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Spring Focus on Sustainability and the Environment : Foliar Nitrogen and Carbon Responses to Experimental Warming in the...
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Permanent Link: http://ufdc.ufl.edu/UF00091523/00620
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Title: Spring Focus on Sustainability and the Environment : Foliar Nitrogen and Carbon Responses to Experimental Warming in the Low Arctic
Series Title: Journal of Undergraduate Research
Physical Description: Serial
Language: English
Creator: Ruben, Rachel
Natali, Susan
Schuur, Edward
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: Spring 2012
 Subjects
Subjects / Keywords: arctic tundra
warming
Δ15N
Δ13C
permafrost
climate change
Genre: serial   ( sobekcm )
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Abstract: Arctic ecosystems are characterized by a short growing season, low nutrient availability, and permafrost soils—factors making them highly vulnerable to climate warming. This study investigates the effects of experimental warming on foliar carbon and nitrogen, variables that reflect water status, photosynthesis, and nutrient dynamics. We deployed open-top chambers and snow fences to simulate three future climate scenarios in interior Alaska: 1) elevated summer air temperatures, 2) elevated winter time soil temperatures, and 3) elevated summer air temperatures and winter time soil temperatures (i.e., annual warming). Leaves from the six dominant plant species in moist acidic tundra were analyzed for percent nitrogen (%N), percent carbon (%C), and isotopic composition (δ15N and δ13C). Leaf mineral nutrition varied across species and functional groups, owing to differences in growth form, water needs, N source, N demand, and mycorrhizal status. Significant treatment effects were observed in four of six species. Highly significant annual warming effects were found in the deciduous shrub B. nana, with a decrease in foliar δ15N and an increase in foliar δ13C. The high responsiveness of B. nana is consistent with global observations of shrub expansion into higher latitudes. Continued changes in N and C dynamics could feed back to the C balance and to plant community diversity, especially if they alter plant competitive ability.
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Holding Location: University of Florida
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Resource Identifier: sobekcm - UF00091523_00602
System ID: UF00091523:00620

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University of Florida | Journal of Un dergraduate Research | Volume 13, Issue 2 | Spring 2012 1 R achel Rubin, Dr. Susan Natali and Dr. Edward Schuur College of Liberal Arts and Sciences, University of Florida Arctic ecosystems are characterized by a short growing seas on, low nutrient availability, and permafrost soils factors making them highly vulnerable to climate warming. This study investigates the effects of experimental warming on foliar carbon and nitrog en, variables that reflect water status, photosynthesis, and nutrient dynamics. We deployed open top chambers and snow fences to simulate three future climate scenarios in interior Alaska: 1) elevated summer air temperatures 2) elevated winter time soil temperatures and 3) elevated summer air temperatures and winter time soil temperatures (i.e. annual warming). Leaves from the six dominant plant species in moist acidic tundra were analyzed for percent nitrogen (%N), percent carbon (%C), and isotopic composition ( form, water needs, N source, N demand, and mycorrhizal status. Significant treatment effects were observed in four of six spe cies. Highly sig nificant annual warming effects were found in the deciduous shrub B. nana and an increase The high responsiveness of B. nana is consistent with global observations of shrub expansion into higher latitudes. C ontinued changes in N and C dynamics could feed back to the C balance and to plant community diversity, especially if they al ter plant competitive ability. INTRODUCTION Detecting plant response to environmental change is critical to understanding arctic ecosystems. Recent models C by the year 2100 (IPCC 2007), with the greatest changes predicted for high latitudes. Change s are already underway and include the loss of permafrost, an increased growing season length, and the advance of shrubs into higher latitudes (Sturm 2001, Walker et al. 2006, Tape et al. 2006), often at the expense of graminoids (Shaver 2001). The objecti ve of this study was to evaluate short term (two years) warming effects on leaf C and N, which can expose