Using functional traits to understand community assembly, responses to drought, and restoration in tropical dry forests

Abstract

Tropical forests have been extensively degraded and deforested. Recent global restoration initiatives, such as the Bonn challenge, have emerged in an attempt to reverse these trends. To ensure these initiatives are effective, continued effort must be made to integrate ecological theory with restoration practice. It is imperative that some of this effort is focused on tropical dry forests (TDFs), as they are critically endangered and their restoration is understudied. Conservation efforts in NW Costa Rica have been effective in passively regenerating extensive areas of TDF, but the presence of degraded Vertisols in this region present a unique challenge and requires an active restoration approach. Furthermore, functional traits have been used to predict the outcomes of applied restoration of tropical wet forests, but their utility had not been evaluated in TDF. The goal of this research was therefore to determine how tree species in passively and actively restored TDFs use different functional strategies, to cope with stressful environmental conditions such as extreme drought and growing in degraded soils. In Chapters 1 and 2 I focused on determining how the functional strategies of tree species drive patterns in passively restored TDFs. In Chapter 1, I studied how abiotic and biotic gradients predict the landscape scale occurrence of TDF tree species, and I found that functional traits clarify community assembly mechanisms along these gradients in passively regenerating TDFs. In Chapter 2, I focused on the hydraulic responses of woody species to extreme drought and I found that trees and lianas have overlapping water-use strategies, but different in their leaf economic traits. In Chapters 3 and 4 I focused on using a similar functional trait-driven approach to actively restore TDF on degraded Vertisols. In Chapter 3, I implemented a 32 species trial to select native TDF species for Vertisol restoration. My results suggest that functional traits most predictive of survivorship and growth in TDF restoration correspond to how species capture carbon and tolerate drought. Finally, for Chapter 4, I used a 6 hectare Vertisol restoration project to conduct the first empirical test of how species with contrasting functional strategies perform at different TDF successional stages. The results from this study suggest that resource acquisition strategies of TDF tree species can be used to predict species’ responses to changes in microclimatic conditions over succession. Collectively these four studies contribute significantly to our understanding of how functional strategies of TDF tree species dictate their responses to drought and gradients in abiotic conditions in both passively and actively restored TDF.