Shrub Expansion Research Articles

How will snow retention and shading from Arctic shrub expansion affect caribou food resources?

ABSTRACT Increased snow cover and attenuation of light due to shrub expansion can lead to changes in the quantity and nutritional quality of food resources for migratory caribou (Rangifer tarandus). To determine how vegetation is affected by snow accumulation and shade, we conducted an experiment that simulated the light and snow conditions created by increased shrub cover at Deception Bay (Nunavik, Canada), within the summer range of the Rivière-aux-Feuilles caribou herd. We estimated the foliar biomass of two major components of the caribou diet (Betula glandulosa and Carex spp.). We also harvested foliar tissue to conduct chemical composition analyses (nitrogen, fibre, total phenolics). Experimental light attenuation was found to increase the nitrogen concentrations in B. glandulosa and Carex spp. throughout the growing season. Phenolic concentration in B. glandulosa decreased in early summer but was otherwise consistent in response to light attenuation and increased snow cover. Increased snow cover combined with ambient light had a positive effect on the foliar biomass of B. glandulosa. Increased snow cover and shade caused by shrub densification may therefore increase forage quantity and quality for caribou. We suggest investigating the effects of shrub expansion on other components of the caribou diet, such as lichens and forbs.

Seasonal thaw and landscape position determine foliar functional traits and whole-plant water use in tall shrubs on the low arctic tundra.

Climate warming is driving tundra shrub expansion with implications for ecosystem function and regional climate. Understanding associations between shrub ecophysiological function, distribution and environment is necessary for predicting consequences of expansion. We evaluated the role of topographic gradients on upland shrub productivity to understand potential constraints on shrub expansion. At a low arctic tundra site near Inuvik, Northwest Territories, Canada, we measured sap flow, stem water potential and productivity-related functional traits in green alder, and environmental predictors (water and nutrient availability and seasonal thaw depth) across a toposequence in alder patches. Seasonal thaw reduced stem sap flow whereas topographic position predicted stem water potential and productivity-related functional traits. Upslope shrubs were more water-limited than those downslope. Shrubs in drainage channels had traits associated with greater productivity than those on the tops of slopes. The effect of thaw depth on sap flow has implications for seasonal water-use patterns and warming impacts on tundra ecohydrology. Topographic variation in functional traits corresponds with observed spatial patterns of tundra shrub expansion along floodplains and concave hillslopes rather than in upland areas. Green alder is expanding rapidly across the low arctic tundra in northwestern North America; thus, anticipating the implications of its expansion is essential for predicting tundra function.

Shrub encroachment balances soil organic carbon pool by increasing carbon recalcitrance in a temperate herbaceous wetland

In recent decades, temperate herbaceous wetlands have been widely experiencing shrub encroachment. By now, little has been known about whether such alteration in plant community would generate updated soil organic carbon (SOC) conservation strategies, and thus alter SOC dynamics. To assess the temporal dynamics of SOC stock and fractions after shrub encroachment into temperate herbaceous wetlands, we measured floor litter mass, fine root biomass, and SOC stock and fractions (i.e. labile C pool I, labile C pool II, and recalcitrant C pool) across the 0–50 cm soil profile in the open wetlands and shrub islands (Betula fruticose and Salix floderusii) with different basal diameter sizes (small, 10 ~ 15 cm; medium, 15 ~ 20 cm; and large, 20 ~ 30 cm) in the Sanjiang Plain of Northeast China. Regardless of shrub species, SOC stock initially decreased after shrub expansion, but gradually recovered to the initial level of herbaceous wetlands. Moreover, SOC stock correlated positively with fine root biomass, but showed no significant relationship with floor litter mass. The responses of SOC fractions to shrub expansion mainly occurred in the surface soils (0–15 cm), despite a slight change in recalcitrant C pool in 15–50 cm layer. In the surface soils, labile C pool decreased substantially, whereas recalcitrant C pool and recalcitrant index of SOC elevated with increasing shrub sizes. Shrub encroachment can recover SOC pool and enhance SOC recalcitrance in temperate herbaceous wetlands, and altered fine root biomass would account for shrub encroachment-induced SOC dynamics.