underlying soil and plant processes in a warmer world. impacts on temperature and water availabi lity and indirect effects on nutrient dynamics. Increased soil temperatures stimulate microbial decomposition of soil organic matter and mineralization of organic nitrogen (N) (Chapin et al. 1995). Typically, this process leads to increased productivity du ring the growing season. However, productivity may be limited as warming increases transpiration, prompting plants to close their stomata to conserve water. Warming mediated changes in plant C and N can f eed back to plant community composition (Schuur et a l. 2007), ultimately determining whether the landscape is net source or sink of C. The Carbon in Permafrost Experimental Heating Research (CiPEHR) field site was established in 2008 near Denali National Park in interior Alaska. The goal of this experiment is to assess ecosystem responses to experimental warming (Natali et al. 2011). During winter, snow fences trap an insulating layer of snow ; during summer, open top chambers behave as greenhouses. Combined, these treatments succeed in warming deep soil and surface air temperatures by 0.5 warming experiments indicate positive effects on plant productivity in as little as two seasons of warming (Walker et al. 2006), and increased net soil N mineralization in as little as three se asons of experimental warming (Aerts et al. 2009). Overall, most studies agree on a positive correlation between temperature and biomass production for some species (Michelsen et al. 1996, Hudson & Henry 2009, Chapin et al. 1995, Lin et al. 2010). Leaf pe rcent C and N (%C and %N) are useful metrics for assessing ecosystem response to climate change because they represent soil nutrient levels, resource allocation, and resource use efficiency (Welker et al. 2005). These values vary greatly among plant specie s and functional groups, owing to differences in physiology, nutrient demand and acquisition strategies (Cornelisen et al. 1997). Thus, the response of plants to changes in climate can range from positive, negative, to no response at all, and they are lik ely to be species specific (Aerts et al. 2009, Chapin et al. 1995, Chapin et al. 1985). However, even small changes in leaf chemistry can affect processes at multiple trophic levels. For example, increased foliar %N is likely to lead to higher rates of soi l N turnover rates

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RACHEL RUBIN DR SUSAN N ATALI AND DR EDWARD S CHUUR University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 2 when leaves senesce and return to the soil as litter, and elevated C:N ratios can reduce herbivore forage quality (Klein et al. 2007, Walsh et al. 1997). Foliar C and N isotopes ( 13 C and 15 N ) index warming mediated changes in the soil environment, suggesting possible mechanisms behind plant community responses. Isotopic values imply discrimination at various stages of biochemical cycles, leading to a unique isotopic signature of source and product pools. As such, the resulting ratio of transitional form (Dawson et al. 2002). N isotopes are expressed as the ratio of 15 N/ 14 N and can be used to infer N sources and pathways (Dawson et al. 2002, Robinson 2011, Evans 2001). Prior s tudies also indicate a positive 15 N values and soil net mineralization rates (Kahmen et al. 2008) as well as inorganic N availability in soil (Garten 1993). Similarly, foliar 13 C/ 12 C is controlled by the C isotope ratio of the C O 2 source and isotope discrimination during plant C assimilation and can be used to integrate plant physiological responses to environmental changes (Michelsen et al. 1996, Dawson et al. 2002). For example, plants undergoing water stress will close their stomata in order to conserve water. This creates a high demand for CO 2 with plants becoming less discriminating against 13 C 13 C. Our aims for this experiment were to quantify the effects of ecosystem warming on foliar C, N, and isotopic 15 13 C) variables that reveal changes in soil nutrient processes and plant functional res ponse to warming. We expected increases in 15 N 13 C and %C in all warming treatments, with the most pronounced effects in annually warmed plots. We also expected %N to remain the same or to decrease slightly, due to a dilution of overall N through incre ased leaf biomass (Schuur et al. 2007). We expected the magnitude of response to warming to vary according to species, which was found in the first year of warming at CiPEHR (Natali et al. 2011). Finally, we stry among species due to differences in physiological pathways, reproductive structures and leaf morphology (Arft et al. 