five rings per plot). We also determined if differences in mean seedling recruitment occurred by functional group (graminoid, forb, legume) as a result of the resource treatments. Differences in mean seedling recruitment or species richness (square root transformed to improve assumptions of normality) that were dependent on gradient location were identified using a t-test analysis for each resource treatment (PROC TTEST, SAS Institute Inc. Version 9.1). In addition, we used the ratio of experimentally sown seedling density in fall to spring as a surrogate estimate of percent survival of seedlings by treatment. The abundance of individual species for each collection period was examined by constructing a frequency distribution histogram of number of occurrences of each species at mesic and xeric locations. Then, differences in seedling species composition between all combinations of resource treatments (W, N, B, C) and collection dates (1, 2, 3, 4) were determined using EstimateS (Version 7.5, R. K. Colwell, http://purl.oclc.org/estimates ). We used the Chao Jaccard procedure, which calculates a probability based dissimilarity index that is more appropriate when many rare species occur in the data set (Chao et al. 2005). The average plot to plot percent dissimilarity was calculated within and between each treatment type as well as within and between gradient locations. In addition, temporal variability in species composition per treatment was assessed from season to season. 22

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Finally, we determined mean differences in the number of seedlings or the number of species per plot in seed rain and seed bank to examine variability in seed supply that could drive patterns of species distribution across the resource treatments. Seed vs. microsite limitation To determine the relative importance of seed availability (seed limitation) and resource availability (microsite limitation) on recruitment rate, we compared mean experimentally sown seedlings across seeding densities and resource treatments. For this analysis, we used a split plot design with resource treatment as the whole plot factor, and seeding density (high or low) as the split plot factor (PROC MIXED, SAS Institute Inc. Version 9.1). Cumulative species count of seedlings per plot in the field (actual recruitment) was compared to cumulative species per plot from seed rain (potential recruitment) to determine if the number of species in the field was limited by seed availability. Potential recruits were obtained from one year of seed rain collection, while actual seedling recruitment was based on seedlings present in November 2006 (one year of field recruitment). In addition, differences in cumulative species counts (log transformed to improve assumptions of normality) that were dependent on gradient location were identified using a t-test analysis for each resource treatment (PROC TTEST, SAS Institute Inc. Version 9.1). Results Patterns of Recruitment Seedlings per treatment Although initially, the number of seedling recruits in the mesic location tended to increase when water was available, after two years, no differences due to treatment were present at either moisture gradient location (Figure 1). However, among functional groups, seedling recruitment by legumes was greatest in watered treatments at both gradient locations (mesic: t = -2.7, p = 23

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0.02, df = 3,12, xeric: t = -3.53, p = 0.004, df = 3,12, Figure 2)), and at the xeric location, a lower mean density of graminoid seedlings was present in nitrogen and nitrogen plus water treatments than in the watered only treatment (F = 4.97, p = 0.02, df = 3,12). The mean number of seedlings did not differ between the xeric and mesic location for any resource treatment (all p > 0.05). The percent survival (seedlings in fall/seedlings in spring) of recruits from experimentally sown treatments did not vary across the resource treatments for either gradient location (p > 0.05). Species per treatment After two years of recruitment, the mean number of species in watered plots was greater than in control plots at both gradient locations (mesic: F= 3.77, p = 0.04, xeric: F = 4.03, p = 0.03, Figure 3). The mean number of species per treatment did not differ in xeric and mesic locations (all p > 0.05) Species distribution across treatments Throughout the study period, the distribution of species across plots was similar at both gradient locations, with a majority of species observed in only one or two plots (Figure 4). As a result of the predominance of rare species, we found considerable plot to plot species dissimilarity between resource treatments (xeric =71%, mesic = 77%). However, within treatment species dissimilarity seemed to differ from between treatment dissimilarity depending on resource availability and gradient location (Table 4). Mean within treatment species dissimilarity was greatest in fertilized plots and lowest in watered plots regardless of gradient location. Species composition across the naturally occurring moisture gradient was 81% dissimilar from the mesic to the xeric locations. Season did not seem to influence differences in species composition between the plots (data not shown). 24

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Seed rain and seed bank patterns of recruitment There was no difference in seedling density or species richness from seed rain across resource treatments or gradient locations (all p > 0.05). More species were identified in seedlings in the field (112 species) than in recruits from seed rain (82 species), but, there was an overlap of 52 species that were observed in both field and seed rain (Appendix). There was also no effect of resource treatment or gradient location on seedling recruitment rate or species richness in germinants from the seed bank (all p > 0.05). Of the 53 species of seedlings that were observed from the seed bank, 45 of those species were also observed in the field (Appendix). Seed vs. Microsite Limitation The relationship between seed density and recruitment rate was dependent on gradient location and water availability (Figure 5). In 2007, the high density seed amendment resulted in greater seedling recruitment than the low density seed treatment regardless of gradient location (mesic: F = 9.41, p = 0.01, df. = 1,12, xeric: F =7.12, p = 0.02, df. = 1, 12). However, in the xeric location, water availability regulated the importance of seed density for seedling recruitment with a five fold increase in recruitment in the high seed density treatments in watered plots but no difference between the seeding densities for any other resource treatment (pairwise comparison, t = 4.73, p = 0.005). The cumulative number of species per plot germinating from seed rain collections was about eight times higher than the cumulative number of species for recruits in the field regardless of resource treatment or gradient location (mesic: p < 0.0001, F = 98.73, and df = 1,15, xeric: p < 0.0001, F = 52.38 and df = 1,15) (Figure 6). 25

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Discussion Relative Importance of Seed and Microsite Limitations The lack of seedling density response to resource treatments suggests that microsite limitation is not a primary driver of recruitment. However, increased seedling density among some functional groups in response to resource treatment or gradient location does imply potential microsite limitation for suites of species. The effect of increased legume seedling density with water addition may drive the observed increase in seedling species richness in water treatments. Similarly, the decreased recruitment of graminoid seedlings at the xeric location with nitrogen addition, even in combination with water treatment, may have important implications for community diversity. Overall, we did observe more seedling recruits when more seeds were sown, which suggests that seed limitation is occurring. However, the importance of the seed limitation effect was dependent on moisture gradient location. At the xeric location, it appeared that the microsite limiting factors (water availability) can override the secondarily important seed limitations, since recruitment rates were only dependent on seed density if supplemental water was added. In addition, because many more species per plot were observed in the seed rain, it suggests that the species composition of the seedling recruits is not limited by seed supply in this system. Community Implications An alternative explanation for the observed patterns of species diversity is that the heterogeneity of recruitment microsites, instead of mere presence, could be a factor. If the establishment of different species is limited by varying minimum moisture levels, watered plots would have a greater amplitude of microsites moisture levels and thus support a greater diversity of seedling recruits (Silvertown et al. 1999). However, it seems likely that if niche heterogeneity is important, it must occur in unison with lottery based seed supply if the high levels of species 26

