Our goal is to contribute to a mechanistic understanding of the key drivers of biodiversity and how those drivers are being altered by human activities
Research in the lab addresses both fundamental and applied questions in the field of plant community ecology, with an emphasis on understanding the drivers of biodiversity in both intact and human-altered tropical forests. Much of our research focuses on processes influencing dynamics of seedlings and saplings, because these early life stages act as a bottleneck in the life cycle of trees and serve as a strong filter on the composition of future plant communities. In addition, spatial and temporal patterns of juvenile survival play a major role in the maintenance of tree species diversity and shape the response of forests to disturbance. We use a variety of approaches to address our research questions, including long-term observational studies, field and greenhouse experiments, and computer simulations. See below for details about current areas of research.
Species coexistence mechanisms in diverse plant communities
How competing species can coexist within highly diverse communities is a long-standing question in ecology. For the past two decades, we have been conducting long-term monitoring of seedling dynamics in central Panama to evaluate potential mechanisms of species coexistence that maintain the high diversity of tropical tree communities. In particular, we focus on two key mechanisms: species' abiotic niche partitioning (i.e., species differ in habitat or resource use, reducing interspecific competition and facilitating coexistence) and conspecific negative density dependence (i.e., individuals have lower survival when surrounded by neighbors of their own species, preventing any one species from dominating). Through a recently funded NSF CAREER award, we are augmenting existing seedling data sets with information on plant functional traits related to species life history strategies. We will also work with collaborators to incorporate our long-term data into novel forest simulation models. This data-model integration will significantly advance understanding of the mechanisms shaping tropical tree communities and will provide a framework for predicting future changes in tropical forest diversity and species composition.
Plant-enemy interactions in tropical forests
Research in the lab also focuses on the integral role that insect herbivores and fungal pathogens play in shaping plant communities. In particular, host-specific natural enemies have been shown to be drivers of conspecific negative density dependence, and thus can help maintain plant diversity. In recent years, we have been examining the role of soil-borne pathogens in driving patterns of species abundance and diversity in tropical forests. Through an NSF-funded collaboration with genomicists, microbial ecologists, and theoreticians, we have been examining how tropical tree species vary in resistance gene diversity, and the implications for pathogen-mediated, conspecific negative density-dependent seedling mortality. Through this project, we found some of the first evidence for within-species specialization by pathogenic soil microbes in a wild tropical tree species in Panama. In another study, we found that interactions between plants and their natural enemies are altered in fragmented tropical forests in southern India, such that the diversifying effects of enemies are lost near forest edges. Currently, we are developing a new project with collaborators in Panama that will involve field and greenhouse experiments to examine how plant-pathogen interactions shift over the course of tropical forest regeneration following deforestation and to test the hypothesis that pathogens interact with changing abiotic factors (i.e., light, moisture) to drive successional dynamics in plant communities.
Tropical tree species responses to drought
Research in the lab also focuses on the role of water availability and drought events in shaping tropical tree communities. Previously, we found high variation in drought resistance among tropical tree species and showed that this variation explains differences in species distributions, abundances, and dynamics across both local and regional moisture gradients in central Panama. Currently, we are examining within-species variation in drought resistance in tropical trees. Understanding the degree to which different populations of the same species vary in response to drought stress is critical for improving predictions of tree species resilience and forest restoration efforts in the face of changing precipitation regimes. To assess within-species variation in drought resistance and its underlying mechanisms, we conducted two large-scale, multi-year, multi-species field experiments that involved transplanting thousands of tree seedlings to sites along a regional rainfall gradient in Panama. Contrary to expectation, we found little evidence for local adaptation to drought within species; seedlings from drier populations had no advantage under drought conditions. Instead, we found that, under wet conditions, seedlings from wetter populations had higher survival than those from drier populations. The survival advantage appears to be the result of local adaptation to insect herbivores in populations at wet, but not dry, sites. This finding highlights the importance of considering not only species responses to abiotic factors, but also shifts in ecological interactions under climate change. We have also been using our long-term observational data on seedling dynamics in Panama to assess community-level and species-specific responses to the severe 2015-16 El Niño drought.