Responses of Tree Islands to Air Temperature Change at Treeline on North-Facing Slopes of the Changbai Mountains

Abstract

A total of 60 scattered tree islands from seven typical plots were collected in the upper treeline on the north-facing slopes of the Changbai Mountains. We selected eight ecological variables related to air temperature, which were simulated based on measured air temperature data and the records at the Tianchi meteorological station. Annual stem recruitment of the Betula ermanii (Ermans Birch) population was calculated using tree-ring analysis and field observations. We linked these ecological variables with annual stem recruitment from 1953 to 2008 to examine the relationship between population dynamics and air temperature change. Our results showed that the mean annual rate of air temperature increase was 0.045° C/yr between 1953 and 2008, which was far higher than that of global average warming, suggesting that the study area may be more sensitive and vulnerable to climate change. Different propagation forms and annual stem recruitment of the Betula ermanii population responded significantly to the increase of air temperature. The population first occupied a point niche using the shrub life form during the period 1953-1988, when air temperature increased at a slow pace (0.007° C/yr on average). In contrast, the population expanded its niche rapidly using a single-trunk life form during the period 1988-2008, when air temperature increased at a much faster pace (0.11° C/yr on average). Under global warming, the population expansion and the domain enlargement of Betula ermanii led to a treeline shift. The population has continued to vary its physiological traits to adapt to air temperature change.

Keywords:

• tree island
• diameter-class structure
• treeline shift
• air temperature change
• population dynamics
• Changbai Mountains
• warming