Document Type

Thesis

Degree

Master of Science

Major

Biology

Date of Defense

2-11-2011

Graduate Advisor

Dr. Amy Zanne

Committee

Jørgensen, Peter M.

Jiménez, Iván

Abstract

A long-standing question in ecology is how so many tree species can coexist. New insight into assembly processes has been gained through functional traits that influence fitness. Such traits include the maximum height and diameter of a species as these describe the plant’s ability to compete for light. Additionally, specific leaf area (SLA) describes the amount of leaf area for light capture per unit of biomass invested. SLA is one of the easiest traits to measure, but it depends on access to fresh leaf material which is not possible for samples from remote areas or historical collections e.g. herbaria. The study examined community assembly patterns based on species functional tratis in a species-rich tropical dry forest at the Madidi National Park (MNP) of Bolivia. In my first chapter, a protocol to predict SLA for fresh leaves from dry leaves was developed. On the same leaf, area was repeatedly measured for fresh and dried leaves to generate four general mixed effects models, varying in their inclusion of the position in the crown where the leaf develops. The accuracy of the models was tested on leaves from an oak-hickory forest in USA. Both models performed well and are readily applicable to other datasets. A protocol for studies predicting SLA from dry leaves was developed. In my second chapter, I investigated the distribution of trait values at plots of different sizes to understand processes that lead to different species assemblages. Deterministic (habitat filtering and competitive exclusion) and stochastic processes are potential drivers of species coexistence in assemblages. The importance of these non-exclusive processes in structuring assemblages at different scales remains unclear. I compared the trait dispersion of SLA (using models from chapter 1), maximum height, and maximum diameter of observed versus null species assemblages with metrics sensitive to deterministic processes. I found evidence for deterministic processes structuring species assemblages in the MNP. Competitive exclusion had greater importance at small grain sizes. Habitat filtering had greater importance at large grain sizes. Ecologically, the results indicate that stabilizing processes promote patterns of species diversity and co-existence in a species-rich tropical dry forest in Bolivia.

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