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[1] Only a few studies to date have collectively examined the vegetation biomass and production of arctic tundra ecosystems and their relationships to broadly ranging climate variables. An additional complicating factor for studying vegetation of arctic tundra is the high spatial variability associated with small patterned-ground features, resulting from intense freeze-thaw processes. In this study, we sampled and analyzed the aboveground plant biomass components of patterned-ground ecosystems in the Arctic of northern Alaska and Canada along an 1800-km north-south gradient that spans approximately 11°C of mean July temperatures. Vegetation biomass was analyzed as functions of the summer warmth index (SWI–sum of mean monthly temperatures > 0°C). The total absolute biomass (g m−2) and biomass of shrubs increased monotonically with SWI, however, biomass of nonvascular species (mosses and lichens), were a parabolic function of SWI, with greatest values at the ends of the gradient. The components of plant biomass on patterned-ground features (i.e., on nonsorted circles or within small polygons) were constrained to a greater degree with colder climate than undisturbed tundra, likely due to the effect of frost heave disturbances on the vegetation. There were also clear differences in the relative abundances of vascular versus nonvascular plants on and off patterned-ground features along the SWI gradient. The spatial patterns of biomass differ among plant functional groups and suggest that plant community responses to temperature, and land-surface processes that produce patterned-ground features, are quite complex.

[1] Recent years have seen a growing interest in measuring and modeling soil CO2 efflux, as this flux represents a large component of ecosystem respiration and is a key determinant of ecosystem carbon balance. Process-based models of soil CO2 production and efflux, commonly based on soil temperature, are limited by nonlinearities such as the observed diurnal hysteresis between soil CO2 concentration ([CO2]) and temperature. Here we quantify the degree to which hysteresis between soil [CO2] and soil temperature is controlled by soil water content in a montane conifer forest, and how this nonlinearity impacts estimates of soil CO2 efflux. A representative model that does not consider hysteresis overestimated soil CO2 efflux for the entire growing season by 19%. At high levels of soil water content, hysteresis imposes organized, daily variability in the relationship between soil [CO2] and soil temperature, and at low levels of soil water content, hysteresis is minimized.

[1] We present empirical evidence for a relationship between the modal (most frequent) soil moisture level and the soil moisture level at which maximum evapotranspiration occurs for twenty-four flux tower sites in North America. We considered correlations and linear regressions between these two variables at annual, seasonal, bimonthly and monthly time scales for unimodal distributions of soil moisture, and found significant relationships between these two soil moisture variables at all time scales. Correlation was stronger during the summer than the winter, suggesting stronger coupling during the growing season. This coupling of modal soil moisture and soil moisture of maximum evapotranspiration suggests that vegetation may be optimizing productivity with respect to water use across different systems.

[1] In many terrestrial ecosystems, vegetation experiences limitation by different resources at different times. These resources include, among others, light, nutrients, and water. Frequently, however, leaf-level modeling frameworks that unite these limitations rely on empirical functions to scale stomatal conductance as a function of water stress. These functions use prescribed values of soil water content to mark the transition between water-stressed and unstressed conditions without accounting for the dependence of such a water content threshold on atmospheric and hydrologic conditions and nutrient availability. To address the phenomenon of a variable threshold to water stress, we combine an existing water-limited stomatal conductance model with an existing assimilation (photosynthesis)-limited stomatal conductance model. In this manner, we simulate variable controls on stomatal conductance and use a combination of the two models to define the threshold at which soil water content becomes limiting to transpiration. Modeled plant processes are used to define this water stress threshold as functionally dependent upon local environmental conditions (light, temperature, and atmospheric vapor pressure), parameters representing different vegetation types, and nutrient status. Simulations demonstrate that as environmental conditions become more favorable for assimilation, the likelihood of water stress increases. Specifically, there exist ranges of leaf temperature, light, and atmospheric humidity for which water stress is maximized.

[1] We explored whether 15N concentration could be predicted from reflectance spectra of fresh leaves, and, if so, whether the spectral features were related to the 15N concentration on a canopy scale. Leaf scale reflectance (R) measurements were conducted in Ghanzi, Botswana using a spectrophotometer in March 2005 and canopy scale leaf R was measured in a series of successional fields in Northern Virginia, USA using the same instrument in September 2005. Results showed that there was a strong correlation between foliar 15N concentration and spectral data in both visible and near-infrared wavelength regions. Stepwise regressions showed that the first-difference of the log1/R [(log 1/R)′] could explain 76 to 92% of the variation in foliar δ15N, providing the most reliable correlations with foliar 15N at bands near 600 and 700 nm. The present study indicates the possibility of estimating fresh leaf 15N abundance from high-resolution reflectance at leaf and canopy levels.

