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Principal Investigators from several sites within the Long Term Ecological Research (LTER) program offer their insights about how long-term research has been effectively sustained from periods ranging from 20 to 40 years. The sites are: Hubbard Brook (New Hampshire), Konza Prairie (Kansas), Niwot Ridge (Colorado), Arctic (Alaska), Palmer Station (Antarctica), and Georgia Coastal Ecosystems (Georgia). The main themes discussed include: the importance of a strong foundation and common vision, creating a culture of collaboration and cooperation, showing the relevance of research to societal needs, managing conflict resolution, encouraging innovation, facilitating an exchange of ideas, working to build collaborations, willingness to adopt new management structures, and careful attention to transitions in leadership. The conclusion summarizes themes based on this chapter as well as other chapters in the book.

Accelerated warming in the Arctic has led to concern regarding the amount of carbon emission potential from Arctic water bodies. Yet, aquatic carbon dioxide (CO2) and methane (CH4) flux measurements remain scarce, particularly at high resolution and over long periods of time. Effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep glacial lake in northern Alaska, were measured for the first time with the direct eddy covariance (EC) flux technique during six ice-free lake periods (2010–2015). CO2 flux estimates from the lake (daily average efflux of 16.7 ± 5.3 mmol m−2 d−1) were in good agreement with earlier estimates from 1975–1989 using different methods. CH4 effluxes in 2010–2015 (averaging 0.13 ± 0.06 mmol m−2 d−1) showed an interannual variation that was 4.1 times greater than median diel variations, but mean fluxes were almost one order of magnitude lower than earlier estimates obtained from single water samples in 1990 and 2011–2012. The overall global warming potential (GWP) of Toolik Lake is thus governed mostly by CO2 effluxes, contributing 86–93% of the ice-free period GWP of 26–90 g CO2,eq m−2. Diel variation in fluxes was also important, with up to a 2-fold (CH4) to 4-fold (CO2) difference between the highest nighttime and lowest daytime effluxes. Within the summer ice-free period, on average, CH4 fluxes increased 2-fold during the first half of the summer, then remained almost constant, whereas CO2 effluxes remained almost constant over the entire summer, ending with a linear increase during the last 1–2 weeks of measurements. Due to the cold bottom temperatures of this 26 m deep lake, and the absence of ebullition and episodic flux events, Toolik Lake and other deep glacial lakes are likely not hot spots for greenhouse gas emissions, but they still contribute to the overall GWP of the Arctic.

Recent warming in the Arctic, which has been amplified during the winter1–3, greatly enhances microbial decomposition of soil organic matter and subsequent release of carbon dioxide (CO2)4. However, the amount of CO2 released in winter is not known and has not been well represented by ecosystem models or empirically based estimates5,6. Here we synthesize regional in situ observations of CO2 flux from Arctic and boreal soils to assess current and future winter carbon losses from the northern permafrost domain. We estimate a contemporary loss of 1,662 TgC per year from the permafrost region during the winter season (October–April). This loss is greater than the average growing season carbon uptake for this region estimated from process models (−1,032 TgC per year). Extending model predictions to warmer conditions up to 2100 indicates that winter CO2 emissions will increase 17% under a moderate mitigation scenario—Representative Concentration Pathway 4.5—and 41% under business-as-usual emissions scenario—Representative Concentration Pathway 8.5. Our results provide a baseline for winter CO2 emissions from northern terrestrial regions and indicate that enhanced soil CO2 loss due to winter warming may offset growing season carbon uptake under future climatic conditions. Winter warming in the Arctic will increase the CO2 flux from soils. A pan-Arctic analysis shows a current loss of 1,662 TgC per year over the winter, exceeding estimated carbon uptake in the growing season; projections suggest a 17% increase under RCP 4.5 and a 41% increase under RCP 8.5 by 2100.

Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current net carbon (C) balance of terrestrial arctic ecosystems is unknown. Recent studies suggest that these ecosystems are sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO2 fluxes over the full annual cycle. Here, we describe a pattern of CO2 loss based on the longest continuous record of direct measurements of CO2 fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. We also report on a shorter time series of continuous measurements from a third ecosystem, tussock tundra. The amount of CO2 loss from both heath and wet sedge ecosystems was related to the timing of freeze-up of the soil active layer in the fall. Wet sedge tundra lost the most CO2 during the anomalously warm autumn periods of September–December 2013–2015, with CH4 emissions contributing little to the overall C budget. Losses of C translated to approximately 4.1 and 1.4% of the total soil C stocks in active layer of the wet sedge and heath tundra, respectively, from 2008 to 2015. Increases in air temperature and soil temperatures at all depths may trigger a new trajectory of CO2 release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.

