Spatiotemporal Variability of Soil Temperature and Moisture across two Contrasting Timberline Ecotones in the Sergyemla Mountains, Southeast Tibet

Simultaneous measurements of soil temperature and moisture and related climate and vegetation variables at high altitudes are rare. Such knowledge is important to predict soil temperature and moisture across heterogeneous alpine landscapes and the impact of climate warming on alpine ecosystems. Based on the four-year observations in 12 plots across two contrasting timberline ecotones (north-facing Abies georgei var. smithii and south-facing Juniperus saltuaria timberlines of a valley) in the Sergyemla Mountains, we aimed to determine the role of altitude, aspect, climate, soil, and vegetation variables affecting the variability of soil temperature and moisture. The two timberlines had similar annual precipitation and seasonal mean air temperature, but the growing-season mean soil temperature differed by 0.8-1.0 K. The spring soil warming date was 20-30 days later on the north-facing slope than on the south-facing slope, which was associated with increased snow and vegetation covers on the north-facing slope. Slope aspect, canopy height, and leaf area index (LAI) rather than altitude were the major determining factors for spatial variability of seasonal mean soil temperatures across plots. A combination of aspect southness and canopy height/LAI explained 56-77% of the variations in the -20 cm mean soil temperatures for the year, growing season, and July across the 12 plots. In contrast, seasonal mean soil moistures did not correlate with altitude, aspect, and stand and soil variables. Furthermore, the -5 cm soil temperature amplitude in the growing season was much lower in the north-facing fir forest than in the south-facing juniper forest, suggesting an explanation for the distribution pattern of both species timberlines on opposite slopes of a valley in the Sergyemla Mountains.