Fertilization via nutrient deposition and agricultural inputs is one of the most important factors driving decreases in plant diversity. However, we still do not fully understand which processes (niche process or neutral process) are more important in leading to decreases in plant diversity caused by fertilization. A hypothesis-based approach was used to test the relative importance of niche versus neutral processes along a fertilization gradient in an alpine meadow community on the eastern Tibetan plateau, China. Niche overlap values were calculated for species biomass, and the null model was used to generate the values of niche overlap expected at random. A linear regression modeling was used to evaluate the relationship between functional traits (specific leaf area, leaf dry matter content, and leaf total nitrogen concentration) and species relative abundance. Our results demonstrated that observed niche overlap for species biomass was significantly higher than expected at lower fertilization gradients. Moreover, we also found a significantly negative correlation between species relative abundance and specific leaf area and leaf dry matter content, but a significantly positive correlation between relative abundance and leaf nitrogen concentration at lower fertilization gradients. However, these relationships were not significant at higher fertilization gradients. We concluded that community assembly is dynamic progression along the environmental gradients, and niche and neutral processes may together determine species diversity loss in response to fertilization.

Soil moisture, and hence precipitation, exerts a dominant control on net primary productivity (NPP; the rate of carbon accumulation by autotrophs) in semiarid to subhumid ecosystems, such as grasslands and shrublands. At the global scale, mean annual precipitation has been shown to account for >50% of the variance in aboveground net primary production (ANPP) in grassland ecosystems (Lauenroth 1979; Le Houerou et al. 1988). Within specific regions, numerous investigators have demonstrated strong positive relationships between mean annual precipitation (MAP) and primary productivity (or some surrogate variable) for various ecosystems globally, including the Great Plains grasslands (Sala et al. 1988; Knapp et al. 2001; Epstein et al. 2002), the Patagonian steppe (Austin and Sala 2002; Jobbagy et al. 2002), African grasslands and savannas (Breman and de Wit 1983; McNaughton et al. 1993; Scanlon et al. 2002), and grasslands and shrublands of Inner Mongolia and northeastern China (Gao and Yu 1998; Yu et al. 2004).

Recent observations of changes in some tundra ecosystems appear to be responses to a warming climate. Several experimental studies have shown that tundra plants and ecosystems can respond strongly to environmental change, including warming; however, most studies were limited to a single location and were of short duration and based on a variety of experimental designs. In addition, comparisons among studies are difficult because a variety of techniques have been used to achieve experimental warming and different measurements have been used to assess responses. We used metaanalysis on plant community measurements from standardized warming experiments at 11 locations across the tundra biome involved in the International Tundra Experiment. The passive warming treatment increased plant-level air temperature by 1-3°C, which is in the range of predicted and observed warming for tundra regions. Responses were rapid and detected in whole plant communities after only two growing seasons. Overall, warming increased height and cover of deciduous shrubs and graminoids, decreased cover of mosses and lichens, and decreased species diversity and evenness. These results predict that warming will cause a decline in biodiversity across a wide variety of tundra, at least in the short term. They also provide rigorous experimental evidence that recently observed increases in shrub cover in many tundra regions are in response to climate warming. These changes have important implications for processes and interactions within tundra ecosystems and between tundra and the atmosphere.

