LTREB: Collaborative Research: Cyclic vs. anthropogenic causes of long-term variation in the regeneration of tropical forests with contrasting latitude and diversity

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Tropical forests are globally important because of their high biodiversity and large contributions to carbon storage. This project studies the timing and success of reproduction in tropical forest plants. Tropical species may use cues of seasonal shifts (for example, in rainfall or light availability) to time the start of reproduction, perhaps to ensure their seeds are dispersed during conditions that will be favorable for the growth and survival of young seedlings. Rainfall, light availability, temperature, and other environmental conditions influence each stage of reproduction- flowering, fruit maturation, seed dispersal, and seedling growth. Beneficial conditions for reproduction can vary within a year, due to seasonality, and among years, due to climate cycles like El Nino. This project focuses on long term data in order to capture periods when conditions are better or worse for various types of plants. The research takes place at multiple tropical forest study sites to separate the effects of local weather variability from large regional climate patterns. By measuring the cues tropical plants use to start reproduction and examining which environmental conditions affect seedling growth and survival, the long-term data from this project will strengthen our ability to predict the future of tropical forests and the benefits these forests confer to society. Training of multiple undergraduate students will occur at the sites, by each of the collaborating universities.These long-term studies will enable tests of hypotheses concerning: (1) what are the environmental cues that induce flowering; (2) how is reproduction in tropical forests affected by disturbances such as wind storms, droughts, and by natural climate cycles such as the El Nino Southern Oscillation, the North Atlantic Oscillation, and the Atlantic Multidecadal Oscillation, which vary at subdecadal to multidecadal time scales; (3) how do species with different traits (e.g., small vs. large seeds, soft vs. dense wood) respond to environmental variation; and (4) how does episodic reproduction ramify through to the structure of adult populations. This research approach will effectively close the demographic gap between studies of early reproduction and the dynamics of sapling and adult tree populations. At each of the field sites, species-specific flower, fruit and seed rain is recorded multiple times each year using seed traps. Additionally, all woody seedlings are recorded in an annual census of hundreds of 1-m2 plots adjacent to seed traps, at each site. The trap and plot censuses take place within large mapped forest dynamics plots where all trees 1 cm in diameter are identified and regularly measured.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.