方枝柏(Sabina saltuaria)是青藏高原特有林线树种。对白马雪山阳坡海拔4 390 m以上林线0.42 hm²样地方枝柏种群进行每木调查,分析了种群结构、数量动态及空间分布格局。结果表明:1)在种群生物学特征、环境因子及人为干扰等综合影响下,林线区方枝柏幼苗数量非常少。幼树在种群中占了很大比重,种群个体数随径级的增加而逐渐减少,密度为成年树>幼树>幼苗;2)种群存活曲线介于Deevey-Ⅱ型和Deevey-Ⅲ型之间,种群结构更接近稳定型。死亡率曲线和消失率曲线都在龄级Ⅲ出现一个高峰,可能是由于方枝柏处于青壮年期,个体对营养空间的需求不断增大,对空间、光照和养分等生存因子的激烈竞争引起的自疏过程,导致死亡率有所上升;3)方枝柏种群各龄级的空间格局基本上是聚集型,随龄级增加,聚集尺度和强度都增加;各龄级关系密切,都呈显著正相关,其中幼苗和成年树的相关性最强,幼苗和幼树相关性最弱。林线方枝柏种群空间分布格局是种群生物学特性、环境条件及人为干扰等因素共同作用的结果,这种格局有利于整个种群的生存和发展,也是高山生态系统恶劣生境中种群的一种适应对策。该研究采用森林罗盘仪与测距仪相结合进行每木定位的方法,准确度高且大大降低工作强度,是高海拔区恶劣条件下开展类似工作的推荐之法。研究结果也再次体现了空间点格局分析方法在格局研究中的有效性和优越性。

Aims Alpine timberlines are ecotones highly sensitive to disturbances and environmental changes that have become a major focus of global climate change research. Sabina saltuaria is an endemic timberline tree in the Qinghai-Tibetan Plateau. It occurs mainly on south-facing slopes and may be useful in ecological restoration of the high-altitude ecosystem. Understanding its ecological characteristics is necessary for understanding its adaptation strategies, as well as the causes and dynamics of timberline patterns in the region.

Methods We established one 0.42 hm² sample plot at timberline >4 390 m on the south-facing slope of Baima Snow Mountain, Northwest Yunnan Province. We recorded height, DBH (diameter at breast height) or diameter at base for seedlings and saplings, and location of each individual. We used a compass and electronic distance-measuring tool to record the location of each individual. We analyzed the size and DBH structure, time-specific life table, survivorship rate, mortality and hazard rate, spatial point pattern of individuals in each age-class and the co-relationships between different age classes.

Important findings Few seedlings occur at timberline in the area of our field survey (about 17 ind·hm^-2, only 1/6 and 1/14 of that of saplings and trees, respectively). Densities declined from trees to saplings to seedlings, and the number of individuals decreased gradually with greater DBH. The survivorship curve was between Deevy type Ⅱ and Ⅲ. The mortality rate curve was similar to the hazard rate curve; both exhibited a peak of mortality at 6-9 cm DBH). Spatial point pattern analysis indicated that most individuals of different age classes had a clumped distribution at different scales (the older the trees, the more aggregative the distribution pattern); however, seedlings had a random distribution at the scale of <14 m. Also the relationships among individuals in different age classes were all significantly correlated with each other. The associations were more significant between seedlings and trees.