Ecological characteristics of Abies georgei population at timberline on the north-facing slope of Baima Snow Mountain, Southwest China
白马雪山阴坡林线长苞冷杉(Abies georgei)种群结构特征

Abies georgei is one of the endemic trees at timberline on the Qinghai-Tibetan Plateau, but little research has been done about its timberline population. Understanding the ecological characteristics is necessary to understand its adaptation strategies and to probe into the causes and dynamics of timberline pattern in the eastern of Qinghai-Tibetan Plateau. One large sample plot was set up for field study at timberline above 4400 m on the north-facing slope of Baima Snow Mountain in the northwest of Yunnan Province, covering an area of 0.8 hm2. Height, DBH (diameter at breast high) or root-collar diameter of seedlings and saplings, and position were recorded for each individual. The size and DBH structure, static life table, survival rate, mortality and hazard rate, and spatial point pattern were further analyzed. The population showed a growing trend, with a large proportion of seedlings and saplings. Number of individuals decreased with increase of DBH, and the densities followed in the order: seedlings>saplings>trees. The survival curve can be described by Deevy-III. Mortality rate curve showed similar dynamics to hazard rate curve. The population had two distinct peaks of mortality: age I (seedlings, DBH = 0-5 cm) and age V-VI (DBH = 20-30 cm). The highest mortality rate of seedlings was due to severe climate at timberline such as low-temperature, high solar radiation and heavy snow. Physiological drought and frozen soil also harmed seedlings. As the saplings grew up, the interspecific and intraspecific competition for space, sunlight and nutrition became stronger and resulted in another mortality peak. Through SPPA (spatial point pattern analysis), individuals of all age-classes showed uniform, random or clumped distribution with the change of scales, however, they had mainly clumped distribution. Moreover, the relationships between individuals in different age-classes were all significantly correlated with each other. Coming from the soil seed bank, and facilitated by the microsites of tree islands, seedlings showed highest clumping intensity and scale.