Objective To verify the formation mechanism of the alpine treeline and explore the limiting factors of the upper limit of tree distribution.

MethodsIn the low altitude coniferous and broad-leaved mixed forest (3500 m), suitable altitude area (3900 m) and high altitude treeline transition zone (4300 m) in Sygera Mountain, the contents of non-structural carbohydrates and C, N, and P in leaves, branches, stems and roots of Abies georgei var. smithii as well as were measured both in the growing season (early July) and non-growing season (end of November).

ResultsThe results showed that the non-structural carbohydrate sources and sinks of Abies georgei var. smithii were stable in different growing seasons, and there was no insufficient carbon supply. There were little effects of altitude on the non-structural carbohydrates and their components in Abies georgei var. smithii . There were significant differences in NSC content among organs, and the physiological activities and functions of leaves, branches, trunks, and roots were significantly different in different growing seasons. In the non-growing season, non-structural carbohydrates and their components were significantly higher than those in the growing season. In the non-growing season, higher soluble sugars and NSC contents enabled the vegetation to resist cold temperature and other harsh conditions. The content of NSC in Abies georgei var. smithii increased with the increase of altitude, which supported the hypothesis “growth limitation” .

Conclusion The growth of Abies georgei var. smithii is more restricted by plant nutrients than by carbon.