Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Vegetation can strongly regulate permafrost degradation at its southern edge through changing surface freeze-thaw processes Weichao Guo a , Hongyan Liu a, ⁎ , Oleg A. Anenkhonov b , Huailiang Shangguan a , Denis V. Sandanov b , Andrey Yu. Korolyuk c , Guozheng Hu a , Xiuchen Wu d a College of Urban and Environmental Science, and MOE Laboratory for Earth Surface Processes, Peking University, Beijing, 100871, China b Institute of General and Experimental Biology, Siberian Branch, Russian Academy of Sciences, Ulan Ude, 670047, Russia c Central Siberian Botanical Garden, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russia d Faculty of Geographical Sciences, Beijing Normal University, Beijing, 100875, China ARTICLE INFO Keywords: Permafrost degradation Freeze-thaw Vegetation Snow accumulation ABSTRACT Permafrost contains twice as much carbon as the atmosphere and the degradation of permafrost due to climatic warming, which could potentially change the global carbon cycle and could also enhance global climate change. It is well studied that permafrost degradation could result in vegetation transition. Aboveground vegetation can act as a buffer for climatic warming, however its role in regulating permafrost degradation remains unclear. In this study we examined how different vegetation types regulated the amplitude and duration of diurnal soil freeze/thaw (FT) cycles and the timing of seasonal soil FT. Soil temperature data (hourly and half hourly) was collected from paired forest-steppe sampling plots spanning a large spatial gradient from northern China to southern Siberia, Russia from 2008 to 2015. FT cycles were found to be larger in amplitude and longer in duration in steppe sites in comparison to forest sites. Soils in the forest sites and steppe sites freeze almost simultaneously, but experience a delay in thawing of approximately 14, 19 and 25 days for deciduous broadleaf forest, evergreen coniferous forest, and deciduous coniferous forest, respectively. Variations in snow accumu- lation due to differences in vegetation structure as opposed to solar radiation were responsible for the disparity in thaw timing. These findings imply that deciduous conifer forest in east Eurasia could reduce carbon emissions more effectively than evergreen conifer forest in west Eurasia by slowing down warming-induced permafrost degradation during spring thaw. 1. Introduction Permafrost (permanently frozen ground) comprises a region un- derlying 23.9% of the land area of the Northern Hemisphere. Currently permafrost contains about twice as much carbon as the atmosphere (Zhang et al., 1999; Zimov et al., 2006). These large quantities of carbon stored in frozen soils can be released into the atmosphere due to warming-induced permafrost degradation, which is further enhanced by a warming climate (Hodgkins et al., 2014; Hollesen et al., 2015; Schuur et al., 2015). This degradation could cause permafrost regions to shift from being a sink to a source of CO 2 by the end of the 21st century (Koven et al., 2011). Permafrost degradation mainly occurs at the southern edge of the permafrost area due to the significant poleward movement of permafrost with climate warming (Guo and Wang, 2016). Soil freezing–thawing (FT) processes are measured by the amplitude and duration of the diurnal soil FT cycle as well as seasonal FT timing and can effectively mediate permafrost degradation. Increasing soil surface thawing days can enhance permafrost degradation by dee- pening active layer thickness (Peng et al., 2017; Wu et al., 2015; Zhang, 2005). The soil suffers remarkable FT cycles during the transition period from cold/warm to warm/cold seasons. The transition between frozen soil to thawing is a continuous process lasting from several days to a few weeks (Cheng et al., 2014; Wang et al., 2013), and is suscep- tible to rapid climate warming, in particular the higher warming rates that occur in winter and spring (IPCC, 2013). Stronger and longer diurnal FT cycles and earlier seasonal permafrost thawing may increase carbon loss from the soil. There has been significant research examining spatio-temporal changes in soil FT processes in response to a warming climate at re- gional and continental scales, mainly using land surface temperature data from satellites (Kim et al., 2014; Li et al., 2012; Zhang, 2003) and land surface model simulations (Guo and Wang, 2014). Despite this, uncertainties still exist in the conclusions of these studies due to the coarse resolution of the available data and poor representation and https://doi.org/10.1016/j.agrformet.2018.01.010 Received 20 July 2017; Received in revised form 25 November 2017; Accepted 5 January 2018 ⁎ Corresponding author. E-mail address: lhy@urban.pku.edu.cn (H. Liu). Agricultural and Forest Meteorology 252 (2018) 10–17 0168-1923/ © 2018 Elsevier B.V. All rights reserved. T