Overstory removal and biological legacies influence long-term forest management outcomes on introduced species and native shrubs

Forests in the western United States are actively managed to increase resilience to fire and extract timber resources. Disturbance from these management activities, however, may reduce forest resilience to invasion and promote shrub expansion, which could compromise treatment efficacy in the long-term. To assess mid- and long-term impacts of forest management practices, we coupled a field experiment with a chronosequence of patch cuts (~0.5 ha cuts) located at Blodgett Forest Research Station in the north-central Sierra Nevada. The field experiment, which was part of the national Fire and Fire Surrogate (FFS) study, compared three fuel treatments to control plots 12–13 years post-treatment. Fuel treatments included mechanical thinning, prescribed fire (that burned twice), and mechanical thinning plus fire. In contrast, the chronosequence assessed the impacts of patch cuts on vegetation across 33 years. The FFS fuel treatments led to increases in introduced species richness initially and in the mid-term. Introduced species richness and cover in the fuel treatments, however, were considerably lower than in patch cuts, which reached over 100% cover six years post-harvesting. In addition, while fuel treatments initially reduced shrub cover and richness, shrubs increased significantly in all fuel treatments in the mid-term. Understory vegetation dynamics in the fuel treatments were influenced by pretreatment flora richness and cover. In contrast, overstory removal was likely the strongest driver of high invasion in patch cuts within the first ten years, suggesting that severe disturbance (e.g., 100% removal of the overstory) may overwhelm biological legacies. Finally, overstory canopy was significantly and negatively associated with introduced species in the fuel treatments and the patch cuts, highlighting that canopy gap dynamics likely influenced invasion in this system. Our study underscored that while pretreatment species composition was a strong determinant of invasion in fuel treatments, management strategies that sustain overstory canopy cover may reduce growth and recruitment of introduced species and native shrubs.

Amino acids dominate diffusive nitrogen fluxes across soil depths in acidic tussock tundra.

Organic nitrogen (N) is abundant in soils, but early conceptual frameworks considered it nonessential for plant growth. It is now well recognised that plants have the potential to take up organic N. However, it is still unclear whether plants supplement their N requirements by taking up organic N insitu: at what rate is organic N diffusing towards roots and are plants taking it up? We combined microdialysis with live-root uptake experiments to measure amino acid speciation and diffusion rates towards roots of Eriophorum vaginatum. Amino acid diffusion rates (321ngNcm-2 h-1 ) were c. 3×higher than those for inorganic N. Positively charged amino acids made up 68% of the N diffusing through soils compared with neutral and negatively charged amino acids. Live-root uptake experiments confirmed that amino acids are taken up by plants (up to 1µgNg-1 min-1 potential net uptake). Amino acids must be considered when forecasting plant-available N, especially when they dominate the N supply, and when acidity favours proteolysis over net N mineralisation. Determining amino acid production pathways and supply rates will become increasingly important in projecting the extent and consequences of shrub expansion, especially considering the higher C:N ratio of plants relative to soil.

Shrub decline and expansion of wetland vegetation revealed by very high resolution land cover change detection in the Siberian lowland tundra

Vegetation change, permafrost degradation and their interactions affect greenhouse gas fluxes, hydrology and surface energy balance in Arctic ecosystems. The Arctic shows an overall “greening” trend (i.e. increased plant biomass and productivity) attributed to expansion of shrub vegetation. However, Arctic shrub dynamics show strong spatial variability and locally “browning” may be observed. Mechanistic understanding of greening and browning trends is necessary to accurately assess the response of Arctic vegetation to a changing climate. In this context, the Siberian Arctic is an understudied region. Between 2010 and 2019, increased browning (as derived from the MODIS Enhanced Vegetation Index) was observed in the Eastern Siberian Indigirka Lowlands. To support interpretation of local greening and browning dynamics, we quantified changes in land cover and transition probabilities in a representative tundra site in the Indigirka Lowlands using a timeseries of three very high resolution (VHR) (0.5 m) satellite images acquired between 2010 and 2019. Using spatiotemporal Potts model regularization, we substantially reduced classification errors related to optical and phenological inconsistencies in the image material. VHR images show that recent browning was associated with declines in shrub, lichen and tussock vegetation and increases in open water, sedge and especially Sphagnum vegetation. Observed formation and expansion of small open water bodies in shrub dominated vegetation suggests abrupt thaw of ice-rich permafrost. Transitions from open water to sedge and Sphagnum, indicate aquatic succession upon disturbance. The overall shift towards open water and wetland vegetation suggests a wetting trend, likely associated with permafrost degradation. Landsat data confirmed widespread expansion of surface water throughout the Indigirka Lowlands. However, the increase in the area of small water bodies observed in VHR data was not visible in Landsat-derived surface water data, which suggests that VHR data is essential for early detection of small-scale disturbances and associated vegetation change in permafrost ecosystems.

