Soil nitrogen pool drives plant tissue traits in alpine treeline ecotones

Abstract

Alpine treeline ecotone is one of the most vulnerable ecosystems and considered an ideal “warning line” for the monitoring of global climate change. However, shifts in the nutrient traits of plant biomass and soils of alpine treeline ecotones along the elevation gradient are poorly investigated. This study aimed to quantify nitrogen (N) and phosphorus (P) traits of Qinghai spruce (Picea crassifolia) and their driving factors for seven typical treelines (timberline - treeline - tree species line) covering 800 km southeast-to-northwest transect within the northeastern Tibetan Plateau. The biomass of plant tissue (leaves, twigs, and fine roots) and the N and P contents in them and soil nutrients (total and available N and P) were determined. Two mechanisms, which can drive the nutrient content in plants, namely, temperature-plant physiology and the sink (growth) limitation hypotheses were checked here. The N and P contents in plant tissue increased with the elevation, and temperature negatively impacted tissue N and N:P in all of the seven typical treelines, which supported both of the hypotheses. Soil N availability increased along the altitudinal gradient in 1.23 times. Soil TN explained 13.4% (p < 0.01), NO3–-N 10.1% (p < 0.05), and NH4+-N 8.6% (p < 0.05) of the variations in plant tissue N and P stoichiometry and biomass. Thus, soil N pools stimulated Qinghai spruce growth and have a strong effect on treeline dynamics.