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This research project will explore how tree regeneration responds to climate anomalies associated with an El Niño event for eight tropical forests. The 2015-16 El Niño event is among the three strongest El Niño events since 1950. Strong El Niño events bring climate anomalies worldwide, including increased temperatures, reduced cloud cover, decreased rainfall, and drought over a large portion of the Tropics. The eight study forests occur along a steep rainfall gradient from the wet Caribbean to the dry Pacific coasts of Panama. The project capitalizes on previously mapped populations of seedlings and trees in each forest. The 2015-16 El Niño event brought reduced cloud cover and increased light availability to all eight forests during the 2015 rainy season, favoring seedlings and trees. The 2015-16 dry season began about one month earlier than normal, promising severe drought. The project will test several hypotheses, including the extent to which tree species from wet and dry forests across the gradient may respond in different ways to reduced rainfall expected during El Nino in 2016. Low light levels limit the metabolism and root development of seedlings in the deeply shaded understory of tropical forests, making understory seedlings particularly vulnerable to climate variation. The net effect drought and