Changes In Arctic Vegetation Research Articles

Large Divergence of Projected High Latitude Vegetation Composition and Productivity Due To Functional Trait Uncertainty

AbstractVegetation distribution and composition are expected to change in northern high latitudes under rapid warming, which regulates ecosystem functions but remains challenging to predict. Vegetation change arises from the interplay of chronic climate trends such as warming and transient demographic processes of recruitment, growth, competition, and mortality. Most predictive models overlooked the role of demographic dynamics controlled by plant traits. Here, we simulate vegetation dynamics at the Kougarok Hillslope site in Alaska under historical and future climates using the E3SM Land Model coupled to the Functionally Assembled Terrestrial Simulator (ELM‐FATES). To evaluate the roles of plant traits, we parameterize the model with 5,265 trait configurations representing diverse physiological and demographic strategies. Results show current modeled biomass, composition, and productivity are most sensitive to traits controlling photosynthetic capacity, carbon allocation, allometry, and phenology. Among all trait configurations, ∼5% reproduce in situ biomass and plant functional type (PFT) composition measured in 2016, that are indistinguishable from these two observed ecosystem states. Notably, these same trait configurations produce diverging biomass, composition, and productivity under future climate, where the uncertainty attributable to traits is twice the change attributable to climate change. The variation of projected productivity arises from emerging PFT composition under novel climate regimes, primarily explained by traits controlling cold‐induced mortality, recruitment, and allometry. Our findings highlight the importance and uncertainty of demographic dynamics and its interaction with climate change in shaping Arctic vegetation change. Improved model predictions will likely benefit from explicit consideration of vegetation demography and better constraints of critical traits.

Top‐Down Regulation by a Reindeer Herding System Limits Climate‐Driven Arctic Vegetation Change at a Regional Scale

AbstractWarming‐driven growth of tall woody vegetation in the Arctic has the potential to accelerate climate change through multiple positive feedbacks. Local‐scale evidence suggests that large herbivores limit this vegetation shift, but there is uncertainty at larger, regional scales whether current herbivory pressure is a major top‐down control on ecosystem structure and functioning. Across a 67,000 km2 region of the Yamal Peninsula in West Siberia, we integrated satellite remote sensing with a novel data set mapping the migrations of herds comprising 151,000 domesticated reindeer. Where reindeer numbers varied over space, higher reindeer herbivory pressure was consistently linked with lower coverage of tall woody vegetation. Within areas dominated by this vegetation type, productivity and climate were increasingly decoupled where reindeer density was higher. Our spaceborne fingerprint detection suggests that large herbivores, at current population densities, counteract Arctic vegetation responses to climate change over large spatial scales.

Improving ecological insights from dendroecological studies of Arctic shrub dynamics: Research gaps and potential solutions

Rapid climate change has been driving changes in Arctic vegetation in recent decades, with increased shrub dominance in many tundra ecosystems. Dendroecological observations of tundra shrubs can provide insight into current and past growth and recruitment patterns, both key components for understanding and predicting ongoing and future Arctic shrub dynamics. However, generalizing these dynamics is challenging as they are highly scale-dependent and vary among sites, species, and individuals. Here, we provide a perspective on how some of these challenges can be overcome. Based on a targeted literature search of dendrochronological studies from 2005 to 2022, we highlight five research gaps that currently limit dendro-based studies from revealing cross-scale ecological insight into shrub dynamics across the Arctic biome. We further discuss the related research priorities, suggesting that future studies could consider: 1) increasing focus on intra- and interspecific variation, 2) including demographic responses other than radial growth, 3) incorporating drivers, in addition to warming, at different spatial and temporal scales, 4) implementing systematic and unbiased sampling approaches, and 5) investigating the cellular mechanisms behind the observed responses. Focusing on these aspects in dendroecological studies could improve the value of the field for addressing cross-scale and plant community-framed ecological questions. We outline how this could be facilitated through the integration of community-based dendroecology and dendroanatomy with remote sensing approaches. Integrating new technologies and a more multidisciplinary approach in dendroecological research could provide key opportunities to close important knowledge gaps in our understanding of scale-dependencies, as well as intra- and inter-specific variation, in vegetation community dynamics across the Arctic tundra.