1999, Nadelhoffer et al. 1996 ). METHODS CiPEHR is located at Eight Mile Lake ( 63 52' 59"N, 149 13' 32"W elevation 700 m) just o utside Denali National Park in the northern foothills of the Alaska Range. The vegetation is moist acidic tundra characterized by tussock forming sedges, forbs and shrubs and underlain by permafrost. The six most common vascular plant species at this sit e are Betula nana (deciduous shrub), Vaccinium uliginosum (deciduous shrub), Eriophorum vaginatum (sedge), Carex bigelowii (sedge), Rubus chaemomorus (forb), and Rhododendron subarcticum (evergreen shrub). CiPEHR was initiated in 2008 with the goal of wa rming surface air and deep soil. During winter, six 1.5 m tall 8 m long snow fences trapped an insulating layer of snow over six winter warming plots ; during the summer, twenty four 0.36 m 2 0.5 m tall open top chambers increased surface air temper atures. The snowpack was removed from the experimental plots in spring to ensure equivalent melt out dates between treatments. Chambers were placed on 12 cm tall bases inserted 5 cm into the soil to improve the seal, and control plots also contained chambe r bases. Combined, this warming regime warmed air and deep soil temperatures by between 0.5C and 1.5C (Natali et al. 2011). Fieldwork for this paper was completed between July 14 and August 18 of 2010. We surveyed vegetation using the point intercep t method in order to determine the six most abundant vascular plant species at the field site. Next, we harvested fully expanded leaves from the six dominant vascular plant species from both experimental and control plots. We collected a minimum of three l eaves from two three individuals in each plot in order to obtain a sample representative of the population. Samples were dried at 60 C before transport to the University of Florida for processing at the Ecosystem Dynamics Lab. We re dried samples at 60C for at least 24 hours, weighed them, and finely chopped each sample with scissors. We weighed approximately 3 mg of homogenized material from each sample and rolled them in Costech 4x6 mm tins. Samples were analyzed for C and N using a Costech Instrum ents EC54010 elemental analyzer and they were combusted in a ThermoFinnigan Delta plus XL mass spectrometer at 1010 C to determine 13 15 N signatures. Stable isotope abundances are reported as the ratio of 15 N/ 14 N or 13 C/ 12 C, expressed in relation to a reference standard (atmospheric N 2 and PeeDee belemnite C). Means and standard errors were calculated in Microsoft Excel, and da ta were then analyzed using a two way ANOVA using JMP 7 software by SAS. Species not fitting the assumptions of normality were transformed using a Box Significant Difference Test) were used to identify c ontributions to the interaction effect between summer and winter warming. Due to the high structural heterogeneity of CiPEHR, tests were analyzed at significance values of P 0.10. RESULTS Nitrogen Responses Warming mediated changes in N dynamics were detected in three out of four functional groups present at CiPEHR. Graminoids and forbs responded as we expected, with increased 15 N values. R. chaemomorus 15 N

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FOLIAR NITROGEN AND CARBON RES PONSES University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 3 increas ed with both summer warming (P=0.04, F=3.04) and winter warming (P=0.09, F=5.03) (F igure 1). 15 N values increased in both graminoid species present at CiPEHR ( E. vaginatum and C. bigelowii ) A shift in E. vaginatum 15 N was driven by an interaction between summer and winter warming (P=0.07, F=3.608), and C. bigelowii 15 N enrichment occurred in summer warming plots (P=0.09, F=3.1). The opposite was found for B. nana a ubiquitous deciduous shrub at CiPEHR. B. nana 15 N d ecreased in annual warming plots, which was driven by a summer x winter interaction (P=0.02, F=6.88) ( Figure 1). Figure 1. 15 N calculated as treatment control. Yellow: summer warming; blue: winter warming; black: annual warming. Values under species label indicate %N of plants in ambient conditions. Overall, leaf N content (%N) remained constant before and after warming in all species studied (Table 1). However, there was considerable var iation in %N among species under ambient conditions, which ranged from 1.57% ( R. subarcticum ) to 2.08% ( C. bigelowii) (Figure 2).