Despite concern about the status of carbon (C) in the Arctic tundra, there is currently little information on how plant respiration varies in response to environmental change in this region. We quantified the impact of long-term nitrogen (N) and phosphorus (P) treatments and greenhouse warming on the short-term temperature (T) response and sensitivity of leaf respiration (R), the high-T threshold of R, and associated traits in shoots of the Arctic shrub Betula nana in experimental plots at Toolik Lake, Alaska. Respiration only acclimated to greenhouse warming in plots provided with both N and P (resulting in a ~30% reduction in carbon efflux in shoots measured at 10 and 20 °C), suggesting a nutrient dependence of metabolic adjustment. Neither greenhouse nor N+P treatments impacted on the respiratory sensitivity to T (Q10 ); overall, Q10 values decreased with increasing measuring T, from ~3.0 at 5 °C to ~1.5 at 35 °C. New high-resolution measurements of R across a range of measuring Ts (25-70 °C) yielded insights into the T at which maximal rates of R occurred (Tmax ). Although growth temperature did not affect Tmax , N+P fertilization increased Tmax values ~5 °C, from 53 to 58 °C. N+P fertilized shoots exhibited greater rates of R than nonfertilized shoots, with this effect diminishing under greenhouse warming. Collectively, our results highlight the nutrient dependence of thermal acclimation of leaf R in B. nana, suggesting that the metabolic efficiency allowed via thermal acclimation may be impaired at current levels of soil nutrient availability. This finding has important implications for predicting carbon fluxes in Arctic ecosystems, particularly if soil N and P become more abundant in the future as the tundra warms.

Long-term measurements of ecological effects of warming are often not statistically significant because of annual variability or signal noise. These are reduced in indicators that filter or reduce the noise around the signal and allow effects of climate warming to emerge. In this way, certain indicators act as medium pass filters integrating the signal over years-to-decades. In the Alaskan Arctic, the 25-year record of warming of air temperature revealed no significant trend, yet environmental and ecological changes prove that warming is affecting the ecosystem. The useful indicators are deep permafrost temperatures, vegetation and shrub biomass, satellite measures of canopy reflectance (NDVI), and chemical measures of soil weathering. In contrast, the 18-year record in the Greenland Arctic revealed an extremely high summer air-warming of 1.3 °C/decade; the cover of some plant species increased while the cover of others decreased. Useful indicators of change are NDVI and the active layer thickness.

We examined the effects of short ( 25 %) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: (1) under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; (2) observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and (3) short-term effects (1–4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments.

I was committed to long-term, site-based, research long before the Arctic (ARC) Long-Term Ecological Research (LTER) site was established in 1987. Working with the LTER program since then has allowed me to reach my goals more easily than would have been possible otherwise. Because of my deep involvement in research in the LTER program, most of the examples I use in teaching now come from LTER sites. For the same reason, most of my communications with the public are about research in the LTER program. I learned the value of collaboration as a graduate student, from my earliest mentors and collaborators. Being a part of the LTER program has helped me to develop a wide array of enjoyable, comfortable, and productive collaborations. A message to students: be generous in all aspects of your research and professional life, because there is much more to be gained from generosity than there is to be lost. I helped set up the ARC site of the LTER program in 1987 and have made it the focus of my scientific career for the past 27 years. My experience with integrated, site-based, multidisciplinary ecosystem research actually began in 1972, however, when as a graduate student I worked with the US Tundra Biome Study at Barrow, Alaska (Brown et al. 1980; Hobbie 1980). The Tundra Biome Study and its umbrella organization, the International Biological Program (IBP), ended officially in 1974, but the ideas developed and lessons learned from these programs were central to the later development of the LTER program (Coleman 2010). These lessons were central to the formation of my own professional worldview; key among them was the idea that long-term approaches, including long-term, whole-ecosystem experiments, were essential to understanding distribution, regulation, and change in populations, communities, and ecosystems everywhere. My dissertation research, on root growth at the Barrow site, benefited greatly from the interactions I had with the diverse group who worked there. I finished my PhD in 1976, during a period when the need for a federally supported program of long-term, multidisciplinary, site-based ecological research was becoming increasingly clear.

Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO(2) flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one-half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO(2) exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (> 10 °C) were net sources of CO(2) , whereas woody sites with cold soils (

Temperature is increasing at unprecedented rates across most of the tundra biome. Remote-sensing data indicate that contemporary climate warming has already resulted in increased productivity over much of the Arctic, but plot-based evidence for vegetation transformation is not widespread. We analysed change in tundra vegetation surveyed between 1980 and 2010 in 158 plant communities spread across 46 locations. We found biome-wide trends of increased height of the plant canopy and maximum observed plant height for most vascular growth forms; increased abundance of litter; increased abundance of evergreen, low-growing and tall shrubs; and decreased abundance of bare ground. Intersite comparisons indicated an association between the degree of summer warming and change in vascular plant abundance, with shrubs, forbs and rushes increasing with warming. However, the association was dependent on the climate zone, the moisture regime and the presence of permafrost. Our data provide plot-scale evidence linking changes in vascular plant abundance to local summer warming in widely dispersed tundra locations across the globe. © 2012 Macmillan Publishers Limited. All rights reserved.

Fires produce land cover changes that have consequences for surface energy balance and temperature. Three eddy covariance towers were setup along a burn severity gradient (i.e. Severely, Moderately, and Unburned tundra) to determine the effect of fire and burn severity on arctic tundra surface energy exchange and temperature for three growing seasons (2008‐2010) following the 2007 Anaktuvuk River fire. The three sites were well matched before the fire, experienced similar weather, and had similar energy budget closure, indicating that the measured energy exchange differences between sites were largely attributable to burn severity. Increased burn severity resulted in decreased vegetation and moss cover, organic layer depth, and the rate of postfire vegetation recovery. Albedo and surface greenness steadily recovered with Moderately matching Unburned tundra by the third growing season. Decreased albedo increased net radiation and partly fueled increased latent and ground heat fluxes, soil temperatures, and thaw depth. Decreases in moss cover and the organic layer also influenced the ground thermal regime and increased latent heat fluxes. These changes either offset or decreased the surface warming effect from decreased albedo, resulting in a small surface warming in Severely and a small surface cooling in Moderately relative to Unburned tundra. These results indicate that fires have a significant impact on surface energy balance and highlight the importance of moss and permafrost thaw in regulating arctic surface energy exchange and temperature.

We examined short- and long-term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen ( 15 N), as a tracer. Tracer levels of 15 NH4 þ were sprayed once onto the tundra at six sites in four tundra types: heath (crest), tussock with high and low water flux (mid- and footslope), and wet sedge (riparian). 15 N in vegetation and soil was monitored to estimate retention and loss over a 3- year period. Nearly all 15 NH4 þ was immediately retained in the surface moss-detritus-plant layer, and .57% of the 15 N added remained in this layer at the end of the second year. Organic soil was the second largest 15 N sink. By the end of the third growing season, the moss-detritus-plant layer and organic soil combined retained � 87% of the 15 N added except at the Midslope site with high water flux, where recovery declined to 68%. At all sites, non-extractable and non- labile-N pools were the principal sinks for added 15 N in the organic soil. Hydrology played an important role in downslope movement of dissolved 15 N. Crest and Midslope with high-water-flux sites were most susceptible to 15 N losses via leaching, perhaps because of deep permeable mineral soil (crest) and high water flow (Midslope with high water flux). Late spring melt season also resulted in downslope dissolved- 15 N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss- detritus-plant layer, rapid incorporation of new N into relatively recalcitrant-soil-N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N-limitation of this ecosystem. An extended snow-free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.

Global environmental change, related to climate change and the deposition of airborne N-containing contaminants, has already resulted in shifts in plant community composition among plant functional types in Arctic and temperate alpine regions. In this paper, we review how key ecosystem processes will be altered by these transformations, the complex biological cascades and feedbacks that might result, and some of the potential broader consequences for the earth system. Firstly, we consider how patterns of growth and allocation, and nutrient uptake, will be altered by the shifts in plant dominance. The ways in which these changes may disproportionately affect the consumer communities, and rates of decomposition, are then discussed. We show that the occurrence of a broad spectrum of plant growth forms in these regions (from cryptogams to deciduous and evergreen dwarf shrubs, graminoids and forbs), together with hypothesized low functional redundancy, will mean that shifts in plant dominance result in a complex series of biotic cascades, couplings and feedbacks which are supplemental to the direct responses of ecosystem components to the primary global change drivers. The nature of these complex interactions is highlighted using the example of the climate-driven increase in shrub cover in low-Arctic tundra, and the contrasting transformations in plant functional composition in mid-latitude alpine systems. Finally, the potential effects of the transformations on ecosystem properties and processes that link with the earth system are reviewed. We conclude that the effects of global change on these ecosystems, and potential climate-change feedbacks, cannot be predicted from simple empirical relationships between processes and driving variables. Rather, the effects of changes in species distributions and dominances on key ecosystem processes and properties must also be considered, based upon best estimates of the trajectories of key transformations, their magnitude and rates of change. © Journal compilation © 2009 Blackwell Publishing.

