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.
Highlights
Rapid increases in air temperature across the Arctic and Boreal region are associated with warming soils, thawing permafrost, and expanding tall-statured shrubs and trees in the tundra (Romanovsky et al 2003, Myers-smith et al 2011, Elmendorf et al 2012)
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 vegetation on cold- and warm-season soil temperature We found that tall shrub tundra had the highest soil temperatures after herb-barren during the warm season (figures 2(a)–(c)) when normalized by air temperature, contrary to observations of local soil cooling associated with shading from shrubs in the warm season (Blok et al 2010, Loranty et al 2018)
Summary
Rapid increases in air temperature across the Arctic and Boreal region are associated with warming soils, thawing permafrost, and expanding tall-statured shrubs and trees in the tundra (Romanovsky et al 2003, Myers-smith et al 2011, Elmendorf et al 2012). Vegetation change across the pan-Arctic is altering land surface energy dynamics with critical implications for soil thermal regimes in the permafrost region (Elmendorf et al 2012, Myers-Smith et al 2015, Martin et al 2017). Experimental manipulations of shrub cover in tundra ecosystems indicate that increasing shrub canopy (‘shrubification’) cover reduces summer soil temperatures and decreases the depth of seasonal thaw via canopy shading (Blok et al 2010, Myers-Smith and Hik 2013). A recent study using eddy covariance measurements suggests that shrubs are associated with warmer summer soil temperatures (Lafleur and Humphreys 2018). The lack of consensus in plot scale observations creates uncertainty regarding potential feedbacks of shrubification in the tundra, and calls for a larger scale assessment of the impacts of vegetation on soil temperature
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