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

Abstract

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.

Recent degradation of interior Alaska permafrost mapped with ground surveys, geophysics, deep drilling, and repeat airborne lidar

“…Previous studies have established that vegetation provides a range of "ecosystem protection" properties for permafrost (Shur and Jorgenson, 2007;Loranty et al, 2018). Recent measurements confirm this and identify strong links between different ecotypes and top-down thaw of permafrost in interior Alaska (Yi et al, 2018;Douglas et al, 2020;Jorgenson et al, 2020;Kropp et al, 2020). In this study, some ecotypes are associated with consistently deeper active layer measurements over time.…”

“…The summer of 2013, when our ERT measurements were made, experienced a total of 14.5 cm of rainfall, which is slightly lower than the 90-year mean of 18.5 cm. The 2013 summer mean temperature of 12.2 • C was close to the 90-year summer mean of 11.8 • C. (Jorgenson et al, 2020). In terms of heating degree days, the summer of 2013 (1133) was slightly above the historical mean of 1090.…”

“…1). The region has a continental climate with a mean annual air temperature of −2.4 • C, typical mean summer temperatures of 20 • C, mean winter temperatures of −20 • C, and yearly extremes ranging from −51 to 38 • C (Jorgenson et al, 2001(Jorgenson et al, , 2020. Mean annual precipitation is 28.0 cm (Wendler and Shulski, 2009) with a typical annual snowfall of 1.7 m (Jorgenson et al, 2001) that represents 40 %-45 % of the annual precipitation (Liston and Hiemstra, 2011).…”

“…This is expected to mostly disappear from the near surface (upper 1 m) by 2100 (Pastick et al, 2015). Mean annual air temperatures in interior Alaska, currently roughly −2 • C (Jorgenson et al, 2020), are projected to increase by 2 • C by 2050 (Douglas et al, 2014) and 5 • C by 2100 (Lader et al, 2017). Roughly half of the discontinuous permafrost in the area represents late Pleistocene ice and organic-carbon-rich "yedoma" Strauss et al, 2016).…”

Shrub expansion in the Arctic may induce large‐scale carbon losses due to changes in plant‐soil interactions

“…For example, leaf litter decomposition experiments suggest that the expansion of deciduous shrubs may promote a negative feedback to climate change due to increases in leaf litter production and the fact that their litter decomposes relatively slowly compared to forb, grass and sedge litter, and only slightly faster than litter from dwarf evergreen shrubs (Cornelissen et al 2007). Aboveground plant traits are also well known to regulate wider ecosystem properties such as soil temperatures (Kropp et al 2020), through interactions between canopy height and snow trapping in the winter (Sturm et al 2005), or canopy leaf area and shading in the summer (Blok et al 2010). Changes in canopy properties could therefore influence permafrost thaw and the exposure of permafrost C to degradation, depending on the relative strength of summer and winter feedbacks.…”