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最普遍的响应并非向上迁移，而是林木密度的提升。然而，林线处林木密度增加对树木间种内互作的影响尚未得到充分阐明。本研究旨在验证气候变暖驱动的林线加密是否会加剧种内竞争。我们对青藏高原东南部两条林线内生长的史密斯冷杉（Smith fir）的个体大小与年龄进行了测绘与测量。将1860年代至今的个体按30年年龄组划分为幼苗、幼树和成树，采用空间点格局分析（spatial point-pattern analysis）与协色散分析（codispersion analyses）描述三者间的空间关联与协变关系。通过双变量标记相关分析（bivariate mark-correlations），可推断竞争对林木高生长与更新的影响。自1950年代以来，两处样地均因气候变暖出现了快速的林线加密现象。随着密度提升，成树与幼树或幼苗间的小尺度竞争逐渐加剧。幼树侵入对成熟林木周边的高生长产生了负面影响，并进一步抑制了成树周围的林木更新。我们推断，在1950年之前，本研究样地的林木更新主要受低温限制；而在气候变暖的影响下，更新过程已转向温度约束更弱的状态。因此，高山林线持续的加密与幼树侵入过程，可能会改变气候驱动林线向亚高山森林转变的路径。