Monitoring of ground hydrothermal regimes and active layer thickness during 2017-2022 in some previously burned areas in hemiboreal forests in Northeast China

Wildfires have important effects on hydrothermal regimes of boreal permafrost. However, they are not yet systematically and extensively studied in Northeast China, except those discrete measurements during the early 1990s. Since 2017, a series of study sites have been built for continuous observation on ground hydrothermal regimes at ten sites of three fire severity (unburned, light burn and severe burn) in three areas (Mo’he, Alongshan and Mangui) on the western flank of the northern Da Xing’anling Mountains, Northeast China. An integrated dataset was compiled with sub-datasets on ground temperatures, soil moisture contents, and active layer thickness (ALT). The study results show evident impacts of wildfires manifested as rapid ground warming, thickening active layer and drying shallow soils. Moreover, the thermally affected depth of wildfires has exceeded 20 m, and ALT has increased by as much as 2.75 m eight years after the severe burn. Post-fire changes were more pronounced in ground temperatures at depths of 0-6 m. In addition, changes in ground hydrothermal regimes became greater with increasing fire severity, and the fire influences lasted more than 30 years. Rapid warming and thawing of permafrost and subsequent loss of SOC after wildfires could have a positive feedback effect on climate warming. Therefore, this study can provide basic data for studies and action plan to support the carbon neutralization initiative and for assessment of ecological safety and management of the permafrost environment.