[1] Old-field succession is a widespread process active in shaping landscapes in the eastern United States, contributing significantly to the terrestrial sink of atmospheric carbon dioxide, particularly at midlatitudes. However, few studies document ecosystem-scale carbon dioxide exchange during the early years of old-field succession, particularly during the temporal transition from cultivation to abandonment. Rates of carbon dioxide exchange were measured for 20 months over a field in Virginia during the transition from an actively cultivated crop field to an unmanaged old field, including one season of crop growth and two seasons of successional growth. Ecosystem carbon respiration exceeded carbon assimilation during growing seasons and dormant periods, resulting in a net flux of carbon dioxide from the biosphere to the atmosphere of between 1.27 and 1.85 kg C m−2 for the entire 20-month period (an average loss to the atmosphere of 2.07 to 3.01 g C m−2 day −1). Crop growth (from 10 January 2001 to 6 June 2001) resulted in a net loss of between 0.22 and 0.32 kg C m−2 to the atmosphere (an average daily loss of 1.5 to 2.2 g C m−2), whereas the two seasons of successional growth combined contributed an additional 1.05 to 1.53 kg C m−2 to the atmosphere (an average daily loss of 2.2 to 3.3 g C m−2). Empirical modeling was used to demonstrate control of ecosystem carbon respiration by soil temperature, soil moisture status, and the status of vegetation growth activity. Tower-based estimates of carbon loss were compared at both short (half hourly) and long (seasonal) timescales to independent, ground-based measurements. Using estimates of carbon exchange from previously published studies, these results are placed in the context of a trajectory of old-field succession.

Changes in the Arctic landscape due to climate warming are evident in greening of the tundra, formation of permafrost thaw lakes and erosion of ice wedge polygons. The changes are caused by atmospheric as well as soil and permafrost warming trends. Freezing and thawing drive many landscape features in the Arctic; cryoturbation being the major force that moves the soil. The interaction between vegetation development and cryoturbation is the objective of our research. In particular we simulate the effects of vegetation on the freezing rate in soils in three-dimensions in order to model non-sorted circle ecosystems that are prevalent in the Arctic. Differential insulation at the soil surface causes the soil to freeze irregularly and determines the strength and location of cryoturbation, caused by ice lens formation. Redistribution of water causes ice accumulation where the soil freezes first, while the thickness of the vegetation and its organic layer slows the freezing process, which dries the soil below vegetated patches. For our research we combined two simulation models, 1) the WIT3D model that simulates the hydrology in the active layer during freezing, and 2) the ArcVeg model that simulates vegetation succession in the tundra. We have analyzed vegetation types along a bioclimatic gradient in the arctic tundra and determined an insulation factor for these vegetation types. The insulation values were used in the combined models and the results show that warming conditions simulated with bare ground, as initial condition, would form a vegetation pattern that is more uniform and would develop fewer non-sorted circles. However when an existing pattern is simulated with a warmer climate as the initial condition, then the pattern does not disintegrate but the vegetation dominates and makes the non-sorted circles smaller. Over a longer time frame the vegetation can eventually dominate the non-sorted circles completely.

A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO 2 ). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.

Bajo condiciones actuales, extensas areas de las zonas templadas del oeste de America del Norte y del sur de America del Sur tienen regimenes climaticos aridos a subhumedos, que son vulnerables a cambios climaticos inducidos por actividades humanas. Predicciones obtenidas a partir de modelos de circulacion global bajo una duplicacion del CO2 atmosferico sugieren grandes cambios en temperatura media anual, y cambios pequenos o nulos en la precipitacion media anual y la proporcion de precipitacion estival. Nuestro objetivo fue evaluar como las predicciones de cambio climatico obtenidas de modelos de circulacion global influiran sobre los patrones climaticos, e inferir a partir de ello la distribucion de los ecosistemas de las zonas templadas de America del Norte y del Sur. Calculos de deficit hidrico anual sugieren que, debido al cambio climatico, se duplicara el area afectada por condiciones muy secas. Esta expansion ocurrira en las cercanias de las zonas aridas actuales. Calculos mensuales de deficit hidrico sugieren que aproximadamente la mitad de la zona templada de cada continente se ve afectada por al menos un mes de deficit. Bajo un clima con doble CO2, estas areas se expandirian y cubririan hasta 77 % de las areas templadas de America del Norte y hasta 80 % de America del Sur. Los cambios en la distribucion de ecosistemas resultantes probablemente seran debidos a la expansion de los desiertos a expensas de los pastizales en America del Norte y del Sur, y la expansion de los pastizales a expensas de los bosques deciduos y boreales en America del Norte. Nuestros analisis asumen que los cambios climaticos futuros estaran abarcados por las predicciones de los tres escenarios de duplicacion de CO2 que utilizamos. La situacion mas probable es que los cambios reales, si es que ocurren, seran distintos a nuestros escenarios. Por lo tanto, nuestros analisis deberan interpretarse como indicaciones de la sensibilidad de partes de las zonas templadas de America del Norte y del Sur a aumentos de temperatura. La principal conclusion de nuestros analisis es que cualquier aumento de temperatura debido a cambios climaticos resultara en una expansion de las porciones mas aridas de ambos continentes

