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.高山林线交错带对全球变暖的最普遍响应并非林线上移，而是树木密度的提升。然而，林线处树木间的相互作用机制受密度升高的影响尚未得到充分解析。本研究旨在检验气候变暖诱导的林线密度增加是否会加剧种内竞争。我们对青藏高原（Tibetan Plateau）东南部两处林线内生长的史密斯冷杉（Smith fir）进行了定位，并测定了其个体大小与树龄。我们采用空间点格局分析（spatial point-pattern analysis）与协散分析（codispersion analysis），对1860年代至今按每30年为一个时段划分的年龄组（幼苗、幼树和成树）间的空间关联与协变关系进行了描述。通过双变量标记相关分析（bivariate mark-correlations），我们推断了竞争对树高生长与种群更新的影响。自1950年代起，两处林线均因气候变暖出现了快速的密度升高现象。随着密度升高，成树与幼树或幼苗之间的小尺度种内竞争显著加剧。林线侵入过程不仅抑制了成树的高生长，还进一步降低了成熟林木周边的幼苗更新量。我们推断，本研究林线的树木更新在1950年前主要受低温限制，而在气候变暖的影响下，其更新已转向受温度约束更弱的模式。因此，高山林线持续发生的密度增加与林线侵入过程，可能会改变气候驱动其向亚高山森林群落演替的路径。