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diversity in this system are to be maintained (Hurtt and Pacala 1995). This explanation is supported by the differences in species composition across the resource treatments. There is the least plot to plot similarity in species composition for treatments without supplemental water (control and nitrogen only). In these cases the difference in species composition is likely due to random chance in recruitment, as lottery based recruitment success is dependent on a limited number of microsites that can provide the minimum water level that is essential for establishment. Meanwhile, the treatments that added water (water only and water + nitrogen) had greater plot to plot similarity in species composition. This may indicate that the additional species that are able to establish in the watered plots are similar across plots. These species are probably present in the seed rain across the gradient, but their recruitment success requires higher water availability then can be provided in the unwatered plots during dry periods (such as our study years). Based on the similar recruitment rates and species richness of seed rain germinants, we conclude that the difference in species richness of seedling recruits across resource treatments was not a factor of differences in seed supply. Trends in greenhouse recruitment did not follow the response to resource availability that we observed in the field, and if anything, pointed towards more viable recruits in plots with added nitrogen. Caveats A factor that may have influenced the results of this study was the extreme drought conditions during the study period. In such dry, hot summer conditions, the water limitations to survival were perhaps just as constraining at the mesic location as that of the xeric location. We saw no evidence of the higher rates of recruitment in the mesic location that we expected based on differences in system productivity and species richness across the gradient. However, in the longleaf pine ecosystem, the long lives of the primarily perennial understory species means that 27

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episodic recruitment is not detrimental, and in dry years it is possible that very little recruitment occurs (Warner and Chesson 1985, Clewell 1989). Under different climatic conditions, resource availability at recruitment microsites may differ across the gradient. Differences in the rates of seedling establishment and mortality across the resource treatments through the hot dry summers is an alternative explanation for the effect of resource availability on species richness (Price and Morgan 2007). However, when we calculated differences in pre and post summer experimentally sown recruits, there was no difference in survival that could be attributed to resource availability regardless of species, gradient location, or year. However, we did not measure seedling mortality directly which limits any conclusions we could draw about establishment success. The impacts of fire are likely to influence patterns of seedling diversity in this system but we did not directly test for the effects of this disturbance. However, the prescribed fire schedule at Ichauway resulted in our plots being burned the spring prior to the start of the study and two years later midway through the study allowing our treatments to occur in a landscape with a historically feasible fire return interval. Conclusion Based on these patterns of recruitment, water availability is the primary driver of species richness in the seedling recruit community and thus is probably the most limiting resource (Stevens and Carson 2002). Although nitrogen is also limiting in this system, its availability does not appear to be closely linked to patterns of community composition in the understory species. This study demonstrates that resource mediated limitations on seedling recruitment can provide a mechanism for the maintenance of species richness within a resource limited yet diverse system. Because seed limitation is secondarily important to resource availability in driving patterns of seedling recruitment we suggest that the conditions are appropriate for lottery 28

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based maintenance of high levels of species diversity (Sale 1977) with potential upper bounds on species composition limited by resource availability (resource heterogeneity theory). This study provides a mechanistic explanation of the positive linear relationship between species richness and system productivity in one of the most diverse ecosystems in North America. 29

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A0510152025May-06Nov-06May-07Nov-07# seedlings per ring A A A B B B B B B B0510152025May-06Nov-06May-07Nov-07Collection period# seedlings per ring Water Both Nitrogen Control Figure 2-1. Number of seedling recruits displayed a trend toward responding to resource availability in the mesic location, but after two years of recruitment there was no effect of resource availability at either gradient location. Letters indicate differences at = 0.05. A) Mesic. B) Xeric. 30

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A05101520253035ForbLegumeGraminoid# of seedlings per ring A B B B B05101520253035ForbLegumeGraminoidFunctional group# of seedlings per ring Water Both Nitrogen Control A B A A B B B B B Figure 2-2. Number of seedlings per functional group varied across resource treatment after two years of recruitment, but the response was dependent on functional group and gradient location. Letters indicate differences at = 0.05. A) Mesic. B) Xeric. 31

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A0123456May-06Nov-06May-07Nov-07# of species per ring A B A B B B B B B0123456May-06Nov-06May-07Nov-07Collection period# of species per ring Water Both Nitrogen Control A A B A B A A B A B B B C C B Figure 2-3. Number of species per ring varied across treatments after two years of recruitment and for most collection periods. Letters indicate differences at = 0.05. A) Mesic. B) Xeric. 32

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A051015202530354045012345678910111213141516Number of species B051015202530354045012345678910111213141516Number of plotsNumber of species Mesic Xeric Figure 2-4. Frequency distribution of the number of plots that each species was observed in after six months of natural recruitment and after two years of natural recruitment (0 plots on the x axis indicates the number of species that are observed only at the mesic or xeric location). A) Two years of recruitment. B) Seed rain for one year. 33

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A012345678MesicXericGradient location# of seedlings per ring A A B B B0246810121416WaterBothNitrogenControlResource treatment# of seedlings per ring High seeding density Low seeding density A A A B A A A A Figure 2-5. Seed density influenced the number of seedling recruits regardless of gradient location after two years of recruitment. However at the xeric location, density was a factor only in the water treatment. Letters indicate differences at = 0.05. A) Density effect. B) Interaction between density and resources in the xeric location. 34

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0102030405060MesicXericGradient locationCumulative species per plot Field recruitment Seed rain B B A A Figure 2-6. More cumulative species per plot were observed in potential recruits (seed rain seedlings) then in actual recruits (field seedlings) and there was no difference in the number of species between gradient locations. Letters indicate differences at = 0.05. 35

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Table 2-1. Results from analysis of mean seedling recruitment across four resource treatments (water addition (W), nitrogen addition (N), water + nitrogen (B), control (C)) and two locations (mesic, xeric) after two years of recruitment Seedlings per treatment (# natural recruits per ring) a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource treatment 0.61 3 12 0.62 0.21 3 12 0.89 Species per treatment (# species per ring) a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource treatment 3.77 3 12 0.04* 4.03 3 12 0.03* b) Planned contrasts F p F p Nno N 1.7 0.22 3.96 0.07 Wno W 9.3 0.01 7.49 0.02 c) Differences of least squares means (species per ring, ddf = 12) t p t p B-C 1.24 0.24 0.53 0.61 B-N 1.76 0.10 1.37 0.20 B-W -1.32 0.21 -1.97 0.07 C-N 0.51 0.62 0.84 0.40 C-W -2.56 0.03 -2.50 0.03 N-W -3.07 0.01 -3.34 0.01 36