The large spatial heterogeneity of arctic landscapes complicates efforts to quantify key processes of these ecosystems, for example productivity, at the landscape level. Robust relationships that help to simplify and explain observed patterns, are thus powerful tools for understanding and predicting vegetation distribution and dynamics. Here we present the same linear relationship between Leaf area index (LAI) and Total foliar nitrogen (TFN), the two factors determining the photosynthetic capacity of vegetation, across a wide range of tundra vegetation types in both northern Sweden and Alaska between leaf area indices of 0 and 1 m(2) m(-2), which is essentially the entire range of leaf area index values for the Arctic as a whole. Surprisingly, this simple relationship arises as an emergent property at the plant community level, whereas at the species level a large variability in leaf traits exists. As the relationship between LAI and TFN exists among such varied ecosystems, the arctic environment must impose tight constraints on vegetation canopy development. This relationship simplifies the quantification of vegetation productivity of arctic vegetation types as the two most important drivers of productivity can be estimated reliably from remotely sensed NDVI images.

Long-term ecosystem-level experiments, in which the environment is manipulated in a controlled manner, are important tools to predict the responses of ecosystem functioning and composition to future global change. We present the results of a meta-analysis performed on the results of long-term ecosystem-level experiments near Toolik Lake, Alaska, and Abisko, Sweden. We quantified aboveground biomass responses of different arctic and subarctic ecosystems to experimental fertilization, warming and shading. We not only analysed the general patterns but also the differences in responsiveness between sites and regions. Aboveground plant biomass showed a broad similarity of responses in both locations, and also showed some important differences. In both locations, aboveground plant biomass, particularly the biomass of deciduous and graminoid plants, responded most strongly to nutrient addition. The biomass of mosses and lichens decreased in both locations as the biomass of vascular plants increased. An important difference between the two regions was the smaller positive aboveground biomass response of deciduous shrubs in Abisko as compared with Toolik Lake. Whereas in Toolik Lake Betula nana increased its dominance and replaced many of the other plant types, in Abisko all vascular plant types increased in abundance without major shifts in relative abundance. The differences between the responses of the dominant vegetation types of the Toolik Lake region, i.e. tussock tundra systems, and that of the Abisko region, i.e. heath systems, may have important implications for ecosystem development under expected patterns of global change. However, there were also large site-specific differences within each region. Several potential mechanistic explanations for the differences between sites and regions are discussed. The response patterns show the need for analyses of joint data sets from many regions and sites, in order to uncover common responses to changes in climate across large arctic regions from regional or local responses.

The activity was designed to provide guidance on future research questions in the Arctic over the next 10-20 years, identifying the key scientific questions that are emerging in different realms of Arctic science and exploring both disciplinary realms (e.g., marine, terrestrial, atmosphere, cryosphere, and social sciences) and cross cutting realms (e.g., integrated systems science and sustainability science). Based on the emerging research questions, the study also helps identify research infrastructure needs and collaboration opportunities. Attention was given to assessing needs where there may be a mismatch between rates of change and the pace of scientific research. Although it is understood that there is no one answer, the committee was asked to explore how agency decision makers might achieve balance in their research portfolios and associated investments. This report is meant to guide future directions in U.S. Arctic research so that research is targeted on critical scientific and societal questions and conducted as effectively as possible. This activity was sponsored by USARC, DOE, NASA, NOAA, NSF, and the Smithsonian Institution.

