Abstract: The treeline of Betula ermanii population had an obviously expanding trend on the northern slope of the Changbai Mountains in recent 50 years. In order to examine the changing mechanism of treeline, in this study, based on the data from Tianchi meteorological station (2624 m altitude) regarded as a referencing site (1953-2007) and the air temperature field measurement, using two eco-climatic preferences which were warmth index (WI) and humid index (HI), the ecotone boundary between Betula ermanii and tundra on the northern slope of the Changbai Mountains was determined by assurance ratio. The results were calculated by using the thresholds at 16 ℃·month and 68.8 mm/℃·month, and they showed that the two treelines were at 1975-2460 m and 1584-2231 m respectively. The interaction influence of WI and HI changes on treeline was not significant (p>0.05). There was a significant negative correlation between WI and HI change (p<0.01). In addition, there was a negative correlation between two treelines changes (r = -0.11<0). The results of correlation and interaction analysis indicated that the two treelines were unable to reach their potential height due to the asynchronous changes of heat and water. Therefore, the range of Betula ermanii treeline was estimated to be 1975-2231 m. The wavelike rise in Betula ermanii treeline (slope was positive, R² <0.8) was discussed by trend and wavelet analyses. The fluctuation process was simple in lower treeline (the characteristic of catastrophe point). However, the complex fluctuation process in upper treeline indicated that fluctuation of treeline shift increased owing to large stresses.

Key words: assurance ratio, eco-climatic preferences, northern slope of Changbai Mountains, treeline shift