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Table 2-2. Results from analysis of mean seedling recruitment by functional group across four resource treatments (water addition (W), nitrogen addition (N), water + nitrogen (B), control(C)) and two moisture gradient locations (mesic, xeric) after two years of recruitment Seedlings per functional group (# seedlings per ring per group) Forbs a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource treatment 0.57 3 12 0.65 0.49 3 12 0.69 Graminoids a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource treatment 0.19 3 12 0.90 0.25 3 12 0.86 b) Planned contrasts f p Nno N 10.37 0.007 Wno W 3.89 0.070 c) Differences of least squares means (species per ring, ddf = 12) t p B-C -0.88 0.400 B-N 0.82 0.400 B-W -2.85 0.010 C-N 1.71 0.110 C-W -1.97 0.070 N-W -3.67 0.003 37

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Table 2-2. Continued Legumes a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource manipulation 5.25 3 12 0.02 6.71 3 12 0.007* b) Planned contrasts f p f p Nno N 8.45 0.01 5.72 0.030 Wno W 3.65 0.08 12.09 0.005 c) Differences of least squares means (species per ring, ddf = 12) t p t p B-C -0.88 0.400 0.77 0.500 B-N 0.82 0.400 1.38 0.190 B-W -2.85 0.010 -2.77 0.020 C-N 1.71 0.110 0.61 0.550 C-W -1.97 0.070 -3.53 0.004 N-W -3.67 0.003 -4.15 0.001 38

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Table 2-3. Results of split plot analysis of experimentally seeded recruitment at two levels of seed density (high = 250 seeds, low = 50 seeds), four resource treatments (water (W), nitrogen (N), water + nitrogen (B), control (C)) and two gradient locations (mesic, xeric) Seedlings per treatment (# natural recruits per ring) a) Test of fixed effects (*significance at =0.05) Mesic Xeric F ndf ddf p F ndf ddf p Resource treatment (whole plot factor) 1.95 3 12 0.17 2.96 3 12 0.08 Seedling level (split plot factor) 9.41 1 12 0.01 7.12 1 12 0.02 Resource*level interaction 1.50 3 12 0.26 5.28 3 12 0.01 b) differences of least squares means for interaction between resource treatment and seeding level in the xeric location Comparison of resource treatments within seeding levels Level High Low treatment comparison t p t p B-C 1.28 0.230 1.79 0.10 B-N 1.9 0.080 1.84 0.09 B-W 1.17 0.260 1.19 0.26 C-N 0.62 0.540 0.05 0.96 C-W -0.309 0.010 -0.60 0.56 N-W -3.71 0.003 -0.65 0.53 Comparison of seeding levels within resource treatments Resource Treatment Water Both Nitrogen Control level comparison t p t p t p t p Low High 4.73 0.0005 -0.03 0.97 -0.13 0.9 0.77 0.45 39

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Table 2-4. Results of ChaoJaccard dissimilarity index calculations of seedling species composition within and between treatments (mean % dissimilarity in plot-to-plot species identity) Mesic location Water Nitrogen Both Control Water 0.62 Nitrogen 0.82 0.89 Both 0.67 0.80 0.63 Control 0.70 0.87 0.78 0.77 Xeric location Water Nitrogen Both Control Water 0.75 Nitrogen 0.89 0.84 Both 0.74 0.86 0.81 Control 0.87 0.88 0.77 0.85 Between locations Mesic Xeric Mesic 0.84 Xeric 0.91 0.88 40

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CHAPTER 3 WIREGRASS PRESENCE MODULATES RECRUITMENT MICROSITES Introduction Facilitative interactions occur when the presence of one plant provides a positive influence on the establishment or success of another plant (Callaway 1995). These relationships have the potential to influence patterns of community composition and identification of the mechanisms and extent of facilitative interactions may be important for understanding the drivers of plant species diversity (Brooker et al. 2008). Facilitation in plant communities appears to be relatively common, particularly in stressful systems where the benefits provided by a neighboring plant outweigh any negative competitive interactions that may occur (Hacker and Bertness 1995, Greenlee and Callaway 1996, Freestone 2006, Brooker et al. 2008). However, the balance between competition and facilitation may vary across environmental stress gradients with competitive effects dominating in mild locations, while facilitation is prevalent in harsher locations (Bruno et al. 2003, Freestone 2006, Brooker et al. 2008). These stress gradients may be temporal (e.g. season (Kikvidze et al. 2006)), or spatial (e.g. gradients in soil moisture (Greenlee and Callaway 1996) or salinity (Hacker and Bertness 1999)). There are several mechanisms by which facilitative interactions can influence plant community composition. Some facilitative effects act by influencing survival and fecundity of adult plants. For instance, the survival of palatable species can be enhanced through association with less palatable neighbors (Smit et al. 2006), or reproductive output may increase in association with a facilitator (Holzapfel and Mahall 1999, Shumway 2000). Alternatively, the facilitative benefit is provided when the presence of a plant (the nurse plant) modulates the surrounding environment, in turn, increasing the probability of successful regeneration of a nearby seedling. In these cases, the nutrient uptake, transpiration, and physical presence of the 41

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nurse plant changes the microenvironment in its immediate vicinity. Water availability for the seedling can be enhanced as shade from the nurse plant lowers seedling transpiration demands or increases soil water availability (Holmgren et al. 1997, Zou et al. 2005). Seedling nutrient supply can also be enhanced directly by the nurse plant through mechanisms such as nitrogen fixation (Shumway 2000), or via indirect effects such as resource concentration beneath plant canopies (Vinton and Burke 1995). Finally, nurse plants can act as facilitators of recruitment if they provide the seedlings with protection from environmental extremes. In this capacity, temperature highs and lows are modulated by the presence of a plant canopy (Shreve 1931), while the mere presence of roots enhances soil stability and helps protect seedlings from wind or water erosion (Holmgren et al. 1997). Despite their putative importance, the role of facilitative interactions in the maintenance of species diversity are still unclear (Brooker et al. 2008). It has been hypothesized that the impact of facilitative effects may be more important in species rich systems, as it could expand the fundamental niche of a species and explain patterns of coexistence that cannot be explained by competition alone (Bruno et al. 2003). One system in which plant diversity does not seem to be driven by competitive exclusion is the longleaf pine (Pinus palustris) savanna of the southeastern U.S. (Kirkman et al. 2001). Several lines of evidence suggest that positive plant to plant interactions are occurring and may play a role in promoting species packing (Kirkman et al. 2001). For instance, there is a positive linear correlation between the extremely rich understory species diversity and annual net primary productivity (Kirkman et al. 2001). In addition, severe water and nutrient limitations are characteristic of this system, and these factors probably interact with frequent fire and high summer temperatures to limit seedling recruitment (Pecot et al. 2007). 42

