Shrub expansion may reduce summer permafrost thaw in Siberian tundra

Abstract

Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.

Erosion of community diversity and stability by herbivore removal under warming

“…First, in Arctic plant communities such as the one in this study, removal of large herbivores may reduce species diversity within the plant community by promoting the expansion of deciduous shrubs [10], which can outcompete forbs for limiting nutrients such as nitrogen, and, through expansion and associated inputs of recalcitrant litter, inhibit their growth through immobilization of soil nitrogen or suppression of nutrient cycling rates [43,44]. Second, expansion of shrubs may result in suppression of soil microbial activity owing to shading and cooling of the soil surface [45], potentially further reducing soil nutrient availability, with adverse consequences for inferior competitors within the plant community [39]. Several results are consistent with the former hypothesis, but data are lacking in this study to address the latter.…”

Ice-age megafauna in Arctic Alaska: extinction, invasion, survival

“…Paleoenvironmental records from arctic Alaska lend support to Guthrie's ideas in the form of evidence for sweeping changes in hillslope erosion, floodplain dynamics, and vegetation e all triggered by increases in effective moisture during the PleistoceneeHolocene transition (Mann et al, 2002(Mann et al, , 2010. Mesice hydric vegetation dominated by sedges and shrubs spread across the region early in post-glacial times (Oswald et al, 1999), replacing the formerly dominant graminoids and forbs (Zazula et al, 2006, and probably lowering soil temperatures (Blok et al, 2010). Shrubs tend to be better defended by anti-herbivory compounds against mammalian herbivores than are grasses and forbs, so range quality for grazers would have declined as moist tundra spread (Guthrie, 2006).…”

Modeling the temperature evolution of Svalbard permafrost during the 20th and 21st century

“…This would lead to more snow becoming potentially trapped, increasing the GST during winter, even if summer GST would decrease because of increased shading (e.g. Blok et al, 2010;Sharkhuu et al, 2007;Hinzman et al, 2005).…”