Abstract: Taking Betula ermanii forests distributed at the altitudes 1800-2050 m on the northern slope of Changbai Mountains as test objects, and using leaf carbon isotope content (δ¹³C) as an indicator of B. ermanii water use efficiency (WUE), this paper studied the effects of altitudinal gradient on the WUE of B. ermanii forests in the area. With the increase of altitude, the soil volumetric water content  (VMC) and the leaf mass per area  (LMA) of B. ermanii increased significantly, while the leaf water content  (LWC) and soil temperature  were in adverse. There was a significant positive correlation between leaf δ¹³C and altitude, with the increment of leaf δ¹³C being 1.013‰·(100 m)^-1, and the leaf δ¹³C was positively correlated with soil VWC and LMA but negatively correlated with soil temperature and LWC. Temperature was not the sole limiting factor for the distribution of treeline in Changbai Mountains, whereas the physiological drought of B. ermanii during its growing season caused by the different water and heat conditions and their interactions along the altitudinal gradient could be the other limiting factors.

Key words: altitude, water stress, treeline, carbon isotope discrimination (δ¹³C), Changbai Mountains