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Wiregrass (Aristida stricta) is the dominant understory species in the southeastern extent of the longleaf pine savanna (Peet and Allard 1993), and it forms a nearly complete groundcover in the regions where annual net primary productivity of the system is highest (Mitchell et al. 1999). There are several mechanisms by which the presence of wiregrass could affect seedling recruitment success. The primary facilitative benefit may be through shading the recruitment microsites and thus promoting positive seedling water relations (Holmgren et al. 1997). Increased soil water availability due to shade directly promotes seedling germination and establishment, especially in arid systems (MacMahon and Schimpf 1981). Meanwhile water loss via transpiration is decreased due to the moister air found in the shade (Abd El Rahman and Batanouny 1965). Alternatively, the presence of a wiregrass clump may influence soil moisture content directly. The only previous study that has examined the effect of wiregrass clumps on soil properties determined that soil carbon pools were elevated beneath the clumps (West and Donovan 2004). However, there is a positive relationship between soil carbon pools and soil water holding capacity (Olness and Archer 2005). In addition, wiregrass protection of seedlings from environmental extremes such as high temperatures may be an important facilitative promoter of seedling recruitment (Fulbright et al. 1995). We have shown that water supply is a strongly limiting resource that regulates seedling recruitment in this system (chapter 1 this thesis). If proximity to wiregrass results in increased water availability, the tradeoff in light reduction may be advantageous if the presence of shade is the difference between the seedlings survival or desiccation, but the effect may be dependent on stress gradients(Greenlee and Callaway 1996, Callaway and Walker 1997, Hacker and Bertness 1999). Nitrogen availability was not considered as an important facilitative effect because patterns of species richness across 43

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the system (Kirkman et al. 2001) and seedling recruitment rates (Chapter 1, this thesis) do not seem to be promoted by nitrogen availability. We propose that microhabitat modulation by wiregrass could facilitate the recruitment of understory species and thus indirectly influence species richness. Here we test the importance of shade provided by wiregrass clumps as a potential facilitative environment for seedling recruitment in the longleaf pine savanna. First, we characterized the abiotic environment of potential seedling recruitment microsites located under and between wiregrass clumps varied spatially (i.e. across a naturally occurring spatial moisture gradient) and temporally (spring vs. summer). Secondly, we experimentally manipulated shade cover over experimentally-sown seed to evaluate how shade and the interaction of shade and water influenced seedling recruitment. Methods Study Site This study was conducted at the Joseph W. Jones Ecological Research Center at Ichauway, located in the Lower Coastal Plain and Flatwoods (LCPF) section of southwest Georgia, USA (31 13 N, 84 29 W) (McNab and Avers 1994). Ichauway property includes 7,500 ha of mature longleaf pine savanna. Historically undisturbed wiregrass-dominated understory has been maintained at the site under a 2-5 year fire return interval for at least the past 90 years and is characterized by notably high levels of fine scale species richness (Drew et al. 1998). Average yearly temperatures range from 5-17 C in the winter to 21-34C in the summer, with approximately 131 cm of precipitation evenly distributed throughout the year (Goebel et al. 1997). A naturally occurring moisture gradient representative of the wide ecological amplitude of the longleaf pine system occurs on Ichauway. Across its range, the longleaf pine/ wiregrass savanna is found across a wide moisture gradient that extends from extremely mesic locations with saturated soils and a perched water table to extremely xeric locations along deep sand 44

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ridges. At Ichauway, seasonally saturated Aquic Arenic Paleudult soils characterize the mesic regions while deep Typic Quartzipsamment ridges are found across the xeric regions. This study took place as part of an on-going, resource manipulation study established in 2002. The long-term experiment includes irrigated plots and no-irrigation control plots installed at the mesic and xeric ends of the natural soil moisture gradient. Eight plots (50 m x 50 m each) were established for each of the two soil moisture conditions, and factor-level treatments (watered, control) were randomly assigned to the plots with four replicates of each treatment. Water addition of approximately 825 mm of irrigation per year (~ 65% increase over natural precipitation levels) maintained the plots at close to 40% field capacity. All irrigation water was treated with reverse osmosis to minimize cation accumulation in the soil. The sites were burned every two years as part of the prescribed fire management regime at Ichauway. Microhabitat Properties To compare environmental conditions in potential seedling recruitment microsites that differed in their proximity to wiregrass, we measured soil and air temperature, soil moisture, relative humidity, and light availability beneath and adjacent to wiregrass clumps. In each control plot of the long term study, we randomly selected a site under a wiregrass clump (hereafter, below) and identified the nearest bare soil with no wiregrass canopy (hereafter, between). In each of these microsites, we estimated volumetric soil moisture at 10 cm depth using time-domain reflectometry (TDR) (Topp et al. 1980). We installed a pair of 11 cm stainless steel rods vertically in each microsite to quantify soil moisture three days per month using a Techtronix cable tester. We measured photosynthetically active radiation (PAR mol.m -2 .s -1 ) at ground level with a quantum line sensor ceptometer (Accupar LP80, Decagon Devices, Pullman WA). The sensor was oriented in a North-South direction and adjusted to integrate the readings over 1/8 th of the rod to obtain localized measurements at the microsites. Above 45

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wiregrass canopy readings were obtained simultaneously at 50 cm above the ground using an external PAR spot sensor. For each observation we calculated the average of three PAR readings and all observations were made between the hours of 10:00 am and 2:00 pm on sunny days. We recorded air temperature and relative humidity at 5cm above ground level below and between each wiregrass clump using two multimode data loggers per plot (HOBO pro H8, Onset Computer Corporations, Bourne MA). We also measured soil temperature, at a depth of 5 cm, with one four channel data logger (HOBO H8, Onset Computer Corporations, Bourne MA) per plot that was fitted with two temperature probes on leads. All data loggers obtained readings at one hour intervals for 36 consecutive hours each month (April-August). Shade Cloth Treatments To simulate the effects of shading by wiregrass, and elucidate the potential importance of interactions between shade and water availability on seedling recruitment rates, we used shade-cloth structures over experimentally sown seedling communities. We used a split-split plot design that manipulated shade levels in both control and watered plots at the wet and dry extremes of the naturally occurring moisture gradient over four collection periods. In 2003, we installed four polyvinylchloride (PVC) rings (30 cm diameter x 10 cm deep) in each plot to exclude vegetative regeneration from the experimentally sown microsites. We first pounded a 30 cm steel ring form into the soil to sever plant roots then extracted the form without disturbing the soil. The PVC rings were inserted into the pre-cut trench so that the top was flush with the ground. In 2005, we removed the standing vegetation within each ring with two treatments of glyphosate (summer and fall). All sites were burned in February of 2004 and 2006. In March 2006, we installed shade structures over two randomly selected rings in each plot and left two rings unshaded in each plot (64 rings total). A 90% reduction in light at the soil surface was provided by multiple layers of shade cloth and screen fitted to wooden frames (55 46

