Formation mechanisms of the alpine Erman's birch (Betula ermanii) treeline on Changbai Mountain in Northeast China

Treeline pattern is an important consideration in exploring the general mechanisms controlling the response of treelines to climatic change. However, most of the present conclusions were derived from evergreen and/or conifer treeline, it is still not clear about the deciduous treeline. This study analyzed concentrations of non-structural carbohydrates (NSC) and their components (total soluble sugars and starch) in tree tissues of the deciduous species Erman's birch (Betula ermanii) at four points along an elevational gradient ranging from 1,908-2,058 m a.s.l at the end of the growing season on Changbai Mountain in Northeast China. The mean 10-cm soil temperature of 8.2 A degrees C under trees across the 129-day growing season at the treeline in this region was higher than that of the average threshold temperature found at treeline positions in the global and China's climate studies. However, altitudinal trends of NSC concentrations increased significantly in all tissue types along the altitudinal gradients, revealing no depletion of carbon reserves at the treeline on Changbai Mountain. At the same time, the pronounced variation of delta C-13 in leaves and aged branches suggested that low temperature and water stress may simultaneously be operating at high altitudes to restrict the growth and NSC accumulation in trees above the treeline. In light of the above, we conclude that treeline formation on Changbai Mountain is no carbon depletion at the end of growing season, and most likely the result of sink limitation reflecting the combined effects of low temperature and water stress that determined the actual position of the treeline.