Upslope release—Downslope receipt? Multi‐year plant uptake of permafrost‐released nitrogen along an arctic hillslope

Abstract As arctic permafrost continues to thaw, previously inaccessible nitrogen (N) becomes available to N‐limited arctic plants. Increased N availability could enhance plant growth and thereby potentially offset climate‐induced carbon release. Arctic plants can take up newly available permafrost‐N locally upon release. However, in a topographically diverse arctic landscape, permafrost‐N may be transported along hillslopes, away from the point‐of‐release. The extent to which topographical N transport can impact arctic vegetation change depends on whether N is retained locally, captured by downslope recipient plant communities, or transported away. We used stable isotope labelling (15N) to simulate upslope release of ammonium (NH4+) and nitrate (NO3−) from thawing permafrost on an arctic hillslope, western Greenland. We tracked the plant species‐specific uptake of simulated permafrost‐released N from the upslope point‐of‐release to the bottom of the slope through 4 years. We found that arctic tundra plants successfully acquired locally released permafrost‐N, even in sloping terrain, and that N was strongly retained in the plant–soil system through multiple years. At the same time, we also importantly demonstrate that permafrost‐N can be transported and taken up by plants up to 30 m downslope from the point‐of‐release. Especially NO3− was more easily redistributed vertically within the soil column compared to NH4+ and therefore potentially more accessible to plants. Specifically, plant species with fast N uptake capacity and deep‐soil foraging strategies may have competitive advantages for capitalising on deep‐soil released and topographically transported permafrost‐N (here exemplified by Equisetum arvense and Salix glauca). Nevertheless, even mosses gained access to permafrost‐N via vertical and lateral redistribution on the slope. Ultimately, the intricate balance between strong local N retention, downslope transport and plant species‐specific uptake strategies may contribute to shaping arctic vegetation change. Synthesis. Across spatially complex arctic ecosystems, arctic plants can take up permafrost‐released N both at the local point‐of‐release and at a considerable distance downslope. The potential for arctic plants to take advantage of topographically transported permafrost‐N could lead to long‐term and landscape‐scale changes in species composition, plant productivity, and ultimately carbon and climate feedbacks across the Arctic.

Shrub expansion in the Arctic may induce large\u2010scale carbon losses due to changes in plant\u2010soil interactions

BackgroundTall deciduous shrubs are increasing in range, size and cover across much of the Arctic, a process commonly assumed to increase carbon (C) storage. Major advances in remote sensing have increased our ability to monitor changes aboveground, improving quantification and understanding of arctic greening. However, the vast majority of C in the Arctic is stored in soils, where changes are more uncertain.ScopeWe present pilot data to argue that shrub expansion will cause changes in rhizosphere processes, including the development of new mycorrhizal associations that have the potential to promote soil C losses that substantially exceed C gains in plant biomass. However, current observations are limited in their spatial extent, and mechanistic understanding is still developing. Extending measurements across different regions and tundra types would greatly increase our ability to predict the biogeochemical consequences of arctic vegetation change, and we present a simple method that would allow such data to be collected.ConclusionsShrub expansion in the Arctic could promote substantial soil C losses that are unlikely to be offset by increases in plant biomass. However, confidence in this prediction is limited by a lack of information on how soil C stocks vary between contrasting Arctic vegetation communities; this needs to be addressed urgently.