Vegetation responses to 26 years of warming at Latnjajaure Field Station, northern Sweden

Climate change is rapidly warming high latitude and high elevation regions influencing plant community composition. Changes in vegetation composition have motivated the coordination of ecological monitoring networks across the Arctic, including the International Tundra Experiment. We have established a long-term passive warming experiment using open-top chambers, which includes five distinct plant communities (Dry Heath; Tussock Tundra; and Dry, Mesic, and Wet Meadow). We measured changes in plant community composition based on relative abundance differences over 26 years. In addition, relative abundance changes in response to fertilization and warming treatments were analyzed based on a seven-year Community-Level Interaction Program experiment. The communities had distinct soil moisture conditions, leading to community-specific responses of the plant growth forms (deciduous shrubs, evergreen shrubs, forbs, and graminoids). Warming significantly affected growth forms, but the direction of the response was not consistent across the communities. Evidence of shrub expansion was found in nearly all communities, with soil moisture determining whether it was driven by deciduous or evergreen shrubs. Graminoids increased in relative abundance in the Dry Meadow due to warming. Growth form responses to warming are likely mediated by edaphic characteristics of the communities and their interactions with climate.

Anthropogenic disturbance modifies long‐term changes of boreal mountain vegetation under contemporary climate warming

AbstractAimsAccelerating high‐latitude climate warming drives shrub expansion in open landscapes and alters species distributions and compositions of plant communities. Simultaneously, various land use practices cause disturbance to the vegetation. However, not much documentation exists on how long‐term intensive land use disturbance modifies high‐latitude vegetation under climate warming. Here, we study how the composition of boreal mountain plant communities has changed during three decades in response to heavy land use disturbance, related to ski resort construction and management, and how these changes compare to those observed in adjacent less disturbed communities.LocationIso‐Syöte, Finland.MethodsWe resurveyed vegetation along four elevational gradients (240–426 m a.s.l.) on a boreal mountain in 2013–14. After the original study in 1980, half of the gradients were subjected to continuous heavy land use disturbance, while the other half remained only slightly disturbed. All the gradients experienced a similar amount of macroclimatic warming over time. We analysed temporal changes in plant group covers, species richness and species’ elevational range means in relation to disturbance levels and elevation.ResultsUnder slight disturbance, the cover of shrubs increased on the originally open upper slopes and elevational range means of several species shifted upward. In contrast, heavy disturbance resulted in a uniform, yet modest, shrub cover increase along the whole elevational gradient and promoted both up‐ and downward shifts of species. Bryophyte cover decreased considerably over time, regardless of the disturbance level. Species richness increased throughout, yet more under heavy disturbance.ConclusionsLong‐term changes in boreal mountain vegetation are substantially influenced by heavy land use disturbance compared to less disturbed sites where the vegetation changes are more comparable to those expected under a warmer climate. Therefore, along with the climatic effects, land use effects on vegetation are important to consider in management actions and in projections of future vegetation.

Increasing shrub damage by invertebrate herbivores in the warming and drying tundra of West Greenland.

Rapid warming is predicted to increase insect herbivory across the tundra biome, yet how this will impact the community and ecosystem dynamics remains poorly understood. Increasing background invertebrate herbivory could impede Arctic greening, by serving as a top-down control on tundra vegetation. Many tundra ecosystems are also susceptible to severe insect herbivory outbreaks which can have lasting effects on vegetation communities. To explore how tundra-insect herbivore systems respond to warming, we measured shrub traits and foliar herbivory damage at 16 sites along a landscape gradient in western Greenland. Here we show that shrub foliar insect herbivory damage on two dominant deciduous shrubs, Salix glauca and Betula nana, was positively correlated with increasing temperatures throughout the first half of the 2017 growing season. We found that the majority of insect herbivory damage occurred in July, which was outside the period of rapid leaf expansion that occurred throughout most of June. Defoliators caused the most foliar damage in both shrub species. Additionally, insect herbivores removed a larger proportion of B. nana leaf biomass in warmer sites, which is due to a combination of increased foliar herbivory with a coinciding decline in foliar biomass. These results suggest that the effects of rising temperatures on both insect herbivores and host species are important to consider when predicting the trajectory of Arctic tundra shrub expansion.

