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.
Overview: This grant supported long-term quantitative studies of plant reproduction and seedling establishment in three Neotropical forests varying in disturbance regime, seasonality and diversity. Such studies are needed to evaluate how tropical species respond to climate change and how reproductive processes contribute to our understanding of the factors which maintain the extremely high plant diversity found in tropical forests. NSF funds were used to maintain studies in Luquillo, Puerto Rico, and Yasuni National Park, Ecuador only. The third site in central Panama (BCI, Barro Colorado Island) is fully funded by the Smithsonian Tropical Research Institute. At each site, flower fall, seed rain, and fruit fall of all plant species are recorded every 1-2.5 weeks using a network of 120-200 traps with a surface area of 0.5 meters2. Seedling establishment, growth and survival are measured annually in seedling plots (3 plots each 2 meters from each seed trap) for seedlings from germination up to 1 cm in diameter. These censuses take place within large Forest Dynamics Plots (16-50 hectares in size) in which all trees and shrubs over 1 centimeter in diameter are regularly mapped, measured, and identified to species at regular intervals. Each of these highly quantitative and detailed sets of data were collected at each of the three tropical study sites using identical methodologies, facilitating comparisons among sites. We report here primarily on the results from the Luquillo site since funding under this grant went exclusively to support the research activities there. Intellectual Merit: Our two major objectives were: (1) to quantify seasonal and inter-annual variation in reproductive output in tropical plant populations and communities; analyze relationships between reproduction and a variety of hypothesized local and regional climate drivers; and evaluate evidence for long-term directional trends which could reflect responses to anthropogenic or other sources of global change. (2) to quantify spatiotemporal variation in seed arrival and seedling recruitment of individual species; analyze associated evidence of how recruitment processes might influence the maintenance of species diversity and evaluate how these processes interact with inter-annual variation in climate and plant reproduction. With respect to our first objective, we found that cloudiness over the past several decades, together with temperature and precipitation, have affected flower production in the tropical forests at Luquillo and BCI. Our results showed that temperature, rather than clouds, is critically important to tropical forest flower production. Warmer temperatures increased flower production over different timescales, contrary to recent evidence that some tropical forests are already near their temperature threshold. Clouds were primarily important in limiting flower production BCI but cloudiness enhanced flower production at Luquillo, probably through increased transmittance of the scattered light through the forest canopy. This last result is a unique contribution to our understanding of the effects of cloudiness on tropical forest reproduction. With respect to our second objective our results suggest that hurricanes shape species composition by altering understorey conditions that differentially influence the success of seedlings (Fig. 1, 2). Thus, predicted increases in the intensity and frequency of hurricanes in the Caribbean will likely alter seedling dynamics and ultimately the species composition in hurricane-impacted forests. In second study, we found that, through effects on seedling dynamics, hurricanes may extend the signature of land use history beyond the average recovery time of forests not subject to intense natural disturbance events. This means that hurricanes and land use signatures may interact to produce novel or non-analogue forest species compositions. Finally, a third study showed that trade-offs in competitive and colonization ability, in addition to successional niche processes, are likely to contribute to species persistence in these hurricane-impacted forests. The strategies species employ to cope with hurricane damage depend on the degree to which species rely on sprouting, repair of adult damage, changes in demographic rates in response to enhanced resource availability after storms, or long distance dispersal as recovery mechanisms. Broader Impacts: Much of the broader impacts of the funded project were achieved in the area of human resources, specifically student training. Over 30 recent college graduates (Fig. 3) were trained in the identification of tropical plant seedlings and in the use of information technology to enter the data they collected and correct entry errors. Approximately 60% of these students were female and about a third were from ethnic groups underrepresented in the sciences. Additional societal value was achieved in the area of the potential management use of information gathered on how tropical trees are responding to global and regional climate change and changes in hurricane frequency and intensity.
