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In the high Arctic, plant community species composition generally responds slowly to climate warming, whereas less is known about the community functional trait responses and consequences for ecosystem functioning. The slow species turnover and large distribution ranges of many Arctic plant species suggest a significant role of intraspecific trait variability in functional responses to climate change. Here we compare taxonomic and functional community compositional responses to a long‐term (17‐year) warming experiment in Svalbard, Norway, replicated across three major high Arctic habitats shaped by topography and contrasting snow regimes. We observed taxonomic compositional changes in all plant communities over time. Still, responses to experimental warming were minor and most pronounced in the drier habitats with relatively early snowmelt timing and long growing seasons (Cassiope and Dryas heaths). The habitats were clearly separated in functional trait space, defined by 12 size‐ and leaf economics‐related traits, primarily due to interspecific trait variation. Functional traits also responded to experimental warming, most prominently in the Dryas heath and mostly due to intraspecific trait variation. Leaf area and mass increased and leaf δ15N decreased in response to the warming treatment. Intraspecific trait variability ranged between 30% and 71% of the total trait variation, reflecting the functional resilience of those communities, dominated by long‐lived plants, due to either phenotypic plasticity or genotypic variation, which most likely underlies the observed resistance of high Arctic vegetation to climate warming. We further explored the consequences of trait variability for ecosystem functioning by measuring peak season CO2 fluxes. Together, environmental, taxonomic, and functional trait variables explained a large proportion of the variation in net ecosystem exchange (NEE), which increased when intraspecific trait variation was accounted for. In contrast, even though ecosystem respiration and gross ecosystem production both increased in response to warming across habitats, they were mainly driven by the direct kinetic impacts of temperature on plant physiology and biochemical processes. Our study shows that long‐term experimental warming has a modest but significant effect on plant community functional trait composition and suggests that intraspecific trait variability is a key feature underlying high Arctic ecosystem resistance to climate warming. [ABSTRACT FROM AUTHOR]

We measured CO 2 and CH 4 fluxes using chambers and eddy covariance (only CO 2) from a moist moss tundra in Svalbard. The average net ecosystem exchange (NEE) during the summer (9 June–31 August) was negative (sink), with - 0.139 ± 0.032 µ mol m -2 s -1 corresponding to - 11.8 g C m -2 for the whole summer. The cumulated NEE over the whole growing season (day no. 160 to 284) was - 2.5 g C m -2. The CH 4 flux during the summer period showed a large spatial and temporal variability. The mean value of all 214 samples was 0.000511 ± 0.000315 µ mol m -2 s -1 , which corresponds to a growing season estimate of 0.04 to 0.16 g CH 4 m -2. Thus, we find that this moss tundra ecosystem is closely in balance with the atmosphere during the growing season when regarding exchanges of CO 2 and CH 4. The sink of CO 2 and the source of CH 4 are small in comparison with other tundra ecosystems in the high Arctic. Air temperature, soil moisture and the greenness index contributed significantly to explaining the variation in ecosystem respiration (Reco), while active layer depth, soil moisture and the greenness index were the variables that best explained CH 4 emissions. An estimate of temperature sensitivity of Reco and gross primary productivity (GPP) showed that the sensitivity is slightly higher for GPP than for Reco in the interval 0–4.5 ∘ C; thereafter, the difference is small up to about 6 ∘ C and then begins to rise rapidly for Reco. The consequence of this, for a small increase in air temperature of 1 ∘ (all other variables assumed unchanged), was that the respiration increased more than photosynthesis turning the small sink into a small source (4.5 g C m -2) during the growing season. Thus, we cannot rule out that the reason why the moss tundra is close to balance today is an effect of the warming that has already taken place in Svalbard. [ABSTRACT FROM AUTHOR]

Summary: Bacterial communities form the basis of biogeochemical processes and determine plant growth and health. Mosses harbour diverse bacterial communities that are involved in nitrogen fixation and carbon cycling. Global climate change is causing changes in aboveground plant biomass and shifting species composition in the Arctic, but little is known about the response of moss microbiomes in these environments.Here, we studied the total and potentially active bacterial communities associated with Racomitrium lanuginosum in response to a 20‐yr in situ warming in an Icelandic heathland. We evaluated the effect of warming and warming‐induced shrub expansion on the moss bacterial community composition and diversity, and nifH gene abundance.Warming changed both the total and the potentially active bacterial community structure, while litter abundance only affected the total bacterial community structure. The abundance of nifH genes was negatively affected by litter abundance. We also found shifts in the potentially nitrogen‐fixing community, with Nostoc decreasing and noncyanobacterial diazotrophs increasing in relative abundance.Our data suggest that the moss microbial community and potentially nitrogen fixing taxa will be sensitive to future warming, partly via changes in litter and shrub abundance. [ABSTRACT FROM AUTHOR]