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.

Shrub changes with proximity to anthropogenic disturbance in boreal wetlands determined using bi-temporal airborne lidar in the Oil Sands Region, Alberta Canada.

In this study, we used bi-temporal airborne lidar data to compare changes in vegetation height proximal to anthropogenic disturbances in the Oil Sands Region of Alberta, Canada. We hypothesize that relatively low-impact disturbances such as seismic lines will increase the fragmentation of wetlands, resulting in shrub growth. Bi-temporal lidar data collected circa 2008 and 2018 were used to identify correspondence between the density of anthropogenic disturbances, wetland shape complexity and changes in vegetation height within >1800 wetlands near Fort McKay, Alberta, Canada. We found that up to 50% of wetlands were disturbed by anthropogenic disturbance in some parts of the region, with the highest proportional disturbance occurring within fens. Areas of dense anthropogenic disturbance in bogs resulted in increased growth and expansion of shrubs, while we found the opposite to occur in fens and swamps during the 10-year period. Up to 30% of bogs had increased shrubification, while shrub changes in fens and swamps varied depending on density of disturbance and did not necessarily correspond with shrub growth. As wetland shapes became increasingly elongated, the prevalence of shrubs declined between the two time periods, which may be associated with hydrological drivers (e.g. elongated may indicate surface and ground-water discharge influences). The results of this study indicate that linear disturbances such as seismic lines, considered to have relatively minimal impacts on ecosystems, can impact proximal wetland shape, fragmentation and vegetation community changes, especially in bogs.

Ancient plant DNA reveals High Arctic greening during the Last Interglacial.

Summer warming is driving a greening trend across the Arctic, with the potential for large-scale amplification of climate change due to vegetation-related feedbacks [Pearson et al., Nat. Clim. Chang. (3), 673-677 (2013)]. Because observational records are sparse and temporally limited, past episodes of Arctic warming can help elucidate the magnitude of vegetation response to temperature change. The Last Interglacial ([LIG], 129,000 to 116,000 y ago) was the most recent episode of Arctic warming on par with predicted 21st century temperature change [Otto-Bliesner et al., Philos. Trans. A Math. Phys. Eng. Sci. (371), 20130097 (2013) and Post et al., SciAdv (5), eaaw9883 (2019)]. However, high-latitude terrestrial records from this period are rare, so LIG vegetation distributions are incompletely known. Pollen-based vegetation reconstructions can be biased by long-distance pollen transport, further obscuring the paleoenvironmental record. Here, we present a LIG vegetation record based on ancient DNA in lake sediment and compare it with fossil pollen. Comprehensive plant community reconstructions through the last and current interglacial (the Holocene) on Baffin Island, Arctic Canada, reveal coherent climate-driven community shifts across both interglacials. Peak LIG warmth featured a ∼400-km northward range shift of dwarf birch, a key woody shrub that is again expanding northward. Greening of the High Arctic-documented here by multiple proxies-likely represented a strong positive feedback on high-latitude LIG warming. Authenticated ancient DNA from this lake sediment also extends the useful preservation window for the technique and highlights the utility of combining traditional and molecular approaches for gleaning paleoenvironmental insights to better anticipate a warmer future.

Differential responses of litter decomposition in the air and on the soil surface to shrub encroachment in a graminoid-dominated temperate wetland

Graminoid-dominated wetlands have been subjected to widespread shrub encroachment, yet the effect of this shift in species composition on litter decomposition remains unclear, especially in the standing-dead stage. We collected labile (Deyeuxia angustifolia) and recalcitrant (Carex schmidtii) graminoid leaf litter from a freshwater wetland in northeast China, and used the litterbag method to characterize litter ash-free dry mass (AFDM) loss and nitrogen (N) release in the air and on the soil surface in open wetlands and two shrub islands (Salix floderusii producing phenol-poor litter and Betula fruticosa producing phenol-rich litter) over 360 days of decomposition. Litter decomposition in the air and on the soil surface responded differentially to shrub expansion. In the air, AFDM loss and N release for both labile and recalcitrant litter were often lower in shrub islands than in open wetlands. On the soil surface, labile litter decomposition was decelerated in the presence of shrubs with phenol-rich litter, but accelerated in the presence of shrubs with phenol-poor litter. Despite the absence of significant differences in N release, recalcitrant litter AFDM loss was lower in shrub islands than in open wetlands after 360 days of decomposition on the soil surface. Shrub encroachment retards litter decomposition in the air, but the effects on litter decomposition on the soil surface vary with shrub type and litter degradability in graminoid-dominated wetlands. Moreover, these findings emphasize that standing litter decomposition should be considered to enhance our understanding of shrub encroachment effects on litter decomposition in these wetlands.