High Arctic Vegetation Change Mediated by Hydrological Conditions

Increasing air temperatures are driving widespread changes to Arctic vegetation. In the high Arctic, these changes are patchy and the causes of heterogeneity are not well understood. In this study, we explore the determinants of high Arctic vegetation change over the last three decades on Banks Island, Northwest Territories. We used Landsat imagery (1984–2014) to map long-term trends in vegetation productivity and regional spatial data to investigate the relationships between trends in productivity and terrain position. Field sampling investigated vegetation community composition in different habitat types. Our analysis shows that vegetation productivity changes are substantial on Banks Island, where productivity has increased across about 80% of the study area. Rising productivity levels can be attributed to increasing biomass of the plant communities in both upland and lowland habitats. Our analysis also shows that the magnitude of greening is mediated by terrain characteristics related to soil moisture. Shifts in tundra vegetation will impact wildlife habitat quality, surface energy balance, permafrost dynamics, and the carbon cycle; additional research is needed to explore the effects of more productive vegetation communities on these processes in the high Arctic.

Complexity revealed in the greening of the Arctic

As the Arctic warms, vegetation is responding, and satellite measures indicate widespread greening at high latitudes. This ‘greening of the Arctic’ is among the world’s most important large-scale ecological responses to global climate change. However, a consensus is emerging that the underlying causes and future dynamics of so-called Arctic greening and browning trends are more complex, variable and inherently scale-dependent than previously thought. Here we summarize the complexities of observing and interpreting high-latitude greening to identify priorities for future research. Incorporating satellite and proximal remote sensing with in-situ data, while accounting for uncertainties and scale issues, will advance the study of past, present and future Arctic vegetation change.

Transferability and the effect of colour calibration during multi-image classification of Arctic vegetation change

Mapping changes in vegetation cover is essential for understanding the consequences of climate change on Arctic ecosystems. Classification of ultra-high spatial-resolution (UHR, < 1 cm) imagery can provide estimates of vegetation cover across space and time. The challenge of this approach is to assure comparability of classification across many images taken at different illumination conditions and locations. With warming, vegetation at higher elevation is expected to resemble current vegetation at lower elevation. To investigate the value of classification of UHR imagery for monitoring vegetation change, we collected visible and near-infrared images from 108 plots with hand-held cameras along an altitudinal gradient in Greenland and examined the classification accuracy of shrub cover on independent images (i.e. classification transferability). We implemented several models to examine if colour calibration improves transferability based on an in-image calibration target. The classifier was trained on different number of images to find the minimum training subset size. With a training set of ~ 20% of the images the overall accuracy levelled off at about 81% and 68% on the non-calibrated training and validation images, respectively. Colour calibration improved the accuracy on training images (1–4%) while it only improved the classifier transferability significantly for training sets < 20%. Linear calibration only based on the target’s grey series improved transferability most. Reasonable transferability of Arctic shrub cover classification can be obtained based only on spectral data and about 20% of all images. This is promising for vegetation monitoring through multi-image classification of UHR imagery acquired with hand-held cameras or Unmanned Aerial Systems.

Status and trends in Arctic vegetation: Evidence from experimental warming and long-term monitoring.

Changes in Arctic vegetation can have important implications for trophic interactions and ecosystem functioning leading to climate feedbacks. Plot-based vegetation surveys provide detailed insight into vegetation changes at sites around the Arctic and improve our ability to predict the impacts of environmental change on tundra ecosystems. Here, we review studies of changes in plant community composition and phenology from both long-term monitoring and warming experiments in Arctic environments. We find that Arctic plant communities and species are generally sensitive to warming, but trends over a period of time are heterogeneous and complex and do not always mirror expectations based on responses to experimental manipulations. Our findings highlight the need for more geographically widespread, integrated, and comprehensive monitoring efforts that can better resolve the interacting effects of warming and other local and regional ecological factors.

