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Vegetation composition shifts, and in particular, shrub expansion across the Arctic tundra are some of the most important and widely observed responses of high-latitude ecosystems to rapid climate warming. These changes in vegetation potentially alter ecosystem carbon balances by affecting a complex set of soil–plant–atmosphere interactions. In this review, we synthesize the literature on (a) observed shrub expansion, (b) key climatic and environmental controls and mechanisms that affect shrub expansion, (c) impacts of shrub expansion on ecosystem carbon balance, and (d) research gaps and future directions to improve process representations in land models. A broad range of evidence, including in-situ observations, warming experiments, and remotely sensed vegetation indices have shown increases in growth and abundance of woody plants, particularly tall deciduous shrubs, and advancing shrublines across the circumpolar Arctic. This recent shrub expansion is affected by several interacting factors including climate warming, accelerated nutrient cycling, changing disturbance regimes, and local variation in topography and hydrology. Under warmer conditions, tall deciduous shrubs can be more competitive than other plant functional types in tundra ecosystems because of their taller maximum canopy heights and often dense canopy structure. Competitive abilities of tall deciduous shrubs vs herbaceous plants are also controlled by variation in traits that affect carbon and nutrient investments and retention strategies in leaves, stems, and roots. Overall, shrub expansion may affect tundra carbon balances by enhancing ecosystem carbon uptake and altering ecosystem respiration, and through complex feedback mechanisms that affect snowpack dynamics, permafrost degradation, surface energy balance, and litter inputs. Observed and projected tall deciduous shrub expansion and the subsequent effects on surface energy and carbon balances may alter feedbacks to the climate system. Land models, including those integrated in Earth System Models, need to account for differences in plant traits that control competitive interactions to accurately predict decadal- to centennial-scale tundra vegetation and carbon dynamics.

Abstract Plant growth and distribution in high-latitude tundra ecosystems is strongly limited by nutrient availability and is critical for quantifying centennial-scale carbon-climate interactions. However, land model representations of plant–nutrient interactions are uncertain, leading to poor comparisons with high-latitude observations. Although it has been recognized for decades in the observational community that plants continue to acquire nutrients well past when aboveground activity has ceased, most large-scale land models ignore this process. Here we address the role tundra plant nutrient acquisition during the non-growing season (NGS) has on centennial-scale vegetation growth and dynamics, with a focus on shrub expansion. We apply a well-tested mechanistic model of coupled plant, microbial, hydrological, and thermal dynamics that explicitly represents nutrient acquisition based on plant and microbial traits, thereby allowing a prognostic assessment of NGS nutrient uptake. We first show that the model accurately represents observed seasonality of NGS plant nutrient uptake in a northern Alaskan tundra site. Applying the model across the North America tundra indicates that NGS nutrient uptake is consistent with observations and ranges between ∼5% and 50% of annual uptake, with large spatial variability and dependence on plant functional type. We show that NGS plant nutrient acquisition strongly enhances modeled 21 st century tundra shrub growth and expansion rates. Our results suggest that without NGS nutrient uptake, total shrub aboveground dominance would be ∼50% lower, limited primarily by their inability to grow tall enough to maximize their inherent capacity for light competition. Evergreen shrubs would be more strongly affected because of their relatively lower capacity for nutrient remobilization and acquisition compared to deciduous shrubs. Our results highlight the importance of NGS plant and soil processes on high-latitude biogeochemistry and vegetation dynamics and motivate new observations and model structures to represent these dynamics.

The expansion of shrubs and trees across high-latitude ecosystems is one of the most dramatic ecological manifestations of climate change. Most of the work quantifying these changes has been done in small areas and over relatively recent time scales. These land-cover transitions are highly spatially variable, and we have limited understanding of the factors underlying this variation. We use repeat photography to generate a data set of land-cover changes in Denali National Park and Preserve, Alaska, stretching back a century and spanning a range of edaphic, topographic, and climatic conditions. Most land-cover classes were quite stable, with low probabilities of transitioning to other land-cover types. The advance of woody vegetation into low-stature tundra, and the spread of conifer trees into shrub-dominated areas, were both more likely at low elevations and in areas without permafrost. Permafrost also reduced the likelihood of herbaceous vegetation transitioning to woody cover. Exceptions to the general trend of relative stability included nearly all (96%) of the broadleaf forest–dominated areas being invaded by conifers, an expected successional trajectory, and many open gravel river bars (17.8%) transitioning to thick shrubs. These floodplain areas were distinctly not at equilibrium, as only 0.1% of shrub-dominated areas converted to gravel. Warming temperatures in coming decades and concomitant declines in the extent of permafrost are predicted to enhance the spread of woody vegetation in Denali further, but only by ∼3%. Land-cover transitions, notably the rapid advance of trees and shrubs observed in other studies, could be less likely and more spatially heterogeneous here than in other high-latitude systems.