We describe a scaling protocol that combines two hierarchically linked mod- els with field surveys, spatially distributed weather data, and remotely sensed images to generate daily predictions of gross primary production (GPP) for a 9200-km 2 arctic wa- tershed. A detailed process-based model of vegetation-atmosphere interactions, which has been tested in a variety of arctic ecosystems against independent hourly gas exchange data, forms the base of the hierarchy. This detailed model was used to construct a second and simpler, ''big-leaf'' model, which was calibrated for arctic conditions and which required many fewer parameters and input data. For landscape forcing data, we derived spatiotem- poral data on weather conditions (maximum and minimum temperature, and irradiance) from weather stations throughout the watershed. Spatiotemporal descriptions of the biotic constraints on production, chiefly leaf area index (LAI) and total foliar nitrogen ( Nf), were derived from field surveys, a land cover database, and normalized difference vegetation index (NDVI) data acquired from satellites. The scaling hierarchy avoided propagation of error via a compensation process, though the procedures involved still introduced uncer- tainty into daily GPP predictions averaging 16% of the growing season daily mean. The construction of the spatiotemporal temperature and irradiance fields introduced uncertainty of 1-2% at spatial and temporal resolutions of 1 km 2 and one day, respectively. The greatest uncertainty was introduced by assignment of LAI across the region, because of the highly heterogeneous landscape and the high sensitivity of production to changes in LAI at the low levels found in the Arctic. Uncertainty in vegetation properties introduced an uncertainty of 615% in basin GPP predictions, a value commensurate with basin net ecosystem pro- duction (NEP). In conclusion, improved characterization of vegetation via remote sensing is required before any bottom-up approach to carbon budgeting can reduce uncertainty to a reasonable level.

The aims of this article are to characterize the nutrient regime in arctic and boreal wetlands and to review published data on various aspects of within-stand nutrient-cycling. Most northern wetlands, such as boreal and arctic ombrogenous bogs and most arctic mesic and wet ecosystem types, are poor in inorganic, plant-available nutrients because mineralization is restricted due to low temperatures and anoxic soils. This is particularly true in the Arctic because nutrient inputs from external sources quite often are small, and nutrient pools in the vegetation also are small. By contrast, the soil and the microbial biomass often contain large stocks of organically fixed nutrients that are largely unavailable to plants. The vegetation is adapted to the low availability of nutrients by the perennial nature of both entire plants and plant tissues, which minimizes the annual nutrient losses and reduces the demand for nutrient uptake to produce new tissues. Furthermore, many of the dominant plant life-forms can ...

Plant species collected from tundra ecosystems located along a north-south transect from central Alaska to the north coast of Alaska showed large and consistent differences in 15N natural abundances. Foliar δ15N values varied by about 10% among species within each of two moist tussock tundra sites. Differences in 15N contents among species or plant groups were consistent across moist tussock tundra at several other sites and across five other tundra types at a single site. Ericaceous species had the lowest δ15N values, ranging between about −8 to −6‰. Foliar 15N contents increased progressively in birch, willows and sedges to maximum δ15N values of about +2‰ in sedges. Soil 15N contents in tundra ecosystems at our two most intensively studied sites increased with depth and δ15N values were usually higher for soils than for plants. Isotopic fractionations during soil N transformations and possibly during plant N uptake could lead to observed differences in 15N contents among plant species and between plants and soils. Patterns of variation in 15N content among species indicate that tundra plants acquire nitrogen in extremely nutrient-poor environments by competitive partitioning of the overall N pool. Differences in plant N sources, rooting depth, mycorrhizal associations, forms of N taken up, and other factors controlling plant N uptake are possible causes of variations in δ15N values of tundra plant species.

Net ecosystem exchange (NEE) of C varies greatly among Arctic ecosystems. Here, we show that approximately 75 per cent of this variation can be accounted for in a single regression model that predicts NEE as a function of leaf area index (LAI), air temperature and photosynthetically active radiation (PAR). The model was developed in concert with a survey of the light response of NEE in Arctic and subarctic tundras in Alaska, Greenland, Svalbard and Sweden. Model parametrizations based on data collected in one part of the Arctic can be used to predict NEE in other parts of the Arctic with accuracy similar to that of predictions based on data collected in the same site where NEE is predicted. The principal requirement for the dataset is that it should contain a sufficiently wide range of measurements of NEE at both high and low values of LAI, air temperature and PAR, to properly constrain the estimates of model parameters. Canopy N content can also be substituted for leaf area in predicting NEE, with equal or greater accuracy, but substitution of soil temperature for air temperature does not improve predictions. Overall, the results suggest a remarkable convergence in regulation of NEE in diverse ecosystem types throughout the Arctic. © 2013 The Author(s) Published by the Royal Society. All rights reserved.

Background: In the Arctic region, temperature increases are expected to be greater under anticipated climate change than the global average. Understanding how dark respiration (Rd) of common Arctic plant species acclimates to changes in the environment is therefore important for predicting changes to the Arctic carbon balance. Aims: The aim of this study is to investigate the influence of genotype and growing environment on Rd, the temperature response (Q10) of Rd, and foliar N (Nleaf) of the Arctic sedge Eriophorum vaginatum. Methods: We measured Rd, and determined its Q10 and Nleaf of E. vaginatum populations that were reciprocally transplanted 30 years previously along a latitudinal transect of 370 km in northern Alaska. Results: Rd and Q10 did not differ among populations (ecotypes) of E. vaginatum, but the local environment had a significant effect on both variables. Rd as well as Nleaf was higher in northern, colder sites, while Q10 was lower there. Conclusions: Rd in the different populations of E....