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cm x 55 cm). This level of shade was chosen to approximate the light reduction provided by wiregrass cover (mean 1SD = 78 1 %, max 99 %, Mitchell, unpublished data). All of the study rings were experimentally seeded with a mixture of common species that included representatives of the major functional groups in the understory. In March 2006, 50 seeds each of Sorghastrum secundum (SOSE), Desmodium strictum (DEST), and Sporobolus junceus (SPJU) were sown. In early May 2006, 50 seeds each of Rudbekia hirta (RUHI) and Desmodium ciliare (DECI) were added to each ring. All unshaded rings were encircled with a 6cm tall, window screen barrier to reduce seed loss by wind. Seedling recruitment was evaluated in May and November 2006 by counting the number of seedlings of each species in each ring. In February 2007, the shade structures were removed from the field so they would not be damaged in the controlled burn, and then replaced in March 2007. Due to low germination rates the previous year, however, a layer of shade cloth was removed to produce an 80% reduction in light at ground level. All rings were reseeded with 50 seeds each of DECI, RUHI, and SOSE. Seedling recruitment was again assessed in May and November 2007. Analyses Microhabitat Properties We tested for monthly differences in mean environmental parameters between microsites using a mixed model procedure (PROC MIXED, SAS Institute Inc. Version 9.1). The ratio of PAR below canopy/ PAR above canopy was used as an estimate of percent incident PAR at the microsite level. TDR measurements were converted to volumetric soil moisture units before analysis. The data was analyzed separately at mesic and xeric locations with plot treated as a random effect and proximity to wiregrass (below or between) as a fixed effect. Statistical significance was noted at p 0.05. 47

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Shade Treatments A mixed model analysis was used to test for mean differences in recruitment rate attributable to shade, water availability, collection period, or potential interactions between these terms. The average number of seedlings per ring was compared for shaded and unshaded treatments across watered and control plots over four collection periods using a split-split plot design (PROC MIXED, SAS Institute Inc. Version 9.1). Xeric and mesic locations were examined separately, with shade treatment (split-split plot factor), water manipulation (whole plot factor), and collection period (split-plot factor) modeled as fixed effects, and plot modeled as a random effect. Planned contrasts of combinations of collection period were used to examine seedling response to season and year. Statistical significance was noted at p 0.05. Results Microhabitat Properties The environmental property most strongly influenced by wiregrass presence was the PAR that reached the potential recruitment microsites (PAR below canopy/ PAR above canopy) regardless of gradient location. There was an approximate 35% reduction in PAR below wiregrass clumps (mean s.d = 0.54 0.19) and between wiregrass clumps (mean s.d = 0.88 0.19) (Figure 1, a). The only exception was the measurements taken in April at the mesic location, where the similar PAR ratio in the two microsite types (mean s.d, below =0.72 0.08, between = 0.86 0.08) was likely attributable to rapid resprouting of Vaccinium sp. immediately following the February burn (G. Iacona, personal observation). Relative humidity, soil temperature, and air temperature also varied in response to proximity to wiregrass but the effect was dependent on gradient location and month (Table 1, b-e). In the xeric location, percent relative humidity was significantly or marginally (p 0.08) greater below wiregrass clumps (mean s.d, below = 69 2.25 %, between = 66 2.25 %) for 48

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all months except June. However, in this location there was no difference in soil temperature with regards to proximity to wiregrass (mean s.d, 27 1.2 C). Air temperature was slightly higher (0.4 C difference) between wiregrass clumps only in May and August (mean s.d, 26 0.7 C). Meanwhile, at the mesic location, there were no differences in relative humidity attributable to proximity to wiregrass clumps (mean s.d, 72 2.25). The average soil temperature was significantly or marginally (p = 0.06 0.08) greater between wiregrass clumps at the mesic location throughout the duration of the study (mean s.d, below, 25 0.7 C, between, 26 0.7 C). Air temperature was also slightly higher (0.4 C difference) between wiregrass clumps in June and August (mean s.d, 25 0.7 C). Finally, there was no observable difference in soil moisture in potential seedling microsites, although there was a slight trend towards higher soil moisture below wiregrass clumps (mean s.d, mesic = 6 1, xeric 4 1). Shade Treatments Seedling recruitment was greater in the irrigated treatment regardless of shade treatment or collection date in the mesic location (p = 0.03) (Figure 6, Table 2). Meanwhile in the xeric location, there were more seedling recruits in unshaded rings (p = 0.002), but, water availability or collection period did not influence recruitment levels. Discussion The presence of a wiregrass clump modulates the immediately adjacent microhabitat in ways that could potentially benefit seedling recruitment. Nevertheless, shade did not provide a facilitative benefit to seedling recruitment in this study. The positive influence of soil moisture on recruitment implies that while shade alone does not promote seedling recruitment rates in this environmentally stressed system, some attribute of wiregrass presence that modulates water availability for seedlings, could potentially provide a facilitative effect. 49

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The most obvious effect of wiregrass presence is the reduction in photosynthetically active radiation (PAR) beneath the canopy of the wiregrass clump (shading). Although PAR reduction may benefit seedlings directly in situations where photoinhibition may occur (Valientebanuet and Ezcurra 1991, Egerton et al. 2000), the more probable benefit of shading by wiregrass is through indirect effects on environmental variables that are beneficial to seedling recruitment. The seedling recruitment microsite variables that are likely to be affected by shade are water availability and soil and air temperature (Holmgren et al. 1997). We observed an increase in relative humidity below the canopy of wiregrass during some measurement periods even though we were not able to discern a difference in volumetric soil moisture. This increase in relative humidity below the canopy is probably due to shade decreasing soil water evaporation rates (Kennedy and Sousa 2006) although alternative mechanisms could include a direct effect of the plant roots increasing soil moisture levels in their vicinity through either the concentration of organic matter in the rhizosphere (Hook et al. 1991) or by deep roots transferring water to the surface layer through hydraulic lift (Zou et al. 2005). We also found that proximity to wiregrass often resulted in decreased soil and air temperatures which can be attributed to shading (Shreve 1931, MacMahon and Schimpf 1981, Moro et al. 1997). Contrary to our predictions, there was no evidence of increased recruitment in the presence of shade. In fact, in the xeric location, there was a negative effect of shade with more seedling recruits occurring in unshaded rings. In addition, although there was no clear pattern of an interaction between water and shade, water alone appeared to be the strongest driver of seedling recruitment. In the mesic location (usually more water available, but debatable in the study year) water addition resulted in more seedling recruits regardless of shading and there was a similar trend in the xeric location. 50