The expansion of shrubs in the Arctic tundra fundamentally modifies land-atmosphere interactions. However, it remains unclear how shrub distribution and expansion differ across key species due to challenges with discriminating tundra plant species at regional scales. Here, we combined multi-scale, multi-platform remote sensing and in situ trait measurements to elucidate the distribution patterns and primary controls of two representative deciduous-tall-shrub (DTS) genera, Alnus and Salix , in low-Arctic tundra. We show that topographic features were a key control on DTSs, creating heterogeneous, but predictable distributions of Alnus and Salix fractional cover (fCover). Alnus was more tolerant of elevation and slope and was found on hilly uplands (slope >10°) within a specific elevational band (200–400 m above sea level [MSL]). In contrast, Salix occurred at lower elevations (50–300 m MSL) on gentler slopes (3-10°) and required adequate soil moisture associated with its profligate water use. We also show that niche differentiation between Alnus and Salix changed with patch size, where larger patches were more specialized in resource requirements than individual plants of Alnus and Salix . To understand what constrains the growth of DTSs at locations with low fCover, we developed environmental limiting factor models, which showed that topography limits the upper bound of Alnus and Salix fCover in 69.2% and 48.7% of the landscape, respectively. These findings highlight a critical need to better understand and represent topography-controlled processes and functional traits in regulating shrub distribution, as well as a need for more detailed species classification to predict shrubification in the Arctic.

The eastern Canadian Subarctic and Arctic are experiencing significant environmental change with widespread implications for the people, plants, and animals living there. In this study, we integrate 10 years of research at the Nakvak Brook watershed in Torngat Mountains National Park of Canada, northern Labrador, to assess the sensitivity of ecological and geomorphological systems to regional climate warming. A time series of the Normalized Difference Vegetation Index indicates that the area has undergone a significant greening trend over the past four decades. Analyses of shrub cross sections suggest that greening has been caused by a combination of rapid establishment and growth that began in the late 1990’s and coincided with warmer growing season temperatures. Recent (2010–2015) vegetation change has been subtle and heavily moderated by soil moisture status. Plant canopy height is greater in wet areas and has an insulating effect on ground surface temperatures during the winter, a consequence of snow trapping by shrub canopies. Observations of subsurface conditions indicate that the study site is best characterized as having discontinuous near-surface permafrost. The importance of subsurface conditions for above-ground vegetation depends on the geomorphological context, with plants in wet areas underlain by fine materials being the most likely to be growth-limited by permafrost, thus being potential hot-spots for future change. With the expectation of sustained climate change, loss of adjacent sea ice, and proximity to the forest-tundra ecotone, it is likely that the Torngat Mountains will continue to be an area of rapid environmental change in the coming decades.

Vegetation is responding to climate change, which is especially prominent in the Arctic. Vegetation change is manifest in different ways and varies regionally, depending on the characteristics of the investigated area. Although vegetation in some Arctic areas has been thoroughly investigated, central Chukotka (NE Siberia) with its highly diverse vegetation, mountainous landscape and deciduous needle-leaf treeline remains poorly explored, despite showing strong greening in remote-sensing products. Here we quantify recent vegetation compositional changes in central Chukotka over 15 years between 2000/2001/2002 and 2016/2017. We numerically related field-derived information on foliage projective cover (percentage cover) of different plant taxa from 52 vegetation plots to remote-sensing derived (Landsat) spectral indices (Normalised Difference Vegetation Index (NDVI), Normalised Difference Water Index (NDWI) and Normalised Difference Snow Index (NDSI)) using constrained ordination. Clustering of ordination scores resulted in four land-cover classes: (1) larch closed-canopy forest, (2) forest tundra and shrub tundra, (3) graminoid tundra and (4) prostrate herb tundra and barren areas. We produced land-cover maps for early (2000, 2001 or 2002) and recent (2016 or 2017) time-slices for four focus regions along the tundra-taiga vegetation gradient. Transition from graminoid tundra to forest tundra and shrub tundra is interpreted as shrubification and amounts to 20% area increase in the tundra-taiga zone and 40% area increase in the northern taiga. Major contributors of shrubification are alder, dwarf birch and some species of the heather family. Land-cover change from the forest tundra and shrub tundra class to the larch closed-canopy forest class is interpreted as tree infilling and is notable in the northern taiga. We find almost no land-cover changes in the present treeless tundra.

In the tundra, woody plants are dispersing towards higher latitudes and altitudes due to increasingly favourable climatic conditions. The coverage and height of woody plants are increasing, which may influence the soils of the tundra ecosystem. Here, we use structural equation modelling to analyse 171 study plots and to examine if the coverage and height of woody plants affect the growing-season topsoil moisture and temperature (< 10 cm) as well as soil organic carbon stocks (< 80 cm). In our study setting, we consider the hierarchy of the ecosystem by controlling for other factors, such as topography, wintertime snow depth and the overall plant coverage that potentially influence woody plants and soil properties in this dwarf shrub-dominated landscape in northern Fennoscandia. We found strong links from topography to both vegetation and soil. Further, we found that woody plants influence multiple soil properties: the dominance of woody plants inversely correlated with soil moisture, soil temperature, and soil organic carbon stocks (standardised regression coefficients = - 0.39; - 0.22; - 0.34, respectively), even when controlling for other landscape features. Our results indicate that the dominance of dwarf shrubs may lead to soils that are drier, colder, and contain less organic carbon. Thus, there are multiple mechanisms through which woody plants may influence tundra soils.