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RACHEL RUBIN DR SUSAN N ATALI AND DR EDWARD S CHUUR University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 4 T able 1 13 15 N from L eaves C ollected at P eak G rowing S ea son at CiPEHR in 2010 Note. Treatments are categorized as follows: Ambient=full control ; Summer=chambers only ; Winter=snow fences only ; Annual=chambers and snow fences. Statistical significance is expressed as follows: S = signi ficant SW effect; W = significant WW effect; SW = significant interaction effect. Figure 2 Change in %N with warming calculated as treatment control. Note. Yellow; sum mer warming, blue; winter warming, black; annual warming. Values under species label indicate %N of plants in ambient conditions. Variable Treatment B. nana C. bigelowii E vaginatum R subarcticum R. chaemomorus V. uliginosum C Ambient 48.0 0.2 44.5 0.2 45.4 0.1 50.7 0.1 s 44.4 0.2 47.6 0.2 (%) Summer 47.9 0.2 44.6 0.2 44.9 0.2 51.1 0.1 44.9 0. 3 48.0 0.2 Winter 47.9 0.2 44.3 0.1 45.4 0.2 50.9 0.1 44.7 0.3 48.1 0.2 Annual 48.0 0.2 44.3 0.1 45.4 0.4 51.1 0.2 45.0 0.3 48.0 0.2 N Ambient 2.0 0.2 2.1 0.1 1.9 0.1 1.6 0.1 2.2 0.1 1.8 0.1 (%) Summer 2.0 0.1 1.9 0.2 1.7 0.1 1.6 0.1 2.3 0.1 1.8 0.1 Winter 2.1 0.1 2.0 0.1 1.9 0.1 1.5 0.0 2.1 0.1 1.8 0.1 Annual 1.8 0.1 2.1 0.2 2.0 0.2 1.6 0.1 2.1 0.1 1.8 0.0 13 C Ambient 28.1 0.2 sxw 25.3 0.3 26.3 0.1 27.0 0.2 s 27.4 0.4 29.4 0.1 Summer 28.0 0.2 25.2 0.4 25.8 0.3 27.5 0.2 27.2 0.2 29.3 0.2 Winter 28.5 0.2 25.1 0.2 26.1 0.1 26.8 0.1 27.1 0.3 29.6 0.1 Annual 27.4 0.3 25.0 0.2 26.0 0.5 27.1 0.1 27.0 0.2 29.3 0.2 15 N Ambient 6.7 0.2 sw 0.7 0.2 s 1.0 0.2 sw 7.1 0.3 0.1 0.2 s,w 5.4 0.2 Summer 6.6 0.3 1.0 0.1 0.9 0.3 7.6 0.2 0.4 0.2 5.2 0.2 Wi nter 6.8 0.3 0.6 0.2 1.1 0.3 7.6 0.4 0.5 0.1 5.4 0.3 Annual 8.2 0.3 0.9 0.2 2.0 0.2 7.3 0.4 0.6 0.2 5.2 0.4

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FOLIAR NITROGEN AND CARBON RES PONSES University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 5 Carbon Responses Summer and winter warming likely increased transpiration (i.e. water stress), as evidenced by the non rand om pa 13 C enrichment for C. bigelowii, E. vaginatum and R. chaemomorus ( Figure 3), although these changes were not statistically significant. Warming 13 C were observed for B. nana and R. subarcticum ( Table 1). B. nana 13 C increased in annual warming plots, which was driven by a summer x winter interaction (P=0.04, F=4.71) ( Figure 4). On the other hand, R. subarcticum 13 C decreased in summer warming plots (P=0.03, F=5.27) ( Figure 4 ) Summer warming also led to a significant increase i n R. subarcticum %C (P=0.03, F=5.48) ( Table 1). Species level 13 C, which ranged from 30.37 ( V. uliginosum ) to 26.26 ( C. bigelowii) as well as for %C, which varied by as much as 5% among species ( Figure 3). Figure 3 Change in %C calculated as treatment control. Note. Yellow: summer warming; blue: winter warming; black: annual warming. Values under species label indicate %C of plants in ambient conditions.

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RACHEL RUBIN DR SUSAN N ATALI AND DR EDWARD S CHUUR University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 6 Figure 4 Change in 13 C calcul ated as treatment control. Note.Yellow; summer warming, blue; winter warming, black; annual warming. Values under species label indicate 13 C of plants in ambient conditions. DISCUSSION This study examined species level variation as well as warming eff ects on leaf chemistry. At CiPEHR, warming mediated shifts occurred in five out of six dominant species. As microbial respiration and photosynthesis are often accompanied by shifts in N availability, it is important to view changes in C and N together rat her than in isolation. Changes in foliar C and N also feed back to larger processes, such as the role of tundra as a net source or net sink of C. Our results indicate a link between leaf chemistry and climate warming, suggesting that leaf responses may for ecast structural changes to the arctic landscape. Effects of Experimental Warming 15 N in E. vaginatum (sedge) and R. chaemomorus (forb). Changes in 15 N values can reflect either a passive response of plants to increas ed N availability and N cycling rates (Kahmen et al. 2008, Craine et al. 2009) or to an active shift in the N source utilized (Robinson 2011, Evans 2001). N inputs 15 14 N isotope is lost through leach ing and denitrification (the production of gaseous N from nitrate by soil bacteria in the absence of O 2 ) (Dawson 2002). Interestingly, all three of 15 N with warming are 15 N values under amb ient conditions ( Figure 1). Since deciduous species lose and gain N faster than other functional groups (Aerts 1995), they may be more responsive to warming mediated increases in soil N. Another possible scenario is that plants could be shifting their N so urces (e.g. from nitrate to ammonium, which is less depleted in 15 N) (Miller & Bowman 2002). 