Unimodal Tree Size Distributions Possibly Result from Relatively Strong Conservatism in Intermediate Size Classes

Tree size distributions have long been of interest to ecologists and foresters because they reflect fundamental demographic processes. Previous studies have assumed that size distributions are often associated with population trends or with the degree of shade tolerance. We tested these associations for 31 tree species in a 20 ha plot in a Dinghushan south subtropical forest in China. These species varied widely in growth form and shade-tolerance. We used 2005 and 2010 census data from that plot. We found that 23 species had reversed J shaped size distributions, and eight species had unimodal size distributions in 2005. On average, modal species had lower recruitment rates than reversed J species, while showing no significant difference in mortality rates, per capita population growth rates or shade-tolerance. We compared the observed size distributions with the equilibrium distributions projected from observed size-dependent growth and mortality. We found that observed distributions generally had the same shape as predicted equilibrium distributions in both unimodal and reversed J species, but there were statistically significant, important quantitative differences between observed and projected equilibrium size distributions in most species, suggesting that these populations are not at equilibrium and that this forest is changing over time. Almost all modal species had U-shaped size-dependent mortality and/or growth functions, with turning points of both mortality and growth at intermediate size classes close to the peak in the size distribution. These results show that modal size distributions do not necessarily indicate either population decline or shade-intolerance. Instead, the modal species in our study were characterized by a life history strategy of relatively strong conservatism in an intermediate size class, leading to very low growth and mortality in that size class, and thus to a peak in the size distribution at intermediate sizes.

林木大小分布长期以来备受生态学家与林学家关注，因其可反映种群核心的统计动态过程。过往研究普遍认为，林木大小分布常与种群动态趋势或耐阴程度相关联。本研究针对中国鼎湖山南亚热带森林20公顷样地内的31个树种，对上述关联假设进行了验证。这些树种在生长型与耐阴性方面差异显著，研究使用了该样地2005年与2010年的森林普查数据。结果显示，2005年有23个树种呈现反J型大小分布，另有8个树种呈现单峰型大小分布。平均而言，单峰型树种的种群补充速率低于反J型树种，但在死亡率、种群人均增长率以及耐阴性方面均无显著差异。研究将实测的大小分布与基于实测的大小依赖型生长和死亡率所预测的平衡分布进行了对比。结果表明，无论是单峰型还是反J型树种，实测大小分布的形状与预测的平衡分布整体一致；但多数树种的实测分布与预测的平衡大小分布之间均存在统计学上显著且具有重要生态学意义的数量差异，这意味着这些种群并未处于平衡状态，该森林正随时间发生动态变化。几乎所有单峰型树种均呈现U型大小依赖型死亡率和/或生长函数，其死亡率与生长的拐点均位于接近大小分布峰值的中等大小径级区间。本研究结果证实，单峰型大小分布并非必然代表种群衰退或耐阴性不足。与之相反，本研究中的单峰型树种其生活史策略表现为在中等大小径级区间具有较强的保守性，使得该径级内的生长与死亡率均极低，最终在中等大小处形成了大小分布的峰值。