Question: In recent decades, high-latitude climate has shown regionally variable trends towards warmer and moister conditions. These changes have been predicted to cause afforestation or shrubification of open tundra, increases of warmth-demanding southern species and plant groups favoured by increased moisture, and decline of species and habitats that are dependent on snow cover. In this study, we explore temporal changes in northern tundra upland plant communities along regional gradients and in local habitats. We ask how vegetation changes are linked with long-term trends in regional climate and grazing pressure. Location: Northern Europe. Methods: In 2013–2014, we resurveyed a total of 108 vegetation plots on wind-exposed and snow-protected tundra habitats in three subareas along a bioclimatic gradient from the northern boreal to the arctic zone. Vegetation plots were originally sampled in 1964–1967. We related observed vegetation changes to changes in temperature, precipitation and grazing pressure, which all showed regionally variable increases over the study period. Results: We found a significant increase of the evergreen dwarf shrub Empetrum nigrum subsp. hermaphroditum in snow-protected communities and a prominent decrease of lichens throughout the study area. No evidence for extensive tree or larger shrub (Betula spp., Salix spp. or Juniperus communis) encroachment despite climatic warming trends was found. Among studied communities, most pronounced changes in vegetation were observed in snow-protected boreal heaths on small isolated uplands, where community composition showed low resemblance to the original composition described decades ago. Changes in plant communities correlated with changes in summer and winter temperatures, summer precipitation and reindeer grazing pressure, yet correlations varied depending on region and habitat. Conclusions: Northern tundra uplands vary in their resistance to on-going climate change and reindeer grazing. Isolated treeless heaths of boreal forest–tundra ecotone appear least resistant to climate change and have already shifted towards new community states.

Plant growth and distribution in high-latitude tundra ecosystems is strongly limited by nutrient availability and is critical for quantifying centennial-scale carbon-climate interactions. However, land model representations of plant–nutrient interactions are uncertain, leading to poor comparisons with high-latitude observations. Although it has been recognized for decades in the observational community that plants continue to acquire nutrients well past when aboveground activity has ceased, most large-scale land models ignore this process. Here we address the role tundra plant nutrient acquisition during the non-growing season (NGS) has on centennial-scale vegetation growth and dynamics, with a focus on shrub expansion. We apply a well-tested mechanistic model of coupled plant, microbial, hydrological, and thermal dynamics that explicitly represents nutrient acquisition based on plant and microbial traits, thereby allowing a prognostic assessment of NGS nutrient uptake. We first show that the model accurately represents observed seasonality of NGS plant nutrient uptake in a northern Alaskan tundra site. Applying the model across the North America tundra indicates that NGS nutrient uptake is consistent with observations and ranges between ∼5% and 50% of annual uptake, with large spatial variability and dependence on plant functional type. We show that NGS plant nutrient acquisition strongly enhances modeled 21 st century tundra shrub growth and expansion rates. Our results suggest that without NGS nutrient uptake, total shrub aboveground dominance would be ∼50% lower, limited primarily by their inability to grow tall enough to maximize their inherent capacity for light competition. Evergreen shrubs would be more strongly affected because of their relatively lower capacity for nutrient remobilization and acquisition compared to deciduous shrubs. Our results highlight the importance of NGS plant and soil processes on high-latitude biogeochemistry and vegetation dynamics and motivate new observations and model structures to represent these dynamics.

To investigate the effects of shrubification—the global phenomena of an increase of shrubs in grasslands—on C, N, and P cycles, the changes in soil organic C, N, and P mineralization in an alpine meadow were compared across five plant communities: grasses and four shrub species–dominated patches. The nutrient content and stoichiometry (C:N:P) of leaves, litter, microbial biomass, and soil organic matter (SOM) were analyzed during vegetation season. Net rates of N and P mineralization were measured in situ in the top 20 cm of soil throughout the growing season, and organic C mineralization was determined under controlled conditions. Microbial C:N, C:P, and N:P ratios in the top 20 cm generally decreased with increasing plant size (height combined with crown diameter), associated with greater input of litter with lower C:nutrient ratios under shrubs. The net N and P mineralization rates in soil under shrubs were about 3- to sevenfold and 4- to 15-fold faster, respectively, compared with those under grasses. The increase in organic C mineralization under shrubs compared with that under grasses was much smaller than the increase of N or P mineralization under shrubs. This indicates faster turnover of nutrients than C leading to decoupling of organic C and nutrient mineralization across plant communities by shrubification. The C:N, C:P, and N:P ratios of organic pools mineralized in soil decreased with increasing plant size, but increased with respective microbial C:N, C:P, and N:P ratios across plant communities. This indicates that specific SOM pools were mineralized depending on plant communities and microbial stoichiometry in soil. Consequently, the decoupling of organic C and nutrient mineralization across plant communities is driven by microbial stoichiometry and increases by shrubification.