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There are several reasons that we may not have observed a positive effect of shade. One reason is that the shade level may have exceeded that for optimum seedling growth. The percent reduction in PAR of this study was within the range of natural conditions; however, it represented the upper range. It is possible that the facilitation of seedling recruitment in this system varies across a shade gradient, and the tradeoff for light versus positive facilitative effects is most effective for seedling growth at the lower levels of shading by wiregrass but competition for water dominates the relationship between seedlings and wiregrass at the higher shade levels that we were replicating. Although our shade treatment results appear to indicate a negative or neutral effect of shade, more insight into how seedling recruitment responds to a gradient of shade densities could help resolve the disconnect between our results and our predictions. Another potential reason why we did not observe positive facilitative effects of shade is that the shade structures may have inhibited water availability by partially preventing precipitation or irrigation from reaching the ground further compounding the light to resources tradeoff. Although every microsite variable we measured varied according to month of measurement, we saw no clear pattern of change in the relationship between microsite factors and proximity to wiregrass as thermal stress increased from spring to summer (temporal stress gradient). In addition, there was no trend of microsite measurements differing between naturally occurring moisture gradient locations (spatial stress gradient). There are several potential explanations for this lack of a clear relationship between stress gradients and observable microsite differences. We chose the April to August temporal stress gradient because the increase in temperature and potential for drought at this time of year could be a limiting factor for seedling establishment. However, later observations suggest that the primary flush of recruitment in this system occurs between September and April (G. Iacona, personal 51

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observation). If this is the case, differences in microsite properties at the time of germination may be most important in driving recruitment success and a gradient of microsite properties from spring to summer may not be important. The lack of clear effect of moisture gradient location (mesic or xeric) on the variation in microsite properties is more confusing. We expected the most apparent differences between versus below wiregrass clumps to appear in the xeric region where the understory vegetation is patchy and bare soil is more prevalent. This was true for the difference in relative humidity, but for soil temperature, there was more of a difference at the mesic location. For all other measurements there did not seem to be any clear effect of gradient location. One possible explanation for this lack of gradient effect could be that the measurements were obtained during an exceptionally dry year in which conditions at the mesic and xeric locations were potentially more similar than in other years. The evidence presented by this study does not disprove the possibility of facilitative interactions occurring between wiregrass and seedling recruits of understory species in this system. However, it does suggest that high levels of shade are not a facilitative mechanism that is present, and that the provision of water is paramount for recruitment microsites. This suggests that an alternative explanation for the previously observed relationship between wiregrass productivity and system species diversity may hinge on community wide effects of wiregrass presence rather than impacts of individual clumps. In the mesic location the density and abundance of wiregrass clumps appears to be much higher than in the xeric location (G.I. personal observation). The non-rhizomatous nature of wiregrass plants precludes them from being significant competitive excluders of other plants in the system (Mulligan and Kirkman 2002, Keddy et al. 2006). Based on these factors an indirect effect of wiregrass abundance could be more important then direct facilitation in influencing seedling recruitment. The trends of PAR 52

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response illustrate a potential manifestation of such an effect. In the mesic location, the difference in light levels between wiregrass versus below wiregrass is not as great as in the xeric location. In cases such as this, the community wide environmental conditions in mesic locations may be more conducive to seedling success and thus could provide more opportunities for seedling recruitment to occur 53

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0.20.30.40.50.60.70.80.91AprilMayJuneJulyAugustMonthRatio of PAR at ground level below mesic between mesic below xeric between xeric *m *x *m *x A x *m *x *m (*)x Figure 3-1. Environmental variables within potential recruitment microsites varied depending on proximity to wiregrass but the effect was dependent on measure, month, and gradient location. Differences between microsite locations are indicated by *m or *x for mesic or xeric location respectively (* = p 0.05, (*) = p 0.08). A) The ratio PAR at ground level: PAR at 1 m. B) Percent volumetric soil moisture at a 10 cm depth. C) Percent relative humidity at seedling microsites. D) Soil temperature at 5 cm depth. E) Air temperature at seedling microsites. 54

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012345678910AprilMayJuneJulyAugustMonthPercent volumetric soil moisture below xeric between xeric below mesic between mesic B Figure 3-1. Continued 55

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505560657075808590AprilMayJuneJulyAugustMonthRelative humidity below mesic between mesic below xeric between xeric C *x *x (*)x *x Figure 3-1. Continued 56

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15171921232527293133AprilMayJuneJulyAugustMonthDegrees C below mesic between mesic below xeric between xeric D (*)m *m *m (*)m *m Figure 3-1. Continued 57

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101520253035AprilMayJuneJulyAugustMonthDegrees C below mesic between mesic below xeric between xeric E *m *x *m *x Figure 3-1. Continued 58

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A012345678910ControlWater# of seedlings per ring B012345678910ControlWaterResource treatment# of seedlings per ring No shade Shade Figure 3-2. Shading influenced recruitment but the response was dependent on gradient location and water availability. A) Mesic. B) Xeric. 59

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Table 3-1. Results from mixed model analysis of variance of microsite measurement values by month and moisture gradient location, fixed factor = proximity to wiregrass clump (below vs. between). Asterisks indicate significance at = 0.05 Photosynthetically active radiation Mesic Xeric F ndf ddf p F ndf ddf p April 1.63 1 3 0.290 19.69 1 3 0.020 May 11.77 1 6 0.040 55.39 1 3 0.005 June 33.37 1 3 0.010 78.38 1 3 0.003 July 15.49 1 3 0.030 26.73 1 3 0.014 August 79.60 1 3 0.003 8.43 1 3 0.060 Volumetric soil moisture Mesic Xeric F ndf ddf p F ndf ddf p April 1.38 1 3 0.33 0.17 1 3 0.71 May 5.55 1 3 0.10 2.94 1 3 0.19 June 0.19 1 3 0.19 6.55 1 3 0.08 July 0.89 1 3 0.42 0.00 1 3 0.97 August 0.26 1 3 0.26 0.13 1 3 0.74 Relative humidity Mesic Xeric F ndf ddf p F ndf ddf p April 0.54 1 3 0.52 7.35 1 3 0.07 May 0.10 1 3 0.77 10.34 1 3 0.05 June 2.56 1 1 0.36 43.17 1 1 0.10 July 0.27 1 3 0.64 27.16 1 3 0.01 August 3.87 1 3 0.14 32.44 1 3 0.01 Soil temperature Mesic Xeric F ndf ddf p F ndf ddf p April 15.08 1 3 0.03 2.14 1 3 0.24 May 7.05 1 3 0.08 4.81 1 3 0.12 June 1083 1 1 0.02 2.29 1 1 0.37 July 8.29 1 3 0.06 4.80 1 3 0.12 August 10.46 1 3 0.05 2.48 1 3 0.21 60

