Dissertation Research: The Influence of Isolated Trees on Forest Succession on a Pasture in the Tropical Wet Lowlands of Costa Rica

9423554个霍尔维茨牧场生态系统现在覆盖了新热带地区的大片地区。这些牧场上的森林演替情况知之甚少，但初步证据表明，牧场上孤立的树木可能成为定居森林物种的“建立中心”。草地孤立乔木的演替格局可以用演替的成核模型来预测。本论文研究的目的是了解孤立的树木如何促进低地热带湿润森林生活区废弃牧场上的森林恢复。该项目将定量研究露天牧场和孤立树木树冠下的木本植物幼苗和种子雨的种类组成，并将研究孤立树种、树木大小和与种子来源的距离对种子雨和四个孤立树种的树冠下木本苗木物种组成的相对重要性。此外，该项目将试图通过将牛排除在孤立的树木地块之外，来确定牛的食草性疾病对孤立树木下木本植物再生的影响，并将试图将孤立树木分类群下不同种子雨的影响与其他可能不同地影响木本植物苗木建立的因素分开。后一项实验将通过收集每个分类群下的种子雨并将其放置在其他分类群的树冠下来完成。最后，本项目将对隔离树种下的光环境进行描述性研究。该项目将扩大我们对热带废弃牧场演替模式的了解，并扩大我们对干扰对热带森林生态系统影响的了解。此外，该项目将能够提出在废弃牧场上有管理地恢复森林的战略。

9423554 Horvitz Pasture ecosystems now cover huge areas in the neotropics. Forest succession on these pastures is poorly understood, but preliminary evidence suggests that isolated trees in pastures may serve as "establishment foci" for colonizing forest species. Patterns of succession stemming from pasture isolated trees can be predicted with a nucleation model of succession. The aim of this dissertation research is to develop understanding of how isolated trees facilitate forest recovery on an abandoned pasture in a lowland tropical wet forest life zone. The project will quantitatively examine the species composition of woody seedlings and the seed rain in open pasture and beneath the canopies of isolated trees and will examine the relative importance of isolated tree species, tree size, and distance to seed sources on seed rain and the woody seedling species composition found under the canopies of four isolated tree taxa. In addition, the project will attempt to determine the effect of cattle herbivory on woody species regrowth under isolated trees by excluding cattle from isolated tree plots, and will also attempt to separate the effects of differential seed rain beneath the isolated tree taxa from other factors that may differentially affect woody seedling establishment. This latter experiment will be done by collecting seed rain under each taxa and placing it under the canopies of the other taxa. Finally, this project will descriptively study the light environment under the isolated tree taxa. This project will expand our understanding of successional patterns in tropical abandoned pastures and also our knowledge on the effects of disturbance on tropical forest ecosystems. In addition, this project will be able to suggest strategies for managed forest recovery on abandoned pastures.