Arctic vegetation is characterized by high spatial variability in plant functional type (PFT) composition and gross primary productivity (P). Despite this variability, the two main drivers of P in sub-Arctic tundra are leaf area index (LT ) and total foliar nitrogen (NT ). LT and NT have been shown to be tightly coupled across PFTs in sub-Arctic tundra vegetation, which simplifies up-scaling by allowing quantification of the main drivers of P from remotely sensed LT . Our objective was to test the LT -NT relationship across multiple Arctic latitudes and to assess LT as a predictor of P for the pan-Arctic. Including PFT-specific parameters in models of LT -NT coupling provided only incremental improvements in model fit, but significant improvements were gained from including site-specific parameters. The degree of curvature in the LT -NT relationship, controlled by a fitted canopy nitrogen extinction co-efficient, was negatively related to average levels of diffuse radiation at a site. This is consistent with theoretical predictions of more uniform vertical canopy N distributions under diffuse light conditions. Higher latitude sites had higher average leaf N content by mass (NM ), and we show for the first time that LT -NT coupling is achieved across latitudes via canopy-scale trade-offs between NM and leaf mass per unit leaf area (LM ). Site-specific parameters provided small but significant improvements in models of P based on LT and moss cover. Our results suggest that differences in LT -NT coupling between sites could be used to improve pan-Arctic models of P and we provide unique evidence that prevailing radiation conditions can significantly affect N allocation over regional scales.

Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the Arctic and Subarctic, despite the well-known importance of lichens and bryophytes to the functioning and climate feedbacks of northern ecosystems. We investigated changes in diversity and abundance of lichens and bryophytes within long-term (9–16 years) warming experiments and along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra. In both Sweden and Alaska, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Bryophytes were less sensitive to experimental warming than lichens, but depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming in terms of both abundance and diversity. Temperature, on both continents, was the main driver of species composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in Alaska, temperature together with soil ammonium availability were the most important factors influencing species composition. Overall, dwarf shrub abundance (deciduous and evergreen) was positively related to warming but so were the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi; the majority of other cryptogams showed a negative relationship to warming. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity and abundance, especially within lichens, is likely to decrease under arctic climate warming. Given the many ecosystem processes affected by cryptogams in high latitudes (e.g. carbon sequestration, N2-fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.

In 2007, an area of more than 1,000 square kilometres of Alaskan tundra was destroyed by a single fire, more than doubling the cumulative area burnt in this region since 1950. Michelle Mack and colleagues now show that, in the process, 2.1 teragrams of carbon was released and about one-third of soil organic matter burned away, thereby potentially exposing permafrost soils to thaw. The amount of carbon released from the entire burn was comparable to the annual net carbon sink of the entire Arctic tundra biome during the past 25 years of the twentieth century. As tundra fires are expected to increase as the climate warms, combustion of 'old growth' tundra soil could constitute a positive climate feedback, by transferring surface soil carbon to the atmosphere and accelerating the thaw and decomposition of deeper permafrost carbon. Arctic tundra soils store large amounts of carbon (C) in organic soil layers hundreds to thousands of years old that insulate, and in some cases maintain, permafrost soils1,2. Fire has been largely absent from most of this biome since the early Holocene epoch3, but its frequency and extent are increasing, probably in response to climate warming4. The effect of fires on the C balance of tundra landscapes, however, remains largely unknown. The Anaktuvuk River fire in 2007 burned 1,039 square kilometres of Alaska’s Arctic slope, making it the largest fire on record for the tundra biome and doubling the cumulative area burned since 1950 (ref. 5). Here we report that tundra ecosystems lost 2,016 ± 435 g C m−2 in the fire, an amount two orders of magnitude larger than annual net C exchange in undisturbed tundra6. Sixty per cent of this C loss was from soil organic matter, and radiocarbon dating of residual soil layers revealed that the maximum age of soil C lost was 50 years. Scaled to the entire burned area, the fire released approximately 2.1 teragrams of C to the atmosphere, an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century7. The magnitude of ecosystem C lost by fire, relative to both ecosystem and biome-scale fluxes, demonstrates that a climate-driven increase in tundra fire disturbance may represent a positive feedback, potentially offsetting Arctic greening8 and influencing the net C balance of the tundra biome.