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Table 3-1 Continued Air temperature Mesic Xeric F ndf ddf p F ndf ddf p April 1.17 1 3 0.36 1.54 1 3 0.30 May 5.62 1 3 0.10 10.03 1 3 0.05 June 242 1 1 0.04 4.90 1 1 0.27 July 2.96 1 3 0.18 3.84 1 3 0.15 August 10.36 1 3 0.05 14.76 1 3 0.03 61

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Table 3-2. Results of split-split plot analysis of seedling recruitment across two shade treatments (shade, no shade), two resource treatments (water, control), and four collection periods (spring and fall, 2006 and 2007). Fixed effects Mesic location Xeric location F ndf ddf P F ndf ddf P Resource treatment (whole plot factor) 9 1 6 0.03 3.46 1 6 0.110 Collection (split plot factor) 4.25 3 18 0.02 2.03 3 18 0.150 Collection*resources 4.01 3 18 0.02 3.02 3 18 0.060 Shade treatment (split-split plot factor) 3.93 1 24 0.06 11.04 1 24 0.003 Resources*shade 3.03 1 24 0.09 1.13 1 24 0.300 Resources*collection*shade 0.65 6 24 0.69 0.33 6 24 0.920 Planned contrasts F P Spring vs. fall 11.62 0.003 2006 vs. 2007 0.88 0.360 Least squares means (seedlings per ring) Estimate SE Estimate SE Resources Control 0.67 1.28 2.41 1.39 Water 6.11 1.28 6.06 1.39 Shading No shade 4.28 1.01 5.80 1.09 Shade 2.50 1.01 2.67 1.09 Collection period 1 4.28 1.19 3.50 1.28 2 1.65 1.19 2.78 1.28 3 5.56 1.19 5.75 1.28 4 2.06 1.19 4.91 1.28 62

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CHAPTER 4 CONCLUSION The response of seedling recruitment to gradients of resource availability and environmental stress is of interest in determining how limitations on seedling recruitment can produce mechanistic explanations for patterns of species diversity (Grubb 1977, Warner and Chesson 1985, Hacker and Bertness 1999). One of the most floristically diverse ecosystems in North America is the longleaf pine savanna (Peet and Allard 1993, Kirkman et al. 2001). This ecosystem extends across a wide environmental stress gradient from wet mesic flatwoods to drought stressed xeric sandy ridges. Across the gradient, species richness is positively correlated with system productivity and soil moisture availability suggesting that resource based recruitment limitations may be important in driving patterns of diversity (Kirkman et al. 2001). In this study we examined how limitations on regeneration could regulate patterns of species diversity across the moisture gradient and potentially explain the positive correlation between species diversity and system productivity. Water availability was the primary regulator of seedling species richness across the moisture gradient although it is unclear whether the response is simply due to more seedlings in certain functional groups, or if effects such as niche differentiation are also important. The increased seedling recruitment with the addition of water may be explained by the removal of microsite limitations in the system, especially in the xeric location. We showed that seed limitation is of secondary importance to resource availability in driving patterns of seedling recruitment, and suggest that the conditions are appropriate for lottery based maintenance of high levels of species diversity. Finally, we present evidence that the presence of a wiregrass clump may marginally increase the moisture availability under its canopy, and also influence other microsite variables that are conducive to seedling microsite suitability. Shading by wiregrass is 63

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probably the primary regulator of microsite properties, although we could isolate no effect of shade alone on seedling recruitment. We present a conceptual model that explains the relationship between an environmental stress gradient and the mechanistic drivers of seedling recruitment success within the longleaf pine savanna (Figure 4-1). Because seed pool composition is equivalent across the gradient, and random dispersal is assumed constant, the availability of water regulates microsite availability across the gradient. At xeric sites, environmental conditions at recruitment microsites are rarely optimum for regeneration. Meanwhile, at more mesic sites, a higher amplitude and frequency of environmental variation over time and space could more commonly result in optimum regeneration conditions. At both locations, seed limitation provides an upper bound on potential recruitment, but at the xeric location, microsite limitation is the primary regulator of recruitment. The results of this study have theoretical implications in explaining the relationship between species diversity and system productivity. The nature of the relationship between system productivity and species diversity has long been contended (Waide et al. 1999, Gillman and Wright 2006). Although positive linear relationships are common in many systems worldwide, the assumed general plot of productivity to diversity is a unimodal, hump-shaped relationship where species diversity decreases due to competitive interactions as system productivity increases (Abrams 1995). Our results describe an alternative mechanism for the maintenance of species richness in which species richness can increase at the highest levels of productivity if biomass accumulation can enhance water availability for seedlings. Although this study provides a mechanistic link between species diversity and resource availability in this system, several questions remain unanswered. One unaddressed component of the seedling recruitment puzzle involves the establishment success of the new seedlings. We 64

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observed differences in initial establishment that can explain patterns of species diversity, but variability in the continued survival of the seedlings from year to year may also depend upon resource availability. More work also needs to be done on the question of whether wiregrass presence can facilitate seedling recruitment success. The wiregrass clumps do provide more hospitable recruitment microsites under their canopy, but it is yet to be determined whether these microsites actually promote seedling success. 65

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Figure 4-1 Conceptual model illustrating episodic recruitment depending on soil water availability. At xeric sites, environmental conditions at recruitment microsites are rarely optimum for regeneration. Meanwhile, at more mesic sites, a higher amplitude and frequency of environmental variation over time and space could result in optimum regeneration conditions more commonly. Once the minimum conditions for establishment of any seedling are attained, the identity of a recruit that occupies a microsite is constrained by niche requirements and chance. At both locations, seed limitation provides an upper bound on potential recruitment, but at the xeric location, microsite limitation is the primary regulator of recruitment. 66

