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

University of Minnesota Ph.D. dissertation. December 2017. Major: Plant and Microbial Biology. Advisor: Jennifer S. Powers. 1 computer file (PDF): xiv, 205 pages.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

Effects of landscape structure on restoration success in tropical premontane forest

Reversing large-scale habitat degradation and deforestation goes beyond what can be achieved by site-level ecological restoration and a landscape ecology perspective is fundamental. Here we assess the relative importance of tree cover and its configuration on forest-dependent birds and late-successional tree seedlings in restoration sites in southern Costa Rica. The abundance and species richness of birds increased in landscapes with more corridors, higher tree cover, and lower levels of fragmentation, highlighting the importance of riparian corridors for connectivity, and continuous tree cover as suitable habitat. Landscape variables affected abundance and species richness of seedlings similarly, but effects were weaker, possibly because seedlings face establishment limitation in addition to dispersal limitation. Moreover, the scale of landscape effects on seedlings was small, likely because proximal individual trees can significantly influence recruitment in restoration plots. Results underscore the importance of incorporating landscape-level metrics to restoration projects, as knowing the extent, and how the landscape may affect restoration outcomes can help to infer what kind of species will arrive to restoration plots.ISSN:2045-232

Assisted restoration interventions drive functional recovery of tropical wet forest tree communities

Choosing appropriate forest restoration interventions is challenging. Natural regeneration can rapidly facilitate forest recovery in many situations. However, barriers such as dispersal limitation and competition with non-native species can require assisted restoration approaches to facilitate plant community recovery. We used a study that has directly compared the outcomes of tropical wet forest restoration interventions across 11 replicate sites in southern Costa Rica. Within this framework, we examined the functional recovery trajectories of recruiting tree sapling communities across a gradient of restoration interventions including low (natural regeneration), intermediate (applied nucleation), and high (plantation) initial resource-investment, which we compared to remnant reference forest. We collated leaf and stem functional traits for tree species that comprised the bulk of recruiting saplings, then determined how community-weighted trait means and functional diversity metrics changed over a decade across treatments. Results show that assisted restoration approaches (applied nucleation, plantation) sped the development of more functionally diverse tree communities, more than tripling the functional richness (FRic) of recruiting communities when compared to natural regeneration. However, functional dispersion (i.e., the trait range of dominant species) was equivalent across interventions, and between 28 and 44% lower than remnant forest, indicating that increases in FRic under assisted restoration were driven by species recruiting in low abundances (<10 individuals across treatments). Recruits in assisted restoration treatments also had 10-15% tougher, less-palatable leaves, and leaves were even tougher in reference forest, which could be driven by increasing herbivory pressure along the gradient of interventions. Results show that tracking simple metrics such as species richness can mask a more mechanistic understanding of ecosystem recovery that is elucidated by taking a functional trait-driven approach toward evaluating outcomes. For example, our work identified a paucity of dense-wooded species recruiting across restoration interventions, wood density was 11-13% lower in restoration treatments than reference forests, underscoring such species as prime targets for enrichment planting. Overall, findings suggest that assisted restoration can catalyze the functional recovery of naturally recruiting tree communities in landscapes that are slow to recover naturally and highlight the importance of evaluating how different components of functional diversity shift over time to fully understand restoration outcomes.ISSN:2624-893

Below-ground traits mediate tree survival in a tropical dry forest restoration

Reforestation is one of our most promising natural climate solutions, and one that addresses the looming biodiversity crisis. Tree planting can catalyse forest community reassembly in degraded landscapes where natural regeneration is slow, however, tree survival rates vary remarkably across projects. Building a trait-based framework for tree survival could streamline species selection in a way that generalizes across ecosystems, thereby increasing the effectiveness of the global restoration movement. We investigated how traits mediated seedling survival in a tropical dry forest restoration, and how traits were coordinated across plant structures. We examined growth and survival of 14 species for 2 years and measured six below-ground and 22 above-ground traits. Species-level survival ranged widely from 7.8% to 90.1%, and a model including growth rate, below-ground traits and their interaction explained more than 73% of this variation. A strong interaction between below-ground traits and growth rate indicated that selecting species with fast growth rates can promote establishment, but this effect was most apparent for species that invest in thick fine roots and deep root structures. Overall, results emphasize the prominent role of below-ground traits in determining early restoration outcomes, and highlight little above- and below-ground trait coordination, providing a path forward for tropical dry forest restoration efforts. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.ISSN:0962-8436ISSN:1471-2970ISSN:0080-462

Leaf habit affects the distribution of drought sensitivity but not water transport efficiency in the tropics

Considering the global intensification of aridity in tropical biomes due to climate change, we need to understand what shapes the distribution of drought sensitivity in tropical plants. We conducted a pantropical data synthesis representing 1117 species to test whether xylem-specific hydraulic conductivity (KS), water potential at leaf turgor loss (ΨTLP) and water potential at 50% loss of KS (ΨP50) varied along climate gradients. The ΨTLP and ΨP50 increased with climatic moisture only for evergreen species, but KS did not. Species with high ΨTLP and ΨP50 values were associated with both dry and wet environments. However, drought-deciduous species showed high ΨTLP and ΨP50 values regardless of water availability, whereas evergreen species only in wet environments. All three traits showed a weak phylogenetic signal and a short half-life. These results suggest strong environmental controls on trait variance, which in turn is modulated by leaf habit along climatic moisture gradients in the tropics.ISSN:1461-023XISSN:1461-024