Boreal shrub and tree expansion promotes soil warming

Soil and air temperature sensors on the tundra.

Soil temperatures are warming and permafrost is thawing across the Arctic and Boreal region. While much of this change is due to recent increases in air temperature, ecosystem properties, such as vegetation, can also affect soil temperature and permafrost. As part of a large collaborative project with members of the Permafrost Carbon Network, we synthesized soil and air temperature data from 106 sites across the northern permafrost region. Our findings show that soil temperatures were higher under tall shrubs and trees compared to other vegetation types. Shrubs and tree-line are expanding northward as the climate become more favorable for larger-structured vegetation in the Arctic. Our findings suggest that changing vegetation distribution in the Arctic may amplify current rates of soil warming and permafrost thaw.

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.

Kropp, H., M. M. Loranty, S. M. Natali, A. L. Kholodov, A. V. Rocha, ... J. A. O'Donnell, et al. 2020. Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems. Environmental Research Letters.