长白山北坡林线岳桦种群空间分布格局

许多自然林线具有的趋同特征之一即乔木树种高生长受限,常演化为矮曲状或类似于灌木的形态(即树种的灌木型)占据高山植被带,因此研究林线树种乔木型与灌木型的结构、功能差异有助于进一步理解林线形成的原因。种群分布格局作为种群相对位置定量化描述的基本特征,可以表征物种对环境适应性选择的结果,反应生态过程的综合作用。利用点格局方法,研究长白山北坡林线岳桦种群各生活史阶段、两种生活型的分布格局,结果表明,长白山北坡林线岳桦树高生长受到限制,1.5—3.0m是一个关键的树高生长阶段;相比于老树、中树,幼苗和灌木型岳桦更为均匀,对空间的异质性选择更弱;林线岳桦发育过程中,存在一个生活型分离的重要阶段。此外,相对于老树,灌木型分布更为均匀,表明低矮、多枝这种相对紧凑的生活型更适宜在过渡带生存,乔木型岳桦和灌木型岳桦可能代表着不同的生存策略。

Spatial patterns of a treeline Betula ermanii Cham. population on the north slope of Changbai Mountain

Treeline as one of the most important indicators of climate change has been extensively researched for the past years. Elucidation of past and present treeline formation can help us to understand how rising temperatures influence the treeline pattern and to predict treeline response to future climate change. On the other hand, global treeline positions have been considered to advance or retreat because of human disturbance and/or changes in local environmental factors combined with climate warming. Every natural treeline has a common convergence characteristic, namely, a limitation of tree height growth because of low temperature or other stress conditions, and this causes a gradual change to elfin or shrubby trees along an elevation gradient. Thus, elucidation of the functional differences between trees and alpine shrubs will facilitate an understanding of the alpine treeline. The spatial distribution pattern is an indicator of the selective adaption of a population to a specific environment, and it is considered to be a crucial character for describing the relative spatial location of the population. The extent to which plant individuals are aggregated can reflect the dispersal strategies of a species and determine how this species utilizes resources. The point process theory provides several statistics containing Riply''s K-function and the paired correlation function pcf or g(r)] to analyze point patterns at different scales. Investigation of the spatial point pattern of a species at the treeline ecotone will provide a valuable insight into ecological processes, for example, the way in which individuals occupy this habitat and the strategies whereby these individuals utilize resources. In the present study, we evaluated the spatial distribution of a Betula ermanii population growing in a 0.64 hm² permanent sample plot at the treeline ecotone on the northern slope of Changbai Mountain, northeastern China. On the basis of the number of branches and tree height, we defined three life stages (adult, medium-sized, and sapling) in single-stemmed B. ermanii individuals and one shrubby shape in multi-stemmed B. ermanii individuals. We subsequently analyzed correlations between these different life stages and tree shape to verify whether life stage and tree shape were related to the spatial pattern. Results showed that 7.5% of the B. ermanii individuals exhibited a tree shape, whereas 32.0% exhibited a shrubby shape except the rest medium-sized and sapling ones. The critical limitation of tree height growth for B. ermanii is 1.5-3.0 m. Larger trees were rarely observed in the study area, indicating that this species is markedly constrained by height growth limitation at the treeline ecotone. We observed that adult and medium-sized trees were clustered in a small area with appropriate micro-topography and/or soil conditions for tree survival, for example, high-quality soil and concave grounds with seasonal snow drift. On the other hand, saplings and shrubby trees showed a weaker tendency for clustering and exerted weaker selective effects on the spatial pattern. Our results imply that the most suitable tree architecture for B. ermanii at the treeline ecotone is a shrubby shape and that an important period of life stage division may exist during the process of tree establishment and development. We further showed a higher degree of clustering among shrubby-shaped individuals than among tree-shaped individuals, implying that a multi-stemmed life shape with low stature is advantageous for survival of B. ermanii in the treeline ecotone. Taken together, our findings provide a valuable insight into the survival strategies of B. ermanii under local environmental conditions.