[1] Here we analyzed a time series of 21-yr satellite data for three bioclimate subzones in northern Alaska and confirmed a long-term trend of increase in vegetation greenness for the Alaskan tundra that has been detected globally for the northern latitudes. There was a 16.9% (±5.6%) increase in peak vegetation greenness across the region that corresponded to simultaneous increases in temperatures. We also examined the changes for four specific vegetation types using an 11-yr finer resolution (1-km) satellite data and found that the temporal changes in peak and time-integrated greenness were greatest in areas of moist nonacidic tundra. These changes in greenness between 1981 and 2001 correspond approximately to a 171 g/m2 (±81 g/m2) increases in aboveground plant biomass for Alaskan tundra. This remotely sensed interpretation is conducted in the absence of long-term biomass records in the region.

[1] We examined the effects of summer warmth on leaf area index (LAI), total aboveground phytomass (TAP), and normalized difference vegetation index (NDVI) across the Arctic bioclimate zone in Alaska and extrapolated our results to the circumpolar Arctic. Phytomass, LAI, and within homogeneous areas of vegetation on acidic and nonacidic soils were regressed against the total summer warmth index (SWI) at 12 climate stations in northern Alaska (SWI = sum of mean monthly temperatures greater than 0°C). SWI varies from 9°C at Barrow to 37°C at Happy Valley. A 5°C increase in the SWI is correlated with about a 120 g m−2 increase in the aboveground phytomass for zonal vegetation on acidic sites and about 60 g m−2 on nonacidic sites. Shrubs account for most of the increase on acidic substrates, whereas mosses account for most of the increase on nonacidic soils. LAI is positively correlated with SWI on acidic sites but not on nonacidic sites. The NDVI is positively correlated with SWI on both acidic and nonacidic soils, but the NDVI on nonacidic parent material is consistently lower than the NDVI on acidic substrates. Extrapolation to the whole Arctic using a five-subzone zonation approach to stratify the circumpolar NDVI and phytomass data showed that 60% of the aboveground phytomass is concentrated in the low-shrub tundra (subzone 5), whereas the high Arctic (subzones 1–3) has only 9% of the total. Estimated phytomass densities in subzones 1–5 are 47, 256, 102, 454, and 791 g m−2, respectively. Climate warming will likely result in increased phytomass, LAI, and NDVI on zonal sites. These changes will be most noticeable in acidic areas with abundant shrub phytomass.

The physical composition of the soil can determine grassland plant responses to rising atmospheric carbon dioxide.

Widespread increases in Arctic tundra productivity have been documented for decades using coarse-scale satellite observations, but finer-scale observations indicate that changes have been very uneven, with a high degree of landscape- and regional-scale heterogeneity. Here we analyze time-series of the Normalized Difference Vegetation Index (NDVI) observed by Landsat (1984–2012), to assess landscape- and regional-scale variability of tundra vegetation dynamics in the northwest Siberian Low Arctic, a little-studied region with varied soils, landscape histories, and permafrost attributes. We also estimate spatio-temporal rates of land-cover change associated with expansion of tall alder (Alnus) shrublands, by integrating Landsat time-series with very-high-resolution imagery dating to the mid-1960s. We compiled Landsat time-series for eleven widely-distributed landscapes, and performed linear regression of NDVI values on a per-pixel basis. We found positive net NDVI trends (‘greening’) in nine of eleven landscapes. Net greening occurred in alder shrublands in all landscapes, and strong greening tended to correspond to shrublands that developed since the 1960s. Much of the spatial variability of greening within landscapes was linked to landscape physiography and permafrost attributes, while between-landscape variability largely corresponded to differences in surficial geology. We conclude that continued increases in tundra productivity in the region are likely in upland tundra landscapes with fine-textured, cryoturbated soils; these areas currently tend to support discontinuous vegetation cover, but are highly susceptible to rapid increases in vegetation cover, as well as land-cover changes associated with the development of tall shrublands.

