## 2 Materials and Methods  
### Overview of the Study Area  
Arxan is located in the northeast of Inner Mongolia Autonomous Region of China, in the southwestern foothills of GKM [28] (119\({}^{\circ}\)30\({}^{\prime}\)-121\({}^{\circ}\)15\({}^{\prime}\) N, 46\({}^{\circ}\)30\({}^{\prime}\)-47\({}^{\circ}\)45\({}^{\prime}\) E; Figure 1a). With an altitude of 490-1750 m, the topography is high in the northeast and low in the southwest. The warm, humid airflow from the southeast ocean and the cold, dry wind from the northwest both have a significant impact on Arxan's climate. It has a cold temperate, continental monsoon climate. The characteristics of the local microclimate are clear [29]. Long and chilly winters alternate with wet summers. Arxan has a short frost-free period and a large daily temperature difference. The annual average air temperature is \(-2\) to 0 \({}^{\circ}\)C, and the extreme minimum temperature can reach \(-45\)\({}^{\circ}\)C [30]. The annual average precipitation is about 450 mm, concentrated from June to August. The snow cover season lasts from October to April of the following year, with the period from January to March seeing the majority of the maximum snow depth [31]. The vegetation coverage rate in Arxan is 81.20%. The vegetation is mainly Eurasian coniferous forest, and there are a few Eurasian grasslands and East Asian summer green broad-leaved forests. There are large areas of Carey and meadows in valleys and depressions, and the thickness of the peat layer in some areas reaches more than 66 cm [32].  
The study area is located in the south boundary of the permafrost region in Northeast China, most of which belong to the zone of seasonally frozen ground and some are permafrost regions. The strong seasonal freezing and thawing effect results in a deep seasonal freezing depth, and the maximum can reach more than 2.5 m [33]. The main ridge of the GKM forms a potent low-temperature arc in the south due to the temperature differential effect brought on by elevation. Peat moss blueberry swamp extends from Arxan to the vast GKM permafrost area in the north. Previously, there was sheet-like permafrost around the Taiping mountains in Arxan. Therefore, the permafrost near Arxan is considered to be the result of the southward extension of the island-like permafrost-thawing area outside the continuous permafrost sub-region at the northern end of the GKM [34]. Under the background of global climate warming and global permafrost degradation, the south boundary of the permafrost in Northeast China is moving northward [35]. These have led to the gradual change in the climate and permafrost in Arxan in recent years. During the period of 2001-2020, the climate in Arxan continued to warm, and the mean annual ground surface temperature (MAGST) and mean annual land surface temperature (MALST) of the study area increased by 0.047 \({}^{\circ}\)C and 0.265 \({}^{\circ}\)C, respectively, per year (Figure 1b). MAGST is calculated based on the monitoring data of the National Meteorological Station (NMS) in Arxan, and MALST is calculated based on the MODIS MOD1A2 V6 ([[https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/)]([https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/)), (accessed on 1 October 2022)) [36] remote sensing data. The permafrost temperature (PT) in the study area increased from \(-2.46\)\({}^{\circ}\)C in 2003-2013 to \(-2.78\)\({}^{\circ}\)C in 2014-2019. More information about PT is provided in Section 2.2.3.  
Figure 1: The geographical location of the study area (Arxan) (**a**). Mean annual ground surface temperature (MAGST) and mean annual land surface temperature (MALST) changes in 2001–2020 (**b**).