n the cold, arctic climate, global warming due to greenhouse gas accumulation in the atmosphere might be expected to increase both primary production and heterotrophic (mainly soil) respiration. Which of these processes will increase more or more rapidly? The answer to this question is critical in understanding the effects of warming on the net carbon balance of arctic ecosystems and of the earth itself. If primary production increases faster than heterotrophic respiration, carbon will be removed from the atmosphere and will accumulate on land. If the reverse happens, carbon will be lost to the atmosphere. Eventually, a new equilibrium may be reached, but this equilibration could take decades or even centuries. Meanwhile, dramatic losses or gains of carbon may occur over the entire arctic region (5.7 x 106 km2; Oechel 1989), with potentially important feedbacks on the global atmospheric concentration of

Leaf area index (LAI) and total foliar nitrogen (TFN) are important canopy characteristics and crucial variables needed to simulate photosynthesis and ecosystem CO2 fluxes. Although plant communities dominated by Cassiope tetragona are widespread in the Arctic, LAI and TFN for this vegetation type have not been accurately quantified. We address this knowledge gap by (i) direct measurements of LAI and TFN for C tetragona, and (ii) determining TFN-LAI and LAI-normalized difference vegetation index (NDVI) relationships for typical C tetragona tundras in the subarctic (Sweden) and High Arctic (Greenland and Svalbard). Leaves of C tetragona are 2-6 mm long and closely appressed to their stems forming parallelepiped shoots. We determined the LAI of C tetragona by measuring the area of the leaves while still attached to the stem, then doubling the resulting one-sided area. TFN was determined from leaf N and biomass. The LAI-NDVI and TFN-LAI relationships showed high correlation and can be used to estimate indirectly LAI and TFN. The LAI-NDVI relationship for C tetragona vegetation differed from a generic LAI-NDVI relationship for arctic tundra, whereas the TFN-LAI relationship did not. Overall, the LAI of C tetragona tundra ranged from 0.4 to 1.1 m(2) m(-2) and TFN from 1.4 to 1.7 g N m(-2).

Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long-term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta-analysis approach to compare the treatment effect size for aboveground biomass among seven long-term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non-acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient-rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.

As a result of the increasing atmospheric concentrations of radiatively active greenhouse gases, global temperatures are expected to rise (Houghton et al., 1990), particularly at high latitudes. Current global temperature trends are consistent with these predictions (Maxwell, 1992). Accompanying this climatic warming will be changes in growing season length, irradiance, permafrost depth, soil moisture, and nutrient availability (Chapin et al., 1992; Kane et al., 1992; Maxwell, 1992), although the exact nature and magnitude of these changes are less certain than those of temperature change. Other, more direct, human impacts on arctic ecosystems may be even more important than climatic change in the next few decades, including air pollution, disturbance associated with resource exploitation, and altered grazing regime due to changing patterns of reindeer husbandry and hunting. Together these global changes in environment are certain to affect arctic communities. The challenge is to predict how community composition will respond to these environmental changes and what will be the consequences for arctic ecosystems. In this chapter we present a framework for predicting how arctic plant communities will respond to global change and discuss the implications for ecosystem functioning. Detailed justification for this approach is provided elsewhere in general terms (Chapin, 1993; Hobbie et al., 1993; Shaver et al., 1992) and for tundra (Chapin et al., 1995).

Projected changes in global climate associated with accumulations of CO2 and other greenhouse gases in the atmosphere might stimulate the release of a substantial portion of the estimated 4.3 × 1016g of carbon stored in wet and moist arctic tundras (Oechel, 1989; Shaver et al., 1992). About 98% of this carbon is in soils and has accumulated over thousands of years as a result of cold, wet conditions that serve to slow decomposition. Increased temperatures will stimulate decomposition and hence release CO2 from soils to the atmosphere. However, increased CO2, increased temperature, and higher amounts of available nitrogen (as a result of faster decomposition) could stimulate rates of production in vegetation and thereby also stimulate CO2 removal from the atmosphere. The key to which of these two opposing processes dominates appears to be closely tied to the nitrogen cycle (Billings et al., 1984; Rastetter et al., 1992; Shaver et al., 1992).