Ecosystems where plant-available nitrogen (N) is limited by constraints on decomposition may be quite capable of retaining additional N. However, there are many factors that will control the quantity of N retained, with potential implications for system carbon and nitrogen storage. We examined the retention and allocation of (15)N 2 years after labeling a semiarid, shortgrass steppe ecosystem in northeastern Colorado. The plant community of the study area is a patchy mixture of C3 (cool-season) and C4 (warm-season) graminoids; we hypothesized that differences in allocation patterns between the two plant types would lead to differing rates of N retention in this grazed system. We found that after three growing seasons (just over 2 years), an average of 28.3% of the original (15)N was retained in our plots, with nearly all of this N in soils (24.9%) rather than plants. Plots dominated by C3 plants had significantly less (15)N retained after 2 years than mixed C3-C4 plots. A high initial rate of retention by C3 plants, combined with a propensity for allocation to shoots rather than roots, likely led to this result in a system that typically has much of its aboveground tissue removed by grazers. In comparing our retention patterns to those of other studies in the shortgrass steppe, we found that the distribution of added (15)N to various ecosystem compartments (plants, mineral soil, soil organic matter) was similar across studies regardless of the experimental conditions, duration of study, and amount of (15)Nretained. We additionally observed the beginning of the formation of "resource islands," with (15)N being physically and biologically redistributed under plants from between plants.

Numerous studies have evaluated the dynamics of Arctic tundra vegetation throughout the past few decades, using remotely sensed proxies of vegetation, such as the normalized difference vegetation index (NDVI). While extremely useful, these coarse-scale satellite-derived measurements give us minimal information with regard to how these changes are being expressed on the ground, in terms of tundra structure and function. In this analysis, we used a strong regression model between NDVI and aboveground tundra phytomass, developed from extensive field-harvested measurements of vegetation biomass, to estimate the biomass dynamics of the circumpolar Arctic tundra over the period of continuous satellite records (1982-2010). We found that the southernmost tundra subzones (C-E) dominate the increases in biomass, ranging from 20 to 26%, although there was a high degree of heterogeneity across regions, floristic provinces, and vegetation types. The estimated increase in carbon of the aboveground live vegetation of 0.40 Pg C over the past three decades is substantial, although quite small relative to anthropogenic C emissions. However, a 19.8% average increase in aboveground biomass has major implications for nearly all aspects of tundra ecosystems including hydrology, active layer depths, permafrost regimes, wildlife and human use of Arctic landscapes. While spatially extensive on-the-ground measurements of tundra biomass were conducted in the development of this analysis, validation is still impossible without more repeated, long-term monitoring of Arctic tundra biomass in the field.

Satellite-based measurements of the normalized difference vegetation index (NDVI; an index of vegetation greenness and photosynthetic capacity) indicate that tundra environments are generally greening and becoming more productive as climates warm in the Arctic. The greening, however, varies and is even negative in some parts of the Arctic. To help interpret the space-based observations, the International Polar Year (IPY) Greening of the Arctic project conducted ground-based surveys along two >1500 km transects that span all five Arctic bioclimate subzones. Here we summarize the climate, soil, vegetation, biomass, and spectral information collected from the North America Arctic transect (NAAT), which has a more continental climate, and the Eurasia Arctic transect (EAT), which has a more oceanic climate. The transects have broadly similar summer temperature regimes and overall vegetation physiognomy, but strong differences in precipitation, especially winter precipitation, soil texture and pH, disturbance regimes, and plant species composition and structure. The results indicate that summer warmth and NDVI increased more strongly along the more continental transect.

Phenology of vegetation is a sensitive and valuable indicator of the dynamic responses of terrestrial ecosystems to climate change. Therefore, to better understand and predict ecosystems dynamics, it is important to reduce uncertainties in detecting phenological changes. Here, changes in phenology over the past several decades across the northern high-latitude region (≥60°N) were examined by calibrating and analyzing time series of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Very High Resolution Radiometer (AVHRR). Over the past decade (2000–10), an expanded length of the growing season (LOS) was detected by MODIS, largely due to an earlier start of the growing season (SOS) by 4.7 days per decade and a delayed end of the growing season (EOS) by 1.6 days per decade over the northern high latitudes. There were significant differences between North America and Eurasia in phenology from 2000 to 2010 based on MODIS data (SOS: d f = 21, F = 49.02, p d f = 21, F = 49.25, p d f = 21, F = 79.40, p d f = 17, F = 14.63, p = 0.0015; EOS: d f = 17, F = 38.69, p d f = 17, F = 16.47, p = 0.0009) from 2000 to 2008 over the northern high latitudes. Thus, further inter-calibration between the sensors is needed to resolve the inconsistency and to better understand long-term trends of vegetation growth in the Arctic.