Footprints from the past: The influence of past human activities on vegetation and soil across five archaeological sites in Greenland

Climate change has irrevocable consequences for the otherwise well-preserved archaeological deposits in the Arctic. Vegetation changes are expected to impact archaeological sites, but currently the effects are poorly understood. In this article we investigate five archaeological sites and the surrounding natural areas along a climate gradient in Southwest Greenland in terms of vegetation types, above- and below-ground biomass, soil geochemistry and spectral properties. The investigations are based on data from site-sampling and optical remote sensing from an unmanned aerial vehicle (UAV) and satellites. Results show that the archaeological sites are dominated by graminoids with approximately two times more above- and below-ground biomass than the surrounding areas, where the vegetation is more heterogeneous. This difference is associated with a 2–6 times higher content of plant available phosphorus and water extractable nitrate and ammonium in the archaeological deposits compared to the surrounding soil. Furthermore, the vegetation at archaeological sites is less affected by the regional climate variations than the surrounding natural areas. This suggests that soil-vegetation interactions at archaeological sites are markedly different from the natural environment. Thus, the long-term vulnerability of buried archaeological remains cannot be assessed based on existing projections of Arctic vegetation change. Finally, the study demonstrates that vegetation within archaeological sites has distinct spectral properties, and there is a great potential for using satellite imagery for large scale vegetation monitoring of archaeological sites and for archaeological prospection in the Arctic.

Legacies of Historical Human Activities in Arctic Woody Plant Dynamics

Recent changes in arctic vegetation might not be driven by climate change alone. Legacies of human activities have received little attention as a contributing factor. We examine the extent to which traditional human activities (hunting, herding, fire, wood extraction, and agriculture) have had lasting effects on arctic woody plant communities and therefore might continue to affect biome-wide responses to climate change. Evidence suggests that legacies are likely to be evident across meters to hundreds of kilometers and for decades, centuries, and millennia. The evidence, however, is currently sparse, and we highlight the potential to develop systematic assessments through a circumarctic collaboratory consisting of a network of interdisciplinary field sites, standardized protocols, participatory research, and new approaches. We suggest that human activities should be brought into consideration to increase our understanding of arctic vegetation dynamics in general and to assess woody plant responses to climate change in particular.

Shrub encroachment in Arctic tundra: Betula nana effects on above- and belowground litter decomposition.

Rapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.

Phenology and species determine growing-season albedo increase at the altitudinal limit of shrub growth in the sub-Arctic.

Arctic warming is resulting in reduced snow cover and increased shrub growth, both of which have been associated with altered land surface-atmospheric feedback processes involving sensible heat flux, ground heat flux and biogeochemical cycling. Using field measurements, we show that two common Arctic shrub species (Betula glandulosa and Salix pulchra), which are largely responsible for shrub encroachment in tundra, differed markedly in albedo and that albedo of both species increased as growing season progressed when measured at their altitudinal limit. A moveable apparatus was used to repeatedly measure albedo at six precise spots during the summer of 2012, and resampled in 2013. Contrary to the generally accepted view of shrub-covered areas having low albedo in tundra, full-canopy prostrate B. glandulosa had almost the highest albedo of all surfaces measured during the peak of the growing season. The higher midsummer albedo is also evident in localized MODIS albedo aggregated from 2000 to 2013, which displays a similar increase in growing-season albedo. Using our field measurements, we show the ensemble summer increase in tundra albedo counteracts the generalized effect of earlier spring snow melt on surface energy balance by approximately 40%. This summer increase in albedo, when viewed in absolute values, is as large as the difference between the forest and tundra transition. These results indicate that near future (<50years) changes in growing-season albedo related to Arctic vegetation change are unlikely to be particularly large and might constitute a negative feedback to climate warming in certain circumstances. Future efforts to calculate energy budgets and a sensible heating feedback in the Arctic will require more detailed information about the relative abundance of different ground cover types, particularly shrub species and their respective growth forms and phenology.

