Objective  Seasonal permafrost degradation directly changes water recharge at the beginning of the growing season, which in turn affects regional forest health. At present, the degradation of permafrost in Southern Part of Greater Khingan Mountains is still unclear, especially the degradation characteristics under climate change.

Method : In the long-term experimental forest of Saihanwula National Nature Reserve in Inner Mongolia, environmental factors such as air temperature, soil temperature, and volumetric soil water content were observed from 2014—2022 to analyze the characteristics of permafrost degradation in forest seasons.

Result : The temperature in Southern Part of Greater Khingan Mountains rose in shortly. The rise rate of annual average temperature was 0.42 ℃·(10 a)⁻¹ in 1997—2022, which was larger 23.5% of the rate of 0.34 ℃·(10 a)⁻¹ in 1973—1996, in Southern Part of Greater Khingan Mountains. It is particularly significant that the average temperature during the freeze-thaw period (i.e., November to June) rose faster with the rate of 0.46 ℃·(10 a)⁻¹. The freeze-thaw pattern of soil was characterized by top-down unidirectional freezing and unidirectional melting; The freezing rate and thawing rate became faster with increasing soil depth, reaching a maximum in the 40～80 cm soil layer (freezing rate 2.23 cm·d⁻¹, thawing rate 4.50 cm·d⁻¹). Seasonally frozen soil continued to degrade, with the observed maximum freezing depth reduced from 80 cm to 40 cm. The freeze-thaw period was significantly shortened, the start of freezing was delayed, and the complete thaw time was advanced, resulting in a shortening of the annual freeze-thaw period by 15.21%. Applying the freeze-thaw multiple linear regression model established in this paper, the contribution of elevated air temperature accumulation to seasonal permafrost degradation in the study area was more than 90%.

Conclusion : In the Southern Part of Greater Khingan Mountains, permafrost degradation is mainly characterized by shortening of the freeze-thaw period, and warmer temperatures are the main driver of seasonal permafrost degradation, making it necessary to monitor changes in seasonal permafrost in the future for better forest management.