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

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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.