15 N increased, the strongest responses were observed in the ubiquitous deciduous shrub at CiPEHR B. nana Previous studies have also shown that B. nana was highly responsive in fertilization studies (Shaver & Chapin 1980, Syndonia Bret Harte et al. 2004) and with experimental warming (Hobbie et al. 1999). One 15 N is heightened mycorrhizal fungi activity. Warming can lead to increased biomass and abundance of ectomycorrhizal fungi, which 15 N depleted N to plant roots (Clemmensen et al. 2006). A long term warming experiment at Toolik Lake found that warming alters the ectomycorrhizal community of B. nana in arctic tundra, suggesting that war ming may facilitate the expansion of B. nana by forming larger mycorrhizal networks (Deslippe et al. 2011). This high responsiveness of B. nana ( Figure 2) is important because woody species are indeed increasing in abundance across the arctic biome (Sturm et al. 2001,

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FOLIAR NITROGEN AND CARBON RES PONSES University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 7 Sturm et al. 2005, Chapin & Shaver 1995, Tape et al. 2006). We did not find any warming induced changes in %N, but this was not unexpected. Studies in Sweden (Aerts et al. 2009 ); Ellesmere Island, Canada (Tolvanen and Henry 2001) ; and T oolik Lake, AK (Hobbie and Chapin 2008) detected no change or minimal decreases in %N with experimental warming. However the biomass of winter warmed plots at CiPEHR in 2010 was 20% higher than in non warmed plots (Natali, unpublished data ). This indicat es that plants were able to acquire enough N to accommodate increased C sequestration. Summer warming elicited a water stressed response from several species, with a positive relationship between temperature and 13 C values. Overall, it appears that 13 C discrimination (increased 13 C values), as evidenced by the non random pattern of 13 C enrichment for C. bigelowii, E. vaginatum and R. chaemomorus ( Figure 4), although these results were no t statistically significant. R. subarcticum (forb) 13 C decreased in summer warmed plots, indicating a decrease in the rate of photosynthetic C fixation relative to diffusion through stomata (Farquhar et al. 1989). Perhaps R. subarcticum is more resilient to the effects of drying tundra. Surprisingly, B. nana was the only species in which 13 C increased significantly during summer warming, indicating that B. nana may have been undergoing water stress. So, while B. nana can be facilitated through increased m ycorrhizal networks, its growth may be constrained by water stress, a side effect of increased temperature. Species D ifferences 15 13 C under ambient conditions. Changes in the soil environment can favor species th at can exploit the most abundant N source (McKane et al. 2002) and have the greatest C storage potential (Schuur et al. 2007). In N limited tundra, plants coexist by partitioning N sources: nitrate, ammonium, and free amino acids (Nadelhoffer et al. 1996 McKane et al. 2002, Miller & Bowman 2001). Thus, the unique 15 N signature among plant species is shaped by a variety of processes, including forms of N taken up, rooting depth, and mycorrhizal associations. For example, shallow rooted species such as B. nana 15 N values because they are more likely to acquire N from sources near the soil surface (e.g. precipitation, runoff and fresh leaf litter). Deep rooted species such as E. vaginatum, on the other hand, compete more effectively for N deeper in soil profiles (Nadelhoffer et al. 1996) Furthermore, 15 N depletion in woody species may also be explained by preferential use of nitrate over ammonium 15 N depleted N by mycorrhizae (Miller and Bowman 2002). Warming mediated changes in soil N status could favor certain species that have improved access to their preferred N source. Under ambient conditions, leaf carbon varied by as much as 5%. This variation may be driven by differences in leaf physiology, growth form, and phenology. For example, shrubs tend to have a higher percent cellulose, lignin, and silica than other plant groups (Klein 2007), and species vary in the amount of C they allocate below ground to roots (Hudson et al. 2011). If plant communities shift to shrub dominated, this could affect the amount of C store d above ground. Furthermore, differences in leaf chemistry among species can affect litter decomposition rates. For example, graminoid litter has been shown to decompose faster than shrubs (Hobbie 1996), leading to increased rates of C and N cycling. Also, 13 C values differed among species and growth forms with values ranging from 30.37 ( V. uliginosum ) to 26.26 ( C. bigelowii) indicating variation in C isotope discrimination, growth rate, and diffusive C uptake Future D irections Short term winter and s ummer warming (two years) resulted in leaf chemistry shifts in five out of six species at CiPEHR. Future study should address whether long term warming will elicit a stronger response or a subdued response as plants acclimate to warmer conditions. Chapin e t al. (1995) found that plant response to short term (three years or less) changes in temperature were mostly a surge in vegetative growth, whereas long term (nine years) warming more accurately represented changes in resource availability. The effects o f warming on plant nutrient dynamics may depend on site characteristics, such as the initial community structure (Jagerbrand et al. 2009, Walker et al. 2006). In Northern Alaska (Welker et al. 2005), moist tundra vegetation was much more responsive to expe rimental warming than dry tundra with up to a 25% increase in leaf N. Furthermore, these results vary globally, 13 C values increasing in C. lapponica and D. integrifolia with 16 years of warming on Ellesmere Island (Hudson et al. 2011) 13 C decr easing in C. lapponica with nine years of summer warming in Northern Sweden (Aerts et al. 2009). Continued sampling across a variety of tundra types may allow a better understanding of plant responses to warming across arctic and subarctic regions. Lastly more emphasis should be placed on the role of mycorrhizal fungi in attenuating N limitation in arctic tundra. Fungal symbioses can supply 61 86% of the nitrogen found in plants by providing access to inorganic and organic N pools (Hobbie & Hobbi e 2006). Depending on the plant species, mycorrhizae may balance N levels in plants by giving N as a function of need. Or, in the case of B. nana mycorrhizae may facilitate shrub expansion across the arctic landscape.

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RACHEL RUBIN DR SUSAN N ATALI AND DR EDWARD S CHUUR University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 8 ACKNOWLEDGEMENTS I thank my mentor, Dr. Sue Natali, for her guidance during all aspects of this study, including project development, fieldwork, laboratory processing, and statistical analyses. I greatly appreciate her patience and attention to detail. I thank my faculty advisor, Dr. Te d Schuur, who has supported me for two years as an undergraduate researcher and provided valuable conceptual assistance with laboratory protocols and to her dedication to all elem ental and mass spectrometry analyses. I am also thankful to the members of the Ecosystem Dynamics Lab for providing feedback on an earlier stage of this work. Travel funds to Alaska were provided by the University Scholars Program through the UF Center for Undergraduate Research. LITERATURE CITED Aerts, R. 1995. The advantages of being evergreen. Tree. 10, 402 407. Arft, A. M., M. 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FOLIAR NITROGEN AND CARBON RES PONSES University of Florida | Journal of Undergraduate Research | Volume 13, Issue 2 | Spring 2012 9 Syndonia Bret Harte, M., E.A. Garcia, V.M. Sacre, J.R. Whorley, J.L. Wagner, S.C. Lipert and F.S. Chapin. 2004. Plant and soil responses to neighbour removal and fertilization in Alaskan tussock tundra. Journal of Ecology. 92, 635 647. Tape, K. M. Sturm and C. Racine. 2006. The Evidence for shrub expansion in Northern Alaska and the Pan Arctic. Global Change Biology. 12, 686 702. Tolvanen, A. and G.H.R. Henry 2001. Responses of carbon and nit rogen concentrations in high arctic plants to experimental warming. Canadian Journal of Botany. 79, 711 718. Wahren, C.H.A., M.D. Walker and M.S. Bret Harte. 2005. Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment. Global Change Biology 11, 537 552. Walker M.D., C.H. Wahren, R.D. Hollister, G.H.R. Henry, L.E. Ahlquist, J.M. Alatalo, M.S. Bret Harte, M.P. Calef, T.V. Callaghan, A.B. Carroll, H.E. Epstein, I.S. Jonsdottir, J.A. Klein, B Magnusson, U. Molau, S.F. Oberbauer, S.P. Rewa, C.H. Robinson, G.R. Shaver, K.N. Suding, C.C. Thompson, A. Tolvanen, O. Totland, P.L. Turner, C.E. Tweedie, P.J. Webber and P.A. Wookey. 2006. Plant community responses to experimental warming across the tu ndra biome. PNAS 103, 1342 1346. Walsh, N.E., T. R. McCabe, J. M. Welker and A.N. Parsons. 1997. Experimental manipulation of snow depth: effects on nutrient content of caribou forage. Global Change Biology. 3, 158 164. Welker, J.M., J.T. Fahnestock, P.F. Sullivan and R.A. Chimner. 2005. Leaf mineral nutrition of Arctic plants in response to warming and deeper snow in northern Alaska. Oikos. 109, 167 177.