Plants Duke It Out in a Warming Arctic

Global warming has significantly influenced tundra ecosystems; thawing permafrost, increasing fire frequency, and altering vegetation. Arctic plant communities are in flux, duking it out for dominance in a changing world. Alongside changes in temperature and precipitation, myriad environmental and biological factors may be influencing plant growth from local to regional scales. Researchers are collaborating to document and predict Arctic vegetation shifts, particularly the phenomenon of shrub encroachment, or shrubification. What do we know about vegetation change in a warming Arctic? Are shrubs taking over, and where are the knowledge gaps? Above latitudinal and altitudinal tree lines, shrubs like birch (Betula), willow (Salix), and alder (Alnus) are often the tallest plants. Their dominance appears to be increasing. Since a May 2001 Nature paper by Matthew Sturm and colleagues drew attention to this phenomenon, research has burgeoned, with shrub encroachment highlighted in the March 2014 Intergovernmental Panel on Climate Change report. Shrubs, explains Isla Myers-Smith, can restructure tundra ecosystems directly and indirectly by altering ecosystem function, creating feedback mechanisms that further advance their own growth and range. Species such as the dwarf birch (Betula nana) can take advantage of warmer temperatures and augmented nutrients by increasing the height and density of their canopy cover. Shrubification may reduce the growth potential of other species as shrub foliage limits competitors’ access to light and modifies snow depth, hydrology, nutrient exchange, carbon balance, albedo, and energy flux. These changes may make the Arctic more like it was millennia ago. “The Arctic was a lot shrubbier in the past,” says Myers-Smith, a Chancellor’s fellow in the Global Change Research Group at the University of Edinburgh. One line of evidence is lake sediments. Sedimentary charcoal (from fires) and pollen indicate that shrubs were a more dominant tundra vegetation in times past, between 6000 and 14,000 years ago. Growth ring analysis from long-lived Arctic shrubs shows that woody species respond to changing conditions. Growth varies among years, often correlating closely with climate variables, especially summer temperature, explains Myers-Smith. More evidence for shrubification comes from tundra experiments. Queen’s University’s Tara Zamin and her colleagues, for example, erected greenhouses during summers at Daring Lake, in Canada’s Northwest Territories, measuring plant responses after 6–8 years. Birch (Betula glandulosa) apical stems grew 2.5 times as much on experimentally warmed than on control patches (doi:10.1088/1748-9326/7/3/034027). “Deciduous shrubs are... able to respond to changes in their community,” says Zamin. In another experiment at the same site, with greenhouses as one treatment and nutrient supplementation as another, they found different effects with evergreen shrubs. Aboveground evergreen biomass increased by 66 percent on plots with greenhouse warming, but decreased by 70 percent on plots with high-level nutrient additions. Their results demonstrate that ongoing Arctic vegetation change is neither uniform nor simple (doi:10.1111/1365-2745.12237). The monitoring of “unmanipulated” (except by climate change) sites with satellite data, repeat photography, and intensively studied quadrats contributes another line of evidence. On Herschel Island, in the Canadian Arctic, Myers-Smith uses repeat photography, vegetation monitoring, and tree ring assessment, noting a dramatic shift. Compared with photographs dating back to the 1890s, when whalers occupied the site, shrubs have increased in size and height, growing from isolated patches to almost continuous cover. The monitoring is ongoing. In 2014, she was surprised to measure an individual with a height of 1 meter, a giant twice as tall as most others. “We named him Gunther, the tallest shrub on Herschel Island,” she says. Gunther grew 20 centimeters in one summer. Shrubs are expanding throughout the Arctic. But as Sarah Elmendorf and her coauthors point out, it is not happening uniformly (doi:10.1038/ nclimate1465). New results from the “shrub hub” research team indicate that climate-sensitive shrubification is a relatively consistent trend in the European Arctic, whereas the Canadian and US Arctic response is more variable. Early indications are that, in addition to temperature, water availability is crucial. At drier Arctic sites, shrubs appear less able to respond. Shifting herbivore communities are also being investigated, with early indications of winners and losers. Willoweating moose may be moving northward beyond the tree line, whereas lichenloving caribou, already in decline in some areas, may face less food as their preferred forage is crowded and shaded out by shrubs. In the Yukon, shrubification is changing the landscape for food, cover, and visibility. Ground squirrels, usually associated with open tundra, are behaving differently. They are “less willing to stay at foraging patches as long in shrubbier habitats,” explains Helen Wheeler, a postdoc at Aarhus University in Denmark. With many unanswered questions about the effects of shrubification on other plant and animal species, the wider tundra ecosystem, and climate feedbacks, the large international research collaborations are continuing. “There’s so much we don’t know,” says Wheeler. Myers-Smith agrees, adding that we may be in for some surprises.