Topographical Controls on Hillslope‐Scale Hydrology Drive Shrub Distributions on the Seward Peninsula, Alaska

AbstractObservations indicate shrubs are expanding across the Arctic tundra, mainly on hillslopes and primarily in response to climate warming. However, the impact topography exerts on hydrology, nutrient dynamics, and plant growth can make untangling the mechanisms behind shrub expansion difficult. We examined the role topography plays in determining shrub expansion by applying a coupled transect version of a mechanistic ecosystem model (ecosys) in a tundra hillslope site in the Seward Peninsula, Alaska. Modeled biomass of the dominant plant functional types agreed well with field measurements (R2 = 0.89) and accurately represented shrub expansion over the past 30 years inferred from satellite observations. In the well‐drained crest position, canopy water potential and plant nitrogen (N) uptake was modeled to be low from plant and microbial water stress. Intermediate soil water content in the mid‐slope position enhanced mineralization and plant N uptake, increasing shrub biomass. The deciduous shrub growth in the mid‐slope position was further enhanced by symbiotic N2 fixation primed by increased root carbon allocation. The gentle slope in the poorly drained lower‐slope position resulted in saturated soil conditions that reduced soil O2 concentrations, leading to lower root O2 uptake and lower nutrient uptake and plant biomass. A simulation that removed topographical interconnectivity between grid cells resulted in (1) a 28% underestimate of mean shrub biomass and (2) over or underestimated shrub productivity at the various hillslope positions. Our results indicate that land models need to account for hillslope‐scale coupled surface and subsurface hydrology to accurately predict current plant distributions and future trajectories in Arctic ecosystems.

Precipitation rather than evapotranspiration determines the warm-season water supply in an alpine shrub and an alpine meadow

Alpine regions are generally referred to as water towers for the lowlands, yet the water balance has not been quantified for alpine catchments with different vegetation types. This paper presented a multi-year time series of evapotranspiration (ET) and water supply (precipitation minus ET) during the warm-season (June to September), measured using eddy covariance techniques for an upper alpine shrub (3400 m) and for a lower alpine meadow (3200 m), on the northeastern Qinghai-Tibetan Plateau. The monthly ET for the shrub averaged 71.8 ± 10.1 mm (Mean ± SD), which was 22.7% lower than for the meadow. ET peaked at 89.0 ± 3.0 mm for the shrub and at 113.9 ± 2.6 mm for the meadow both in July. The monthly water supply was nearly neutral from June to July and represented a surplus in August and September at both sites. The mean warm-season ET and water supply were 287.0 ± 17.6 mm and 59.6 ± 16.3 mm for the shrub, and 352.2 ± 15.7 mm and 22.5 ± 32.5 mm for the meadow, respectively. Piecewise structural equation models showed that variability of daily ET was dominated more by net radiation (Rn) than by vapor pressure deficit (VPD) at both sites. However, net radiation was 32.6% higher for the shrub than for the meadow, but this did not drive higher ET for the shrub. This was explained by the difference in the energy partitioning strategy (Bowen ratio), which was attributable to the difference in the bulk surface resistance, rather than the difference in the climatological resistance (which is proportional to VPD/Rn). The seasonal and annual variability of water supply was determined by precipitation rather than by ET. The linear slopes of water supply with precipitation were all close to one, regardless of vegetation type. This suggested that the water yield efficiency was consistent at the two sites. Our results highlighted the different effects of vegetation type on ET and water supply for humid alpine regions. The lower ET loss and consequent higher water yield, but with less digestible forage in the shrub, would present a dilemma for the balance between water supply and forage production under current shrub expansion in alpine rangeland.