Understanding the responses of the arctic tundra biome to a changing climate requires knowledge of the complex interactions among the climate, soils and biological system. This study investigates the individual and interaction effects of climate change and reindeer grazing across a variety of climate zones and soil texture types on tundra vegetation community dynamics using an arctic vegetation model that incorporates the reindeer diet, where grazing is a function of both foliar nitrogen concentration and reindeer forage preference. We found that grazing is important, in addition to the latitudinal climate gradient, in controlling tundra plant community composition, explaining about 13% of the total variance in model simulations for all arctic tundra subzones. The decrease in biomass of lichen, deciduous shrub and graminoid plant functional types caused by grazing is potentially dampened by climate warming. Moss biomass had a nonlinear response to increased grazing intensity, and such responses were stronger when warming was present. Our results suggest that evergreen shrubs may benefit from increased grazing intensity due to their low palatability, yet a growth rate sensitivity analysis suggests that changes in nutrient uptake rates may result in different shrub responses to grazing pressure. Heavy grazing caused plant communities to shift from shrub tundra toward moss, graminoid-dominated tundra in subzones C and D when evergreen shrub growth rates were decreased in the model. The response of moss, lichen and forbs to warming varied across the different subzones. Initial vegetation responses to climate change during transient warming are different from the long term equilibrium responses due to shifts in the controlling mechanisms (nutrient limitation versus competition) within tundra plant communities.

In the Chihuahuan Desert of Mexico and New Mexico, shrub invasion is a common problem, and once-abundant grassland ecosystems are being replaced by shrub-dominated habitat. The spatial arrangement of grasses and shrubs in these arid grasslands can provide better insight into community dynamics and can provide information on grass shrub interactions. To better understand the dynamics of the Chihuahuan Desert ecosystem and to provide information regarding the interactions between grasses and shrubs, we examined the spatial patterns of grasses and shrubs in remaining grass-dominated areas, interspersed with some shrubs. We developed 18, 10 × 20 m vegetation distribution plots by mapping the location of all grasses and shrubs on each and repeating the measurements three years later. Spatial patterns were then assessed for each plot using a second-order spatial statistic, Ripley's K-function, as well as any observed changes in the spatial patterns over a three-year period. We observed clumped grass distributions, indicating a lack of competition among grasses; random shrub distributions; and even grass distribution with respect to shrub locations, indicating competition between grasses and shrubs. We also observed a tendency for grass distributions to become more even over time, and grasses to become less even with respect to shrub locations over time. These changes occurred during a period of greater than average rainfall, indicating that greater water availability may lead to increased competition among grasses and decreased competition between grasses and shrubs.

Micrometeorological flux towers provide spatially integrated estimates of net ecosystem production (NEP) of carbon over areas ranging from several hectares to several square kilometers, but they do so at the expense of spatially explicit information within the footprint of the tower. This finer-scale information is crucial for understanding how physical and biological factors interact and give rise to tower-measured fluxes in complex landscapes. We present a simple approach for quantifying and evaluating the spatial heterogeneity of cumulative growing season NEP for complex landscapes. Our method is based on spatially distributed information about physical and biological landscape variables and knowledge of functional relationships between constituent fluxes and these variables. We present a case study from a complex landscape in the Rocky Mountains of Montana (US) to demonstrate that the spatial distribution of cumulative growing season NEP is rather large and bears the imprint of the topographic and vegetation variables that characterize this complex landscape. Net carbon sources and net carbon sinks were distributed across the landscape in manner predictable by the intersection of these landscape variables. We simulated year-to-year climate variability and found that some portions of the landscape were consistently either carbon sinks or carbon sources, but other portions transitioned between sink and source. Our findings reveal that this emergent behavior is a unique characteristic of complex landscapes derived from the interaction of topography and vegetation. These findings offer new insight for interpreting spatially integrated carbon fluxes measured over complex landscapes.