Arctic greening can cause earlier seasonality of Arctic amplification

AbstractAs global temperatures rise, vegetation types will change, particularly in the northern high latitudes. Under a warming scenario, shrub and grasslands over the Arctic are expected to shift to boreal forests. This study compares the impact of such a change in Arctic vegetation type with that of CO2 doubling on the seasonality of Arctic warming. Even though vegetation is changed throughout the year, the effect of the surface albedo change is maximum in boreal summer when the incoming solar radiation is largest. Evapotranspiration changes are also maximized in the summer, when the photosynthesis rate is highest. As a result, when Arctic vegetation change is considered in addition to doubled CO2, Arctic amplification is maximized earlier in the annual cycle.

Caribou exclusion during a population low increases deciduous and evergreen shrub species biomass and nitrogen pools in low Arctic tundra

Summary Increased shrub growth has been observed across the Arctic with recent climate warming trends, whilst many populations of caribou and reindeer (Rangifer) have been in decline. Paradoxically, our current understanding of the impacts of caribou and reindeer in altering trajectories of Arctic vegetation change is almost entirely dependent on studies with relatively high density semi‐domesticated herds in Fennoscandia. With many wild herd populations across the Arctic presently at much lower densities, it is important to understand the impact of low intensity browsing on shrub biomass and the soil nutrient pools that fuel it. We used exclosures that have been in place for 5 years in mesic birch hummock tundra in the central Canadian low Arctic to investigate the impact of caribou exclusion on tundra shrub biomass, tissue chemistry and plant and soil nitrogen (N) pools. Over the study period, the migratory tundra caribou herd in the region declined from 25% to 7% of its previous population maximum. Caribou exclusion significantly enhanced the above‐ground biomass components of one deciduous shrub (Betula glandulosa) and two evergreen shrubs (Vaccinium vitis‐idaea and Rhododendron subarcticum). In particular, exclusion doubled B. glandulosa leaf biomass and increased V. vitis‐idaea old leaf biomass 1.7 times, with the strongest effects in evergreens present in tissues &gt; 1 year old, indicating a legacy of browsing from the earlier years of the experiment when the caribou population was higher. Meanwhile, Vaccinium uliginosum biomass and overall vascular plant diversity tended to decline with exclusion. Caribou exclusion increased B. glandulosa leaf N pools by 0.15 g N m−2 (equivalent to 12% of the total vascular plant community annual N requirement for apical growth). Altogether exclusion did not alter total above‐ground N pools, but rather led to a redistribution of shoot biomass and N, enhancing spatial variability in a key growth‐limiting resource for tundra plants. Synthesis. Excluding caribou during a population low resulted in ecologically significant changes in the distribution of plant above‐ground biomass and nitrogen, further increasing the dominance of the three most abundant shrubs. These findings demonstrate that, despite uncertainty in herd recovery, Rangifer browsing impacts to both deciduous and evergreen shrub species should be considered for more robust projections of Arctic vegetation change.

