Browsing by Subject "Tropical dry forest"

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    Assessing forest structure, biodiversity, and ecosystem functions between public and private tropical dry secondary forests, a case study in Guanacaste, Costa Rica
    (2014-08) McClellan, Moana L.
    For this dissertation, I was interested by how human decisions may affect forest ecology and, in turn, how those ecological effects may feedback to influence social systems. Humans are the dominant force driving global environmental changes, yet we are still working towards quantifying how humans affect ecosystem functions, environmental services, and biodiversity across varying environmental and human land use gradients. I conducted a case study in Guanacaste, Costa Rica to evaluate ecological differences in public and private tropical dry secondary forests and assessed what factors influenced private landowners' decisions to allow forest regeneration on their farms. For the forest study, I found that public and private forests did not differ significantly in integrative metrics of forest structure or biodiversity. Yet there were tree species compositional differences with tell-tale signs of a human signature: highly prized timber species were more abundant in public forest, suggesting high-grading in private forests and the most abundant species in private forests was a species favored and consumed by cattle, Guazuma ulmifolia. In the farmer land-use study, I found that landowners appreciated the multiple benefits and services that their forests provided for them and for society; however, landowners noted that market prices were the main factor affecting their farm land use decisions regarding production expansion or contraction. Finally, I was interested in conducting a combined assessment of ecological and human use traits that may differentially affect ecosystem functions in private versus public lands. I expected that the patchy forest characteristics on private lands would favor plant species that have low seed mass, fast relative growth rate, and high leaf N. I acquired functional plant trait data for the majority of tree species in the forest inventory analysis from international databases. I also created a human use plant trait summary for traits I expected to be less evident in private forests (`harvest traits' e.g. timber) and traits that I expected to be more evident private forests (`on farm use traits' e.g. ornamentals, living fences). Seed mass was the only trait that was significantly different between public and private forests: public forests had higher seed mass relative to private. Likely, this is due to differences in animal dispersal vector behaviors between continuous and patchy forest. I also found a trend of `on farm use traits' more evident in private forests, so the species that people select to use on their farms may be more abundant in nearby naturally regenerating forests. I found differences in species composition, seed mass, and `on farm use traits' between public and private forests that appear to be caused by differences in management yet, overall, these differences to not appear to influence ecological function between public and private forests.
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    Using functional traits to understand community assembly, responses to drought, and restoration in tropical dry forests
    (2017-12) Werden, Leland Kendall
    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.