Tundra vegetation change and impacts on permafrost

Abstract

Tundra vegetation productivity and composition are responding rapidly to climatic changes in the Arctic. These changes can, in turn, mitigate or amplify permafrost thaw. In this Review, we synthesize remotely-sensed and field-observed vegetation change across the tundra biome, and outline how these shifts could influence permafrost thaw. Permafrost ice content appears to be an important control on local vegetation changes; woody vegetation generally increases in ice-poor uplands, whereas replacement of woody vegetation by (aquatic) graminoids following abrupt permafrost thaw is more frequent in ice-rich Arctic lowlands. These locally observed vegetation changes contribute to regional satellite-observed greening trends, although the interpretation of greening and browning is complicated. Increases in vegetation cover and height generally mitigate permafrost thaw in summer, yet increase annual soil temperatures through snow-related winter soil warming effects. Strong vegetation-soil feedbacks currently alleviate the consequences of thaw-related disturbances. However, if the increasing scale and frequency of disturbances in a warming Arctic exceeds the capacity for vegetation and permafrost recovery, changes to Arctic ecosystems could be irreversible. To better disentangle vegetation-soil-permafrost interactions, ecological field studies remain crucial, but require better integration with geophysical assessments. [H1] IntroductionArctic tundra is changing rapidly, with a pervasive trend toward more abundant and taller vegetation as shrubs and trees expand northward 1 . Field and satellite observations suggest that tundra vegetation has become more productive, a phenomenon known as tundra greening. Such increases in the biomass and stature of Arctic tundra vegetation can alter the thermal properties of the ground surface. Canopies can mediate the effect of increasing summer air temperatures on soil temperatures 2-4 and contribute to insulation of soils in winter through trapping of snow [5][6][7][8] .Vegetation and soil characteristics also influence surface energy partitioning and the thermal diffusivity of the soil 9,10 . Permafrost (permanently frozen ground) underlies soil and vegetation, and is the foundation of Arctic tundra ecosystems. In turn, vegetation and near-surface soils insulate permafrost 11 , regulating the effects of atmospheric conditions. However, the Arctic is warming more than twice as fast as the global average, amplified by loss of sea ice cover 1 . Even if Arctic temperatures were to stabilize at 2°C of warming, as aimed for with the Paris Agreement, approximately 40% of near-surface permafrost is still projected to thaw 12 . Permafrost-dominated ecosystems are thus at risk 13 , even under modest CO2 emission scenarios 1 , with consequences for Arctic inhabitants 14 .

The activity and functions of soil microbial communities in the Finnish sub-Arctic vary across vegetation types

“…Major ecological disturbances have already been observed in Arctic ecosystems and are expected to become more frequent over the coming decades even if anthropogenic greenhouse gas (GHG) emissions are curbed (Post et al ., 2019). For example, a systematic greening of the Arctic tundra has been observed over the last decades, accompanied by increased plant productivity and the northward and upslope expansion of tall shrubs and trees into this otherwise treeless biome (Frost & Epstein, 2014; Heijmans et al ., 2022). In addition to regional disturbances, the effects of Arctic climate change might have wider global consequences due to the vast amounts of carbon (C) and nitrogen (N) stored in frozen tundra soils (Mackelprang et al ., 2011; Johnston et al ., 2019).…”

“…90% of the Arctic (Walker et al ., 2005). This knowledge gap is relevant as most of the Arctic is greening and the typically low-growing Arctic vegetation is being gradually replaced by taller woody plants, a development known as shrubification (Mod & Luoto 2016, Myers-Smith et al ., 2020; Heijmans et al ., 2022). Shifts in vegetation and, in particular, shrub expansion across the Arctic tundra are some of the most important ecosystem responses to climate change.…”

Fire-vegetation interactions in Arctic tundra and their spatial variability

“…These findings have profound implications in the context of climate change. Under the strong Arctic warming, most of the Arctic tundra biome is experiencing substantial "greening" and shrubs are a major contributor to this process 7,16,54 . Even though both warming and wildfire have been suggested to boost the shrubification, our results show that during the 15-year period between 2006 and 2020, the increases in NDVImax at the high severity burned sites (indicated by the red lines in Fig.…”

“…It has become increasingly clear that the Arctic tundra, despite being seemingly homogeneous, is in fact quite variable at the spatial scales that affect ecosystem functioning. This is reflected by the spatial heterogeneity in species compositions 51,54 , geophysical conditions 57,58 , and the resultant vegetation-land-climate interactions 16,[58][59][60][61][62][63] . Comparatively, we have a much poorer understanding of the role played by wildfires in tundra ecosystems.…”

Root‐associated fungi not tree density influences stand nitrogen dynamics at the larch forest–tundra ecotone

“…This increases the soil volume, decomposition rate and the release of N from the active layer and newly thawed permafrost soils, which in turn is available for plant nutrient acquisition (Keuper et al, 2012; Salmon et al, 2018). However, warming also triggers long‐term vegetation shifts, like increased tree density at the TTE and shrub expansion into tundra, that increase soil shading, reduce soil temperature and active layer thickness (Heijmans et al, 2022), therefore slowing decomposition and constraining soil volume available for nutrient capture. As such, increased tree density and an associated decrease in active layer thickness likely represents a scenario of reduced soil N supply, greater vegetation N demand, and thus, greater competition for N.…”