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APPENDIX SPECIES LIST Genus Species Field Seed Rain Seed Bank 1 Acalypha gracilens X X X 2 Ambrosia artemisiifolia X X 3 Andropogon virginicus X X X 4 Aristida purpurascens X X 5 Aristida stricta X 6 Asimina angustifolia X 7 Baccharis halimifolia X X 8 Bulbostylis ciliatifolia X X X 9 Centrosema virginianum X X 10 Chamaecrista fasciculata X X 11 Chamaecrista nictitans X 12 Chrysopsis mariana X 13 Clitoria mariana X X 14 Cnidoscolus stimulosus X 15 Commelina erecta X 16 Conyza canadensis X X X 17 Croptilon divaricatum X 18 Crotalaria purshii X X 19 Crotalaria rotundifolia X X 20 Croton argyranthemus X X 21 Cyperus filiculmis X X X 22 Cyperus odoratus X X 23 Cyperus plukenetii X 24 Cyperus pseudovegetus X 25 Cyperus retrorsus X X 26 Dalea pinnata X 27 Desmodium ciliare X 28 Desmodium lineatum X 29 Desmodium strictum X 30 Dichanthelium aciculare X X X 31 Dichanthelium acuminata X X 32 Dichanthelium boscii X X 33 Dichanthelium commutatum X X X 34 Dichanthelium ovale X X 35 Dichanthelium sphaerocarpon X X X 36 Dichanthelium strigosum X X X 37 Dichanthelium tenue X X X 67

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Species list: continued Genus Species Field Seed Rain Seed Bank 38 Digitaria filiformis X 39 Diodia teres X X 40 Dyschoriste oblongifolia X X X 41 Elephantopus elatus X X 42 Eragrostis capillaris X 43 Eragrostis hirsuta X 44 Eragrostis virginica X 45 Eupatorium capillifolium X X X 46 Eupatorium compositifolium X X X 47 Eupatorium hyssopifolium X X 48 Eupatorium leptophyllum X 49 Eupatorium mohrii X 50 Euphorbia pubentissima X 51 Galactia erecta X X 52 Galium pilosum X X 53 Gamochaeta falcata X 54 Gamochaeta purpurea X X 55 Gaura filipes X X 56 Helianthus angustifolius X X 57 Helianthus radula X 58 Hieracium gronovii X 59 Houstonia procumbens X X X 60 Hypericum crux-andreae X X X 61 Hypericum gentianoides X X X 62 Hypericum gymnanthum X X 63 Hypericum suffruticosum X X X 64 Hypoxis curtisii X 65 Hypoxis juncea X 66 Hypoxis wrightii X X X 67 Juncus bufonius X 68 Lechea minor X X X 69 Lespedeza procumbens X 70 Lespedeza repens X 71 Liatris gracilis X 72 Liatris graminifolia X X 73 Linaria canadensis X X 74 Linaria texana X X 75 Lygodesmia aphylla X 76 Lygodium japonicum X 68

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Species list: continued Genus Species Field Seed Rain Seed Bank 77 Mecardonia acuminata X 78 Mimosa quadrivalvis X X X 79 Muhlenbergia capillaris X 80 Myrica cerifera X X 81 Opuntia humifusa X 82 Oxalis corniculata X 83 Paronychia rugelii X 84 Paspalum bifidum X 85 Paspalum setaceum X X X 86 Pediomelum canescens X 87 Penstemon australis X 88 Physalis pubescens X 89 Pinus elliottii X 90 Pinus palustris X X 91 Pirequeta cistoides X X 92 Pityopsis graminifolia X 93 Pluchea camphorata X 94 Polygala grandiflora X X X 95 Polygala lutea X 96 Polygala nana X X X 97 Polypremum procumbens X X X 98 Pseudognaphalium obtusifolium X X X 99 Pteridium aquilinum X 100 Pterocaulon pycnostachyum X 101 Quercus laurifolia X 102 Rhexia mariana X 103 Rhus copallinum X 104 Rhynchosia reniformis X X 105 Rhynchosia tomentosa X 106 Rhynchospora filifolia X 107 Rhynchospora harveyi X 108 Rubus cuneifolius X X X 109 Rudbeckia hirta X X X 110 Salvia lyrata X X 111 Schizachyrium tenerum X X 112 Scleria ciliata X X 113 Scleria georgiana X 114 Scleria reticularis X X 115 Scutellaria integrifolia X 69

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Species list: continued Genus Species Field Seed Rain Seed Bank 116 Sericocarpus tortifolius X 117 Sesbania herbacia X 118 Seymeria cassioides X 119 Sisyrinchium angustifolium X X 120 Smilax bona-nox X 121 Smilax smallii X 122 Solidago leavenworthii X 123 Solidago odora X X 124 Solidago stricta X 125 Solidago tortifolia X 126 Sorghastrum secundum X X 127 Sporobolis junceus X X 128 Stipulicida setacea X X X 129 Stylisma patens X 130 Stylosanthes biflora X X 131 Symphyotrichum adnatum X 132 Symphyotrichum concolor X 133 Symphyotrichum dumosum X X X 134 Tradescantia hirsutiflora X 135 Tragia smallii X 136 Tragia urens X 137 Tragia urticifolia X 138 Trichostema dichotomum X X 139 Triplasis americana X 140 Typha latifolia X 141 Vaccinium myrsinites X 142 Vernonia angustifolia X 143 Veronica arvensis X X 144 Viola lanceolata X 145 Viola palmata X X X 146 Wahlenbergia marginata X 147 Woodwardia virginica X 70

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BIOGRAPHICAL SKETCH Gwen Iacona grew up on the east coast of Florida where as a homeschooling student she gained much of her early education from the piney woods around her home. At seventeen, she set foot in a classroom for the first time when she enrolled in classes at Brevard Community College. At BCC the mentorship of Sue Phillips and Dr. Martin McClinton inspired Gwen to pursue a degree in science, and in 2003 she graduated Summa cum Laude from Florida Institute of Technology with a BS in biology. After a short time pursuing extra-curricular activities such as house remodeling, giving pony rides, and showing tourists alligators and ecosystems, Gwen received a cooperative assistantship from the J.W. Jones Ecological Research Center and the University of Florida Department of Wildlife Ecology and Conservation for graduate study in plant ecology. During her MS thesis research, Gwen learned more about the drivers of seedling recruitment than organic vegetable gardening ever was able to teach her. Learning and loving to identify the diverse understory species of the longleaf pine savanna has led Gwen to a new adventure after graduation as a field biologist for the Florida Natural Areas Inventory in Tallahassee, FL. 78