Recently, there have been several studies using open top chambers ( OTCs) or cloches to examine the response of Arctic plant communities to artificially elevated temperatures. Few, however, have investigated multitrophic systems, or the effects of both temperature and vertebrate grazing treatments on invertebrates. This study investigated trophic interactions between an herbivorous insect ( Sitobion calvulum, Aphididae), a woody perennial host plant ( Salix polaris) and a selective vertebrate grazer (barnacle geese, Branta leucopsis). In a factorial experiment, the responses of the insect and its host to elevated temperatures using open top chambers ( OTCs) and to three levels of goose grazing pressure were assessed over two summer growing seasons (2004 and 2005). OTCs significantly enhanced the leaf phenology of Salix in both years and there was a significant OTC by goose presence interaction in 2004. Salix leaf number was unaffected by treatments in both years, but OTCs increased leaf size and mass in 2005. Salix reproduction and the phenology of flowers were unaffected by both treatments. Aphid densities were increased by OTCs but unaffected by goose presence in both years. While goose presence had little effect on aphid density or host plant phenology in this system, the OTC effects provide interesting insights into the possibility of phenological synchrony disruption. The advanced phenology of Salix effectively lengthens the growing season for the plant, but despite a close association with leaf maturity, the population dynamics of the aphid appeared to lack a similar phenological response, except for the increased population observed. [ABSTRACT FROM AUTHOR]

1. Understanding the impact of disturbance on vegetation and the resilience of plant communities to disturbance is imperative to ecological theory and environmental management. In this study predictors of community resilience to a simulated natural disturbance are investigated. Responses to disturbance are examined at the community, plant functional type and species level. 2. Field experiments were set up in seven tundra plant communities, simulating disturbance based on the impact of grubbing by an increasing herbivore population of pink-footed geese ( Anser brachyrhynchus). The short-term resilience of communities was assessed by comparing community dissimilarity between control plots and plots subject to three disturbance intensities based on the foraging impact of these geese. Potential for long-term recovery was evaluated across different disturbance patch sizes. 3. Resilience to disturbance varied between communities; those with higher moss cover and higher soil moisture, such as wetlands and mires, were most resilient to disturbance. 4. The wetter communities demonstrated greater long-term recovery potential following disturbance. In wetland communities, vegetative recovery of vascular plants and moss was greater in smaller disturbed patches and at the edges of patches. 5. The response of vegetation to disturbance varied with intensity of disturbance, plant community and plant species. The use of functional type classifications only partially explained the variation in species responses to disturbance across communities, thus their use in predicting community changes was limited. 6. Synthesis. The impact of disturbance is shown to be plant-community specific and related to the initial abiotic and biotic properties of the community. By showing that resilience is partly predictable, the identification of disturbance-susceptible communities is possible, which is of relevance for ecosystem management. [ABSTRACT FROM AUTHOR]

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 degrees 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.

Environmental manipulation experiments showed that species respond individualistically to each environmental-change variable. The greatest responses of plants were generally to nutrient, particularly nitrogen, addition. Summer warming experiments showed that woody plant responses were dominant and that mosses and lichens became less abundant. Responses to warming were controlled by moisture availability and snow cover. Many invertebrates increased population growth in response to summer warming, as long as desiccation was not induced. CO2 and UV-B enrichment experiments showed that plant and animal responses were small. However, some microorganisms and species of fungi were sensitive to increased UV-B and some intensive mutagenic actions could, perhaps, lead to unexpected epidemic outbreaks. Tundra soil heating, CO2 enrichment and amendment with mineral nutrients generally accelerated microbial activity. Algae are likely to dominate cyanobacteria in milder climates. Expected increases in winter freeze-thaw cycles leading to ice-crust formation are likely to severely reduce winter survival rate and disrupt the population dynamics of many terrestrial animals. A deeper snow cover is likely to restrict access to winter pastures by reindeer/caribou and their ability to flee from predators while any earlier onset of the snow-free period is likely to stimulate increased plant growth. Initial species responses to climate change might occur at the sub-species level: an Arctic plant or animal species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do so also in the future. Indigenous knowledge, air photographs, satellite images and monitoring show that changes in the distributions of some species are already occurring: Arctic vegetation is becoming more shrubby and more productive, there have been recent changes in the ranges of caribou, and "new" species of insects and birds previously associated with areas south of the treeline have been recorded. In contrast, almost all Arctic breeding bird species are declining and models predict further quite dramatic reductions of the populations of tundra birds due to warming. Species-climate response surface models predict potential future ranges of current Arctic species that are often markedly reduced and displaced northwards in response to warming. In contrast, invertebrates and microorganisms are very likely to quickly expand their ranges northwards into the Arctic.