Allocation strategy of mobile carbohydrates of Qinghai spruce in the alpine treeline ecotone of the northeastern Tibetan Plateau

Alpine treeline ecotone is considered an ideal “warning line” for monitoring global climate change. However, the shifts in the nonstructural carbohydrates (NSC) of tree organs within alpine treeline ecotones along the gradient from timberline–treeline–tree species line remain elusive. This study aimed to quantify the soluble sugar, starch, and NSC contents of Qinghai spruce and their driving factors across six typical treeline ecotones of the Qilian Mountains, northeastern Tibetan Plateau. The soluble sugar, starch, and NSC contents were the highest in leaves, followed by fine roots and twigs. The NSC contents in tree leaves and twigs peaked with an increase in elevation from the timberline to the treeline. At the tree species line, the NSC content was the highest in fine roots. Namely, the aboveground carbon investment decreased whereas the underground carbon allocation increased after exceeding the upper limit of treeline. The NSC contents in leaves and twigs were all higher and in fine roots were lower in the western part of the Qilian Mountains (an extremely arid region) than in the eastern and central parts (semi-arid region). The NSC contents were positively correlated with mean annual temperature and negatively correlated with mean annual precipitation. The redundancy analysis results showed that the soil factors contributed to 66.3% of the variation in tree NSC contents, with soil NO3–-N (26.6%) and NH4+-N (24.2%) being the dominant factors influencing the changes in tree NSC in the alpine treeline ecotone. This study provides novel insights into carbon metabolism and growth adaptation strategies of trees in alpine arid mountainous ecosystems.