Forest buffers soil temperature but divergent influences on soil freeze–thaw state at the edges of an arid mountain forest

Arid montane forests exhibit distinct upper (UFE) and lower forest edges (LFE). Changes in soil temperature at these edges are crucial for soil biological processes and hydrological cycles. Here, we used multiple years of temperature data from 0-80 cm soil depths in paired forest versus shrubland and forest versus grassland sites at UFE and LFE, respectively, in the Qilian Mountains of northwestern China to determine the influence of the forest canopy on soil temperature dynamics at the forest edge. Generally, the average soil temperature in each layer in the forest was higher than that of shrubland and grassland at UFE and LFE. Further, the forest had a buffering effect on soil temperature, and the forest at LFE had a higher buffering effect than at UFE in cold seasons. Soil thawing and freezing were delayed in the forest compared with shrubland respectively by 2–3 and 7 days at UFE; however, compared with grassland, soil thawing and freezing advanced in the forest respectively by 2 and 5 days at LFE. In addition, the number of freezing days was greater than that of thawing days at UFE, and the number of freezing days of shrubland was significantly greater than that of the forest. The number of thawing days was greater than freezing days at LFE, and the number of freezing days in the forest was significantly smaller than that of the grassland, especially at the 80 cm soil depth. These results, based mainly on soil temperature, suggested that forest expansion will change the shallow soil temperature regimes regardless of UFE and LFE, which further affect the complexity of soil freeze–thaw cycle characteristics in arid mountains under climate warming.