白马雪山阴坡林线长苞冷杉(Abies georgei)种群结构特征

Ecological characteristics of Abies georgei population at timberline on the north-facing slope of Baima Snow Mountain,Southwest China

长苞冷杉(Abies georgei)是青藏高原特有林线树种。对白马雪山阴坡海拔4 400 m以上林线0.8 hm2样地长苞冷杉种群进行每木调查,分析其种群结构、数量特征及空间分布格局。结果表明:(1)结构呈典型金字塔型,幼苗和幼树在种群中所占比重大,表现为增长型种群,种群个体数随径级的增加而减少,密度为幼苗>幼树>成年树;(2)存活曲线接近Deevy-III型,高径级种群趋于稳定,种群具两个死亡高峰,低径级种群尤其幼苗死亡率高达90%,这是林线区的气候条件如低温、强光照、积雪及冬季冻害等综合作用的结果,种群另一死亡高峰出现于V^VI龄级,种内和种间对空间、光照和养分等生存因子的激烈竞争引起自疏,导致死亡率再度上升;(3)长苞冷杉种群各龄级空间点格局在不同尺度上表现为聚集、随机和均匀分布,以聚集分布为主,由于幼苗来源于种子库且依赖成年树的微生境,幼苗聚集强度和尺度都最大;各龄级关系密切,在不同尺度上表现出显著的相关性。

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 hm^2. 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.