Permafrost-affected tundra soils are large carbon (C) and nitrogen (N) reservoirs. However, N is largely bound in soil organic matter (SOM), and ecosystems generally have low N availability. Therefore, microbial induced N-cycling processes and N losses were considered negligible. Recent studies show that microbial N processing rates, inorganic N availability, and lateral N losses from thawing permafrost increase when vegetation cover is disturbed, resulting in reduced N uptake or increased N input from thawing permafrost. In this review, we describe currently known N hotspots, particularly bare patches in permafrost peatland or permafrost soils affected by thermokarst, and their microbiogeochemical characteristics, and present evidence for previously unrecorded N hotspots in the tundra. We summarize the current understanding of microbial N cycling processes that promote the release of the potent greenhouse gas (GHG) nitrous oxide (N2O) and the translocation of inorganic N from terrestrial into aquatic ecosystems. We suggest that certain soil characteristics and microbial traits can be used as indicators of N availability and N losses. Identifying N hotspots in permafrost soils is key to assessing the potential for N release from permafrost-affected soils under global warming, as well as the impact of increased N availability on emissions of carbon-containing GHGs.

Boreal peatlands are critical ecosystems globally because they house 30%-40% of terrestrial carbon (C), much of which is stored in permafrost soil vulnerable to climate warming-induced thaw. Permafrost thaw leads to thickening of the active (seasonally thawed) layer and alters nutrient and light availability. These physical changes may influence community-level plant functional traits through intraspecific trait variation and/or species turnover. As permafrost thaw is expected to cause an efflux of carbon dioxide (CO2) and methane (CH4) from the soil to the atmosphere, it is important to understand thaw-induced changes in plant community productivity to evaluate whether these changes may offset some of the anticipated increases in C emissions. To this end, we collected vascular plant community composition and foliar functional trait data along gradients in aboveground tree biomass and active layer thickness (ALT) in a rapidly thawing boreal peatland, with the expectation that changes in above- and belowground conditions are indicative of altered resource availability. We aimed to determine whether community-level traits vary across these gradients, and whether these changes are dominated by intraspecific trait variation, species turnover, or both. Our results highlight that variability in community-level traits was largely attributable to species turnover and that both community composition and traits were predominantly driven by ALT. Specifically, thicker active layers associated with permafrost-free peatlands (i.e., bogs and fens) shifted community composition from slower-growing evergreen shrubs to faster-growing graminoids and forbs with a corresponding shift toward more productive trait values. The results from this rapidly thawing peatland suggest that continued warming-induced permafrost thaw and thermokarst development alter plant community composition and community-level traits and thus ecosystem productivity. Increased productivity may help to mitigate anticipated CO2 efflux from thawing permafrost, at least in the short term, though this response may be swamped by increase CH4 release.

The populations, species, and communities in high elevation mountainous regions at or above tree line are being impacted by the changing climate. Mountain systems have been recognized as both resilient and extremely threatened by climate change, requiring a more nuanced understanding of potential trajectories of the biotic communities. For high elevation systems in particular, we need to consider how the interactions among climate drivers and topography currently structure the diversity, species composition, and life-history strategies of these communities. Further, predicting biotic responses to changing climate requires knowledge of intra- and inter-specific climate associations within the context of topographically heterogenous landscapes. Changes in temperature, snow, and rain characteristics at regional scales are amplified or attenuated by slope, aspect, and wind patterns occurring at local scales that are often under a hectare or even a meter in extent. Community assemblages are structured by the soil moisture and growing season duration at these local sites, and directional climate change has the potential to alter these two drivers together, independently, or in opposition to one another due to local, intervening variables. Changes threaten species whose water and growing season duration requirements are locally extirpated or species who may be outcompeted by nearby faster-growing, warmer/drier adapted species. However, barring non-analogue climate conditions, species may also be able to more easily track required resource regimes in topographically heterogenous landscapes. New species arrivals composed of competitors, predators and pathogens can further mediate the direct impacts of the changing climate. Plants are moving uphill, demonstrating primary succession with the emergence of new habitats from snow and rock, but these shifts are constrained over the short term by soil limitations and microbes and ultimately by the lack of colonizable terrestrial surfaces. Meanwhile, both subalpine herbaceous and woody species pose threats to more cold-adapted species. Overall, the multiple interacting direct and indirect effects of the changing climate on high elevation systems may lead to multiple potential trajectories for these systems.

Warming environmental conditions are often credited with increasing Arctic shrub growth and altering abundance and distribution, yet it is unclear whether tundra shrub expansion will continue into future decades. Water availability may begin to limit Arctic shrub growth if increasing air temperatures create drier soil conditions due to increased evapotranspiration and permafrost-thaw-induced soil drainage. However, few studies have effectively considered how dominant tundra shrub species respond to variations in both temperature and moisture. To better understand the key effects of temperature variation and soil moisture on two dominant circumpolar deciduous shrubs, we studied shrub growth along a natural landscape gradient in West Greenland, which is a region observed to be drying due to ongoing warming. We found that the growth forms of both grey willow (Salix glauca) and dwarf birch (Betula nana) were sensitive to warmer and drier conditions. For both species, increases in air temperature positively correlated with greater shrub volume, with the doubling of canopy volume due to increased woody biomass. Leaf biomass was best predicted by edaphic features including extractable ammonium, which was positively related to soil moisture, and bulk density. Warmer soils tended to be drier, suggesting that ongoing warming in the area could lead to significant water limitation. Our findings suggest that drier soil conditions might be limiting foliar production despite warming temperatures for two circumpolar dominant shrubs,Betula nanaandSalix glauca, which could have wide-ranging, biome-level consequences about ongoing and predicted shrub growth and expansion.

Aim Litter humification is vital for carbon sequestration in terrestrial ecosystems. Probing the litter humification of treeline ecotone will be helpful to understand soil carbon afflux in alpine regions under climate change. Methods Foliar litter of six plant functional groups was chosen in an alpine treeline ecotone of the eastern Tibetan Plateau, and a field litterbag decomposition experiment (669 days) was conducted in an alpine shrubland (AS) and a coniferous forest (CF). Environmental factors, litter quality, humus concentrations (total humus, Huc; humic acid, HAc; and fulvic acid, FAc) and hue coefficient (Delta logK and E4/E6) were measured to explore litter humification processes. Results Litter humification was controlled by both litter stoichiometric traits and local-environment conditions, while stoichiometric traits played a more obvious regulatory role. Significant discrepancies in litter humus were detected among six plant functional groups; more precisely, litter of evergreen conifer and shrubs showed a net accumulation of Huc and FAc during winter, whereas others experienced more mineralization than accumulation. Huc, HAc, and hue coefficient were mainly controlled by cellulose/N, cellulose/P, C/N, lignin/P, lignin/N, etc., yet FAc was more susceptible to local-environment conditions. Meanwhile, Huc, HAc and FAc, as well as humification degree and E4/E6 differed between AS and CF, with faster humification in AS. Conclusion We suggest that litter stoichiometric traits are more responsible for regulating litter humification than environmental conditions in elevational gradients. Furthermore, potential upward shifts by plants may accelerate litter humification in alpine ecosystems.