The arctic tundra is an ecosystem type in which both vegetation composition and primary production are strongly limited by an extreme physical environment, including limitation by low temperatures, low light, a short growing season, and extremes of soil moisture (Billings and Mooney 1968; Bliss et al. 1981; Chapin and Shaver 1985; Chapin et al. 1992). In this physically stressful environment, both nutritional and biotic resources like soil-available nutrients or pollinators are also in short supply and thus contribute to its overall stressfulness. Finally, arctic tundras are frequently and extensively disturbed by freeze and thaw processes, and thus both the environment and the vegetation are continually changing on a fine scale even though on a coarse scale they may appear to be relatively stable (Sigafoos 1952; Churchill and Hanson 1958; Bliss and Peterson 1992).

To determine the role lemmings play in structuring plant communities and their contribution to the 'greening of the Arctic', we measured plant cover and biomass in 50 + year old lemming exclosures and control plots in the coastal tundra near Barrow, Alaska. The response of plant functional types to herbivore exclusion varied among land cover types. In general, the abundance of lichens and bryophytes increased with the exclusion of lemmings, whereas graminoids decreased, although the magnitude of these responses varied among land cover types. These results suggest that sustained lemming activity promotes a higher biomass of vascular plant functional types than would be expected without their presence and highlights the importance of considering herbivory when interpreting patterns of greening in the Arctic. In light of the rapid environmental change ongoing in the Arctic and the potential regional to global implications of this change, further exploration regarding the long-term influence of arvicoline rodents on ecosystem function (e.g. carbon and energy balance) should be considered a research priority.

F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology: A Challenge for the Future. The Arctic System: B. Maxwell, Arctic Climate: Potential for Change under Global Warming. D.L. Kane, L.D. Hinzman, M. Woo, and K.R. Everett, Arctic Hydrology and Climate Change. L.C. Bliss and N.V. Matveyeva, Circumpolar Arctic Vegetation. W.D. Billings, Phytogeographic and Evolutionary Potential for the Arctic Flora and Vegetation in a Changing Climate. L.C. Bliss and K.M. Peterson, Plant Succession, Competition, and the Physiological Constraints of Species in the Arctic. Carbon Balance: W.C. Oechel and W.D. Billings, Effects of Global Change on the Carbon Balance of Arctic Plants and Ecosystems. O.A. Semikhatova, T.V. Gerasimenko, and T.I. Ivanova, Photosynthesis, Respiration, and Growth of Plants in the Soviet Arctic. G.R. Shaver and J. Kummerow, Phenology, Resource Allocation, and Growth of Arctic Vascular Plants. J.D. Tenhunen, O.L. Lange, S. Hahn, R. Siegwolf, and S.F. Oberbauer, The Ecosystem Role of Poikilohydric Tundra Plants. B. Sveinbj~adornsson, Arctic Tree Line in a Changing Climate. Water and Nutrient Balance: S.F. Oberbauer and T.E. Dawson, Water Relations of Arctic Vascular Plants. K.J. Nadelhoffer, A.E. Giblin, G.R. Shaver, and A.E. Linkins, Microbial Processes and Plant Nutrient Availability in Arctic Soils. D.M. Chapin and C.S. Bledsoe, Nitrogen Fixation in Arctic Plant Communities. K. Kielland and F.S. Chapin III, Nutrient Absorption and Accumulation in Arctic Plants. F. Berendse and S. Jonasson, Nutrient Use and Nutrient Cycling in Northern Ecosystems. Interactions: J.B. McGraw and N. Fetcher, Response of Tundra Plant Populations to Climatic Change. J.P. Bryant and P.B. Reichardt, Controls over Secondary Metabolite Production by Arctic Woody Plants. R.L. Jefferies, J. Svoboda, G. Henry, M. Raillard, and R. Ruess, Tundra Grazing Systems and Climatic Change. J.F. Reynolds and P.W. Leadley, Modeling the Response of Arctic Plants to Changing Climate. F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology in an Ecosystem Context. Index.

Deciduous species (graminoids, forbs and deciduous shrubs) completed accumulation of biomass, nitrogen (N) and phosphorus (P) in new leaves and current-year's twigs by midsummer in arctic Alaska, whereas these processes continued throughout the growing season in the same tissue of evergreen shrubs. Nand P-use-efficiency increased from deciduous species to evergreen shrubs to lichens and mosses. Fertilization with N-plus-P had no effect on N-use-efficiency in deciduous leaves but caused a decline in N-useefficiency in evergreen leaves, mosses and lichens. P-use-efficiency declined with nutrient addition in leaves of all growth forms. Approximately half of the maximum pools of leaf N and P were withdrawn from leaves prior to autumn senescence, but this resorption was unaffected by nutrient addition treatment or by growth form. Key-words: Arctic, deciduous, evergreen, growth, nitrogen, phosphorus, resorption, tundra