The spatial variation of alpine timberlines and their biogeographical characteristics in the northern Tianshan Mountains of China

Alpine timberline is a proper indicator of climate change and has received much attention in ecological research during the past decades. In this study, variations of alpine timberlines and their biogeographic characteristics were investigated by a combination of field surveys, climate indices, and soil nutrient analyses in the northern Tianshan Mountains (NTM) of northwest China. The timberline altitudes were probed to range from 2,530 to 2,800 m a.s.l. in the NTM, gradually descending from west to east along the V-shaped Ili valley (from Zhaosu to Xinyuan), and ascending from west to east across the northernmost range of the study area (from Bole to Balikun). The vertical widths between the upper and lower limits of the timberlines were widest in the middle NTM (Kuytun to Changji) and the Ili valley (Zhaosu to Xinyuan). Interactions between the terrain and air currents resulted in variant climates within the NTM, which contributed to significant timberline distribution differences. Although vertical width was positively correlated to soil nutrient (especially organic matter, r = 0.78), and timberline altitudes were positively related to the soil pH values, the temperature during the growing season primarily determines both altitude and vertical width. In conclusion, timberline altitudes were dominated by the combined effects of local topography, soil nutrient properties, water, and thermal conditions. Identifying the dominating influencing factors of NTM timberlines provided potential insights to better understand the effects of climate change on the alpine ecosystem over central Asia.