While, tropical forests support perhaps 60% of all flowering plant species and 50% of all terrestrial NPP and carbon stored in terrestrial biomass, few long-term studies of tropical forests have been conducted, particularly studies of factors affecting flowering, seed production and seedling establishment, growth and mortality. Knowledge of the forces that maintain diversity during the reproduction and regeneration life history stages is critical to understanding and maintaining tropical forest diversity. The absence of long-term, quantitative studies of reproduction and early regeneration of tropical forest plants has made it difficult to identify relationships with natural climate variation as well as possible long-term trends caused by anthropogenic forcing. The research will maintain long-term quantitative studies of plant reproduction and seedling establishment in three Neotropical forests varying in disturbance regime, seasonality and diversity. Ecuador, Panama and Puerto Rico vary in the influence of climatic forcing by the El Nino Southern Oscillation, the North Atlantic Oscillation, hurricane disturbance and the potential effect of anthropogenic global change. These data on temporal and spatial patterns of seed rain and seedling recruitment allow tests of hypotheses concerning causes of interannual variation and roles of different mechanisms in facilitating species coexistence including recruitment limitation, competition-colonization trade-offs, density-dependence and regeneration niche differentiation; all mechanisms hypothesized to be important to species coexistence and forest dynamics. This LTREB project will support the training of undergraduate and graduate students, provide new data to the scientific community, and inform the general public on the importance of global change research. Publishing all data and associated analyses on a web site here will insure maximum societal benefit from the data collected.
Tropical forests account for approximately 60% of the earth’s flowering plant species and 25% of terrestrial carbon stores. Understanding what drives the dynamics and maintains the diversity of tropical forest is of fundamental and applied importance in ecology. Despite this, there are few long-term studies of tropical forests, particularly of flowering, seed production, and seedling establishment and survival. This makes it difficult to identify relationships between forest dynamics and natural climate variation or long-term trends associated with global climate change. The seed and seedling stages are particularly vulnerable bottlenecks in a plant’s life, and processes that affect patterns of successful flowering, seed production, or seedling survival play a large role in determining patterns of forest diversity. This study has compared long-term data on plant reproduction and seedling establishment in three Neotropical forests in Puerto Rico, Ecuador and Panama. These forests vary dramatically in climate and diversity. At each site, flowers and seeds of all species falling into 120-200 traps are recorded every 1-2.5 weeks and seedlings present in 360-600 plots adjacent to the traps are measured annually. Our seed traps and seedling plots are arrayed within large (16-50 ha) forest dynamics plots where all trees are regularly mapped and identified to species, so that we may connect patterns of flowering, fruiting and seedling establishment to patterns of abundance and diversity of the neighboring adult trees. Our research has made several significant contributions to our understanding of how high levels of forest diversity in the tropics are maintained, and how plant reproduction fluctuates with variation in climate. We have found that seedling establishment is highly variable in space and time, and depends on the density or proximity of neighbors, characteristics of the physical environment, and differences among species in seed dispersal distances. Our data have also supported the development of new theoretical models that examine trade-offs in species’ reproductive outputs, the ability to withstand environmental stresses, and strategies for responding to disturbances. Our research provides important tests of these models and contributes to our understanding of changes in species’ abundances and the maintenance of diverse regeneration strategies. We have also examined our long-term data on seed production and found that plant reproduction shows similar patterns of year-to-year variation for sites that are affected by the El Niño Southern Oscillation. Other sites that do not experience El Niño-related climate variation are equally variable year-to-year in seed production, but the patterns are not correlated among sites. More information on these findings and links to peer-reviewed journal articles or book chapters that we have published using data collected with this funding are available at www.ctfs.si.edu/floss/. Our research is important to understanding how natural and anthropogenic climatic variability drives regeneration in tropical forests, with consequences for managing forests for silviculture or conservation as well as for assessment of the potential impacts of global climate change. This project trained more than 25 undergraduates (a significant proportion of which were Hispanics and/or women) in field methods, data management, tropical plant identification, and the scientific method. We have mentored several undergraduates from under-represented groups in independent research projects as part of Research Experience for Undergraduates grants from NSF associated with this project. The project has aided in the training and professional development of at least three graduate students and four post-docs. We have also trained five Ecuadorian biological technicians in various field methods and data management.
