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.

Ground temperature plays a significant role in the interaction between the land surface and atmosphere on the Tibetan Plateau (TP). Under the background of temperature warming, the TP has witnessed an accelerated warming trend in frozen ground temperature, an increasing active layer thickness, and the melting of underground ice. Based on high-resolution ground temperature data observed from 1997 to 2012 on the northern TP, the trend of ground temperature at each observation site and its response to climate change were analyzed. The results showed that while the ground temperature at different soil depths showed a strong warming trend over the observation period, the warming in winter is more significant than that in summer. The warming rate of daily minimum ground temperature was greater than that of daily maximum ground temperature at the TTH and MS3608 sites. During the study period, thawing occurred earlier, whereas freezing happened later, resulting in shortened freezing season and a thinner frozen layer at the BJ site. And a zero-curtain effect develops when the soil begins to thaw or freeze in spring and autumn. From 1997 to 2012, the average summer air temperature and precipitation in summer and winter from six meteorological stations along the Qinghai-Tibet highway also demonstrated an increasing trend, with a more significant temperature increase in winter than in summer. The ground temperature showed an obvious response to air temperature warming, but the trend varied significantly with soil depths due to soil heterogeneity.

Ground temperature plays a significant role in the interaction between the land surface and atmosphere on the Tibetan Plateau (TP). Under the background of temperature warming, the TP has witnessed an accelerated warming trend in frozen ground temperature, an increasing active layer thickness, and the melting of underground ice. Based on high-resolution ground temperature data observed from 1997 to 2012 on the northern TP, the trend of ground temperature at each observation site and its response to climate change were analyzed. The results showed that while the ground temperature at different soil depths showed a strong warming trend over the observation period, the warming in winter is more significant than that in summer. The warming rate of daily minimum ground temperature was greater than that of daily maximum ground temperature at the TTH and MS3608 sites. During the study period, thawing occurred earlier, whereas freezing happened later, resulting in shortened freezing season and a thinner frozen layer at the BJ site. And a zero-curtain effect develops when the soil begins to thaw or freeze in spring and autumn. From 1997 to 2012, the average summer air temperature and precipitation in summer and winter from six meteorological stations along the Qinghai-Tibet highway also demonstrated an increasing trend, with a more significant temperature increase in winter than in summer. The ground temperature showed an obvious response to air temperature warming, but the trend varied significantly with soil depths due to soil heterogeneity.