The accelerated or decelerated freezing-thawing processes of the active layer in Xing'an permafrost regions are crucial for the protection of permafrost. To better understand the freezing-thawing processes of the active layer and its driving factors, according to the observation from 2017 to 2020 of soil temperature and water content in the active layer of forest and peatland in two representative hemiboreal ecosystems in the Da Xing'anling Mountains, Northeast China, the study explored in detail the effects of climatic conditions and local factors on the hydrothermal and freezing-thawing processes of active layer soils. The results showed that during the freezing-thawing cycles of 2017-2020, freezing and thawing start times in the peatland and forest ecosystems soils were generally delayed, and it took longer for the active layer soil to completely thaw than to freeze. The annual average soil temperature in the peatland's active layer (5-80 cm) was 0.7-2.0 degrees C lower than that in the forest, and the annual average soil moisture content on the peatland was 5.5%-26.7% higher than that in the forest. Compared with the forest ecosystem soils, the ground surface freezing time of the peatland was delayed by 3-10 d, and the freezing rate decreased by 1.1-1.5 cm d-1, while the beginning time of thawing was advanced by 22-27 d, and the thawing rate decreased by 1.3-1.4 cm d-1. In the process of decreasing soil temperature and increasing soil moisture content, the freezing and thawing rate of the active layer would be reduced, decelerating the freezing-thawing processes of the active layer in the process of decreasing soil temperature and increasing soil moisture content. The results provide the key original data for studying the formation and evolution of active layer and permafrost in the Xing'an permafrost regions in Northeast China and can be used to validate the prediction of ecosystem succession under the combined influences of climate change and permafrost degradation.

In the face of increasing temperature at high latitudes, ecosystem respiration (RE) is a key to determining sink and source dynamics of a boreal forest. In this paper, we analyzed four-year RE data obtained in an open black spruce forest-a typical boreal forest ecosystem with permafrost in Interior Alaska. RE measured as nighttime CO2 effluxes for both the ecosystem and the forest floor were clearly situated along the exponential temperature dependent curve, except for the data obtained in extremely dry conditions in mid-summer. More than 93 % of RE data measured at nighttime with high vapor pressure deficit (VPD > 400 Pa) were lower than the values predicted from the temperature-dependent curve. Consequently, the year 2013 (with an unusually dry summer) had a 15 % lower amount of annual RE than that could be expected from the temperature-dependent curve without considering the effect of high VPD. The suppression of RE under dry conditions was also related to decreases in soil moisture and net ecosystem productivity. Finally, assuming daytime RE could be extrapolated from the temperature-dependent curves, annual daytime RE estimated with the effect of high VPD was decreased by up to 62% from RE estimated without the VPD effect. The down-regulation of RE presented in this study postulates a possible negative feedback for the carbon budget of boreal forests in response to climate warming.