Sustainability of tundra vegetation under changing climate on the Yamal Peninsula, northwestern Siberia, home to the world’s largest area of reindeer husbandry, is of crucial importance to the local native community. An integrated investigation is needed for better understanding of the effects of soils, climate change and grazing on tundra vegetation in the Yamal region. In this study we applied a nutrient-based plant community model—ArcVeg—to evaluate how two factors (soil organic nitrogen (SON) levels and grazing) interact to affect tundra responses to climate warming across a latitudinal climatic gradient on the Yamal Peninsula. Model simulations were driven by field-collected soil data and expected grazing patterns along the Yamal Arctic Transect (YAT), within bioclimate subzones C (high arctic), D (northern low arctic) and E (southern low arctic). Plant biomass and NPP (net primary productivity) were significantly increased with warmer bioclimate subzones, greater soil nutrient levels and temporal climate warming, while they declined with higher grazing frequency. Temporal climate warming of 2 ◦ C caused an increase of 665 g m −2 in total biomass at the high SON site in subzone E, but only 298 g m −2 at the low SON site. When grazing frequency was also increased, total biomass increased by only 369 g m −2 at the high SON site in contrast to 184 g m −2 at the low SON site in subzone E. Our results suggest that high SON can support greater plant biomass and plant responses to climate warming, while low SON and grazing may limit plant response to climate change. In addition to the first order factors (SON, bioclimate subzones, grazing and temporal climate warming), interactions among these significantly affect plant biomass and productivity in the arctic tundra and should not be ignored in regional scale studies.

This study is presented within the context that climate warming and sea-ice decline has been occurring throughout much of the Arctic over the past several decades, and that terrestrial ecosystems at high latitudes are sensitive to the resultant alterations in surface temperatures. Results are from analyzing interannual satellite records of vegetation greenness across a bioclimate gradient of the Canadian Arctic over the period of 1982-2006. Here, we combine multi-scale sub-pixel analysis and remote sensing time-series analysis to investigate recent decadal changes in vegetation greenness along spatial gradients of summer temperature and vegetation. Linear autoregression temporal analysis of vegetation greenness was performed with relatively "pure" vegetation pixels of Advanced Very High Resolution Radiometer (AVHRR) data, spanning Low Arctic, High Arctic and polar desert ecosystems. Vegetation greenness generally increased over tundra ecosystems in the past two decades. Peak annual greenness increased 0.49-0.79%/yr over the High Arctic where prostrate dwarf shrubs, forbs, mosses and lichens dominate and 0.46-0.67%/yr over the Low Arctic where erect dwarf shrubs and graminoids dominate. However, magnitudes of vegetation greenness differ with length of time series and periods considered, indicating a nonlinear response of terrestrial ecosystems to climate change. The decadal increases of greenness reflect increasing vegetation production during the peak of the growing season, and were likely driven by the recent warming.

We performed an analysis of the climatic patterns of the temperate zones in North and South America using a global database of monthly precipitation and temperature. Three synthetic variables, identified by a principal component analysis (PCA) of the monthly data, were used: mean annual precipitation, mean annual temperature and the proportion of the precipitation falling during summer. We displayed the spatial gradient of the three variables by constructing a com- posite colour raster image. We used a parallelepiped classification algorithm to locate areas in both continents that are climatically similar to five North American Long Term Ecological Research sites and to two South American long- term ecological research sites. The same algorithm was used to identify areas in South America which are climatically similar to some of the major grassland and shrubland types of North America. There is substantial overlap between the climates of North and South America. Most of the climatic patterns found in South America are well represented in North America. How- ever, there are certain climates in North America that are not found in South America. An example is a climate with relatively low mean annual temperature and high summer precipitation. The climatic signatures of three North American LTER sites (Cedar Creek, CPER and Sevilleta) were not found in South America. The climatic signatures of two LTER sites (Konza and Jornada) had some representation in South America. Two South American research sites (Rio Mayo and Las Chilcas) were well represented climatically in North America. The climates of six out of seven selected North American grassland and shrubland types were represented in South America. The northern mixed prairie type was not represented climatically in South America. Our analysis sug- gests that comparisons of North and South America can provide a powerful test of climatic control over vegetation.

The arctic is a sensitive system undergoing dramatic changes related to recent warming trends. Vegetation dynamics - increases in the quantity of green vegetation and a northward migration of trees into the arctic tundra - are a component of this change. Although field studies over long time periods can be logistically problematic, simulation modeling provides a means for projecting changes in arctic and subarctic vegetation caused by environmental variations.