Does fire always accelerate shrub expansion in Arctic tundra? Examining a novel grass-dominated successional trajectory on the Seward Peninsula

ABSTRACT Over the last century in the circumpolar north, notable terrestrial ecosystem changes include shrub expansion and an intensifying wildfire regime. Shrub invasion into tundra may be further accelerated by wildfire disturbance, which creates opportunities for establishment where recruitment is otherwise rare. The Seward Peninsula currently experiences more frequent and larger fires than other tundra regions in Alaska. There are areas of overlapping burn scars dating back to the 1950s. Using a chronosequence approach, we examined vegetation and ecosystem dynamics in tussock tundra. Increasing burn severity and fire frequency corresponded with an increase in grass cover and a decrease in shrub basal area. We used multivariate ordination analysis to create a single integrator variable of fire effect that accounted for time after fire, burn severity, and number of times burned. This fire effect was significantly associated with decreases in soil organic layer thickness and overall plant biomass. Unlike previous studies in Arctic Alaska tundra, we found that increases in fire frequency and severity did not increase shrub cover and biomass. Instead, intensifying fire disturbance, and particularly repeat fires, led to grass dominance. Our findings support the hypothesis that intensifying tundra fire regimes initiate alternative post-fire trajectories that are not shrub dominated and that are structurally and functionally quite different from sedge or shrub-dominated tundra.

Stability among cyclic change in an Antipodean pond and bolster heath system 1983–2017

A long-term study of a bolster heath–pond system at Newdegate Pass, Mount Field, Tasmania, was designed to test a theory of cyclical dynamics and to determine why there was constancy in the proportions of species in stable mosaic bolster heath, while their individual distributions changed. Groups of permanent plots were monitored approximately once every 5 years between 1983 and 2017, in apparently stable, apparently colonising, apparently recovering and apparently degrading bolster heath. The putatively stable sites exhibited an average of 37% spatial turnover in the three early 5-year periods, with reciprocal replacements between many pairs of species; however, there was little evidence of long-term directional change except for expansion of a gymnosperm shrub across the top of the bolster mosaic at one site. This pattern suggests a cyclical succession of species in the stable mosaic; however, the mechanism driving cyclicity remains unclear, with facilitation, competition and stochastic processes being implicated at different times and locations. These cyclic changes were likely to have been largely endogenous, given the variability and autocorrelations in relative growth rates of the stable mosaic over short horizontal distances, conforming with neutral theory. The process of dam vegetation loss after draining was rapid, as was vegetation colonisation of the floor of one dam and recovery from fire. On a putatively colonising site on rocky ground, changes were subdued but directionally consistent with the previously proposed model, which was consistent with all long-term observations.

Vegetation science during hectic times

Vegetation science during hectic times

Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems

Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (>40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.

Impacts of Arctic Shrubs on Root Traits and Belowground Nutrient Cycles Across a Northern Alaskan Climate Gradient.

Deciduous shrubs are expanding across the graminoid-dominated nutrient-poor arctic tundra. Absorptive root traits of shrubs are key determinants of nutrient acquisition strategy from tundra soils, but the variations of shrub root traits within and among common shrub genera across the arctic climatic gradient are not well resolved. Consequently, the impacts of arctic shrub expansion on belowground nutrient cycling remain largely unclear. Here, we collected roots from 170 plots of three commonly distributed shrub genera (Alnus, Betula, and Salix) and a widespread sedge (Eriophorum vaginatum) along a climatic gradient in northern Alaska. Absorptive root traits that are relevant to the strategy of plant nutrient acquisition were determined. The influence of aboveground dominant vegetation cover on the standing root biomass, root productivity, vertical rooting profile, as well as the soil nitrogen (N) pool in the active soil layer was examined. We found consistent root trait variation among arctic plant genera along the sampling transect. Alnus and Betula had relatively thicker and less branched, but more frequently ectomycorrhizal colonized absorptive roots than Salix, suggesting complementarity between root efficiency and ectomycorrhizal dependence among the co-existing shrubs. Shrub-dominated plots tended to have more productive absorptive roots than sedge-dominated plots. At the northern sites, deep absorptive roots (>20 cm depth) were more frequent in birch-dominated plots. We also found shrub roots extensively proliferated into the adjacent sedge-dominated plots. The soil N pool in the active layer generally decreased from south to north but did not vary among plots dominated by different shrub or sedge genera. Our results reveal diverse nutrient acquisition strategies and belowground impacts among different arctic shrubs, suggesting that further identifying the specific shrub genera in the tundra landscape will ultimately provide better predictions of belowground dynamics across the changing arctic.