Age and Size of Smith Firs at Treeline in Tibet 1700-2013

The most widespread response to global warming among alpine treeline ecotones is not an upward shift, but an increase in tree density. However, the impact of increasing density on interactions among trees at treeline is not well understood. Here, we test if treeline densification induced by climatic warming leads to increasing intraspecific competition. We mapped and measured the size and age of Smith fir trees growing in two treelines located in the southeastern Tibetan Plateau. We used spatial point-pattern and codispersion analyses to describe the spatial association and covariation among seedlings, juveniles, and adults grouped in 30-year age classes from the 1860s to the present. Effects of competition on tree height and regeneration were inferred from bivariate mark-correlations. Since the 1950s, a rapid densification occurred at both sites in response to climatic warming. Competition between adults and juveniles or seedlings at small scales intensified as density increased. Encroachment negatively affected height growth and further reduced recruitment around mature trees. We infer that tree recruitment at the study treelines was more cold-limited prior to 1950 and shifted to a less temperature-constrained regime in response to climatic warming. Therefore, the ongoing densification and encroachment of alpine treelines could alter the way climate drives their transitions towards subalpine forests.

高山林线交错带（alpine treeline ecotones）对全球变暖最普遍的响应并非向更高海拔抬升，而是林木密度的提升。然而，林线处林木密度增加对树木间相互作用的影响尚未得到充分阐释。本研究旨在检验气候变暖引发的林线致密化是否会加剧种内竞争（intraspecific competition）。我们对青藏高原东南部两处林线内的史密斯冷杉（Smith fir）进行了测绘，并测量了其树体尺寸与年龄。我们采用空间点格局分析（spatial point-pattern analysis）与协弥散分析（codispersion analysis），对1860年代至今以30年为间隔划分的年龄组内的幼苗、幼树和成树的空间关联与协变特征进行了描述。通过双变量标记相关分析（bivariate mark-correlation analysis），我们推断了竞争对树高生长与林木更新的影响。自1950年代起，两处林线均因气候变暖出现了快速的林木致密化现象。随着密度提升，成树与幼树或幼苗间在小尺度上的种内竞争显著加剧。幼树侵占（encroachment）对成熟树木的高生长产生了负面影响，并进一步抑制了成熟树周边的林木更新。我们推断，在1950年之前，研究区域林线的林木更新主要受低温限制；而随着气候变暖，其更新已转向受温度约束较弱的状态。因此，高山林线持续发生的致密化与幼树侵占现象，可能会改变气候驱动林线向亚高山森林（subalpine forests）转变的路径。