Environmental Heterogeneity and Woody Species Diversity in Low-elevation Tropical Secondary Forests

研究了影响热带低海拔次生林演替过程中树木更新性质和多样性的基本生态过程。假设演替过程中幼苗的补充在很大程度上取决于资源的可用性和异质性。随着演替林龄的增长，林冠和林隙结构的变化增加了地面的光有效性和光异质性。预计再生池的物种丰富度会因整体光利用率和光环境的多样性而增强，这些光环境的多样性代表了老的、结构异质的林分。与空隙或动物活动有关的扰动土壤斑块的可用性进一步加强了幼苗的招募。这种跨学科方法将热带森林演替模式的生态系统级研究与产生这些模式的林内过程的详细分析以及对代表性物种的生理和生长反应的研究结合起来。森林组成和结构的变化在幼苗物种丰富度和林内环境异质性中被记录在哥斯达黎加东北部次生林和原生林前雨林的时间序列中。将沿着这一时间序列对选定森林的光有效性、土壤扰动和凋落物覆盖的空间变化进行详细测量。在这些林分中，将通过取样研究地点的土壤种子库，并通过比较幼苗和树苗多样性与种子源距离的关系，来评估距离种子源对繁殖体多样性的影响。研究人员将在15-25年生林分中，通过调节光效和土壤扰动，研究资源有效性和异质性对幼苗建立和生长的影响。在重复的单因素（4个处理）设计中，我们将通过创造或大或小的冠层间隙和去除林下灌木来控制光照，从而评估对自然幼苗招募和物种丰富度的影响。在林隙和封闭林冠场地进行的其他操作研究将考察光和土壤干扰对自然幼苗招募和选定物种种子萌发和存活的相互作用。利用实验和野外观测数据，利用森林动态的空间机制模拟模型，模拟林分结构和光效对幼苗丰度和多样性的影响。为了评价树种间差异对次生林林冠层透光性的影响，本文对林分中5种丰富的冠层树种进行了冠层结构和透光性的比较研究。该模型还将模拟空间异质性对幼苗再生多样性的影响，该模型将基于测量的生态生理和生长响应。模型输入将基于时间序列中有代表性的站点的现场数据。这一建模工作将在热带森林资源可得性的林分水平变化的背景下综合测量的生物体反应。项目成果将有助于维持潮湿热带森林生物多样性的基础生态研究，直接适用于热带森林恢复、次生林管理和可持续热带森林的研究。

This study addresses basic ecological processes that influence the nature and diversity of tree regeneration during secondary forest succession in low-elevation tropical forest. It is hypothesized that seedling recruitment during succession is strongly dependent on both resource availability and heterogeneity. Specifically, it is suggested that changes in canopy and gap structure, as successional forests age, increase both light availability and light heterogeneity at ground level. It is expected that species richness of the regeneration pool is enhanced by both overall light availability and the diversity of light environments represented in older, structurally heterogeneous stands. Seedling recruitment is further enhanced by availability of patches of disturbed soil associated with gaps or animal activity. This interdisciplinary approach integrates ecosystem-level studies of the patterns of tropical forest succession with detailed analyses of within-stand processes producing those patterns and studies of physiological and growth responses of representative species. Changes in forest composition and structure with be documented in the species richness of seedlings, and in within-stand environmental heterogeneity in a chronosequence of secondary and old-growth premontane rain forests of northeastern Costa Rica. Detailed measurements of the spatial variation of light availability, soil disturbance, and litter cover in selected forests will be performed along this chronosequence. The influence of distance to seed sources on propagule diversity will be assessed in these stands by sampling soil seed stocks in the study sites and by comparing the diversity of seedlings and saplings as a function of distance from seed sources. Investigators will experimentally examine the effects of resource availability and heterogeneity on seedling establishment and growth by manipulating light availability and soil disturbance in a series of 15-25 year old stands. Light will be manipulated by creating large and small canopy gaps and removing understory shrubs in a replicated single-factor (4 treatments) design in which we will assess the effects on natural seedling recruitment and species richness. Additional manipulative studies in gaps and closed-canopy sites will examine the interactive effects of light and soil disturbance on natural seedling recruitment and the germination and survival of seeds of selected species. Experimental and observational field data will be used to model the effects of stand structure and light availability of seedling abundance and diversity using a spatial, mechanistic, simulation model of forest dynamics. To evaluate the role of species-specific differences in canopy light transmission within secondary forest, a comparative study of canopy structure and light transmission will be conducted for five abundant canopy tree species in the stands. The model will also simulate the effect of spatial heterogeneity on the diversity of seedling regeneration, based on measured ecophysiological and growth responses of selected species. Model inputs will be based on field data from representative stands within the chronosequence. This modelling effort will integrate measured organismal responses in the context of stand-level changes in resource availability in tropical forests. Project results will contribute to basic ecological research on the maintenance of biological diversity in moist tropical forests, with direct applicability to studies of tropical forest restoration, secondary forest management, and sustainable tropical forestry.