Changes in arctic vegetation on Jan Mayen Island over 19 and 80 years

AbstractQuestionsCan recent vegetation changes on an isolated, grazer‐free island be explained by recent climate change? Are observed changes consistent when focusing on two different time scales?LocationJan Mayen, an arctic volcanic island in the North Atlantic Ocean.MethodsWe re‐surveyed two botanical studies conducted 19 and 80 yr earlier to explore changes in species frequency, cover and co‐occurrence with other species. The observed changes were statistically evaluated using restricted permutation tests and were compared for the two time scales considered using Pearson correlation tests.ResultsTotal number of species did not significantly change over the two time periods considered. One species (Botrychium lunaria) was found new to the island. The dwarf‐shrub Salix herbacea and several graminoids increased in frequency or cover, or both, whereas species linked to snowbeds (e.g. Saxifraga spp., Oxyria digyna, Cerastium cerastoides) decreased. Changes over 19 yr were significantly correlated with 80‐yr changes considering species frequency, but not when comparing changes in cover and species co‐occurrences. Observed changes were more pronounced in the 80‐yr comparison.ConclusionsOur findings from the virtually grazer‐free island of Jan Mayen are in line with other studies on short‐ and long‐term vegetation changes in the Arctic and confirm that indirect effects of climate change (e.g. longer growing season, altered soil moisture conditions, increased nutrient availability) may be the main driver of the observed changes in arctic vegetation composition. However, whereas our study found the main trend to be similar over both time scales considered, discrepancies in the trends of some species suggest that long‐term changes are only partly predictable from short‐term studies.

Changes in Arctic vegetation amplify high-latitude warming through the greenhouse effect

Arctic climate is projected to change dramatically in the next 100 years and increases in temperature will likely lead to changes in the distribution and makeup of the Arctic biosphere. A largely deciduous ecosystem has been suggested as a possible landscape for future Arctic vegetation and is seen in paleo-records of warm times in the past. Here we use a global climate model with an interactive terrestrial biosphere to investigate the effects of adding deciduous trees on bare ground at high northern latitudes. We find that the top-of-atmosphere radiative imbalance from enhanced transpiration (associated with the expanded forest cover) is up to 1.5 times larger than the forcing due to albedo change from the forest. Furthermore, the greenhouse warming by additional water vapor melts sea-ice and triggers a positive feedback through changes in ocean albedo and evaporation. Land surface albedo change is considered to be the dominant mechanism by which trees directly modify climate at high-latitudes, but our findings suggest an additional mechanism through transpiration of water vapor and feedbacks from the ocean and sea-ice.

Relating Biomass and Leaf Area Index to Non-destructive Measurements in Order to Monitor Changes in Arctic Vegetation

This paper reports an alternative method for seasonal and long-term monitoring of biomass and the leaf area index (LAI) at Arctic tundra sites. Information related to the historical and projected change in abundance and distribution of biomass and LAI is required to address numerous environmental and resource management issues. Observations of earth from satellites could potentially be used to derive seasonal and long-term changes in biomass and the LAI. To realize this potential, seasonal and long-term ground monitoring data for validation are essential; however, the conventional destructive sampling method for measuring biomass and the LAI does not allow repetitive measurements at the same plots and thus is not suitable for monitoring change over time. Alternative methods, such as sampling nearby similar plots, can be laborious and easily subject to large sampling errors, especially in Arctic tundra sites with low vegetation cover. In this study, we developed a practical method for relating non-destructive measurements (percent cover and mean height) to biomass and the LAI for 13 major Arctic plant groups, or seven plant functional types, on the basis of measurements at 196 plots across Canada’s Arctic tundra ecosystems. Using the method at the plant group level to estimate plot total vascular aboveground biomass, foliage biomass, and LAI, we had r2 = 0.91–0.95 and relative mean absolute error of 25–29%. By this method, one could monitor seasonal and long-term changes in biomass and the LAI through repeated, non-destructive observations of percent cover and mean height at the same permanent plots.

Summer Differences among Arctic Ecosystems in Regional Climate Forcing

Biome differences in surface energy balance strongly affect climate. However, arctic vegetation is considered sufficiently uniform that only a single arctic land surface type is generally used in climate models. Field measurements in northern Alaska show large differences among arctic ecosystem types in summer energy absorption and partitioning. Simulations with the Arctic Regional Climate System Model demonstrate that these variations in land surface parameters and ecological processes cause variation in surface fluxes that is sufficiently large to affect the regional climate. Plausible changes in arctic vegetation in response to high-latitude warming would feed back positively to local summer warming. This local warming could extend into the boreal zone. Climate feedbacks that operate during the growing season are particularly likely to impact vegetation and ecosystem properties. These field and model results suggest that vegetation changes within a biome could be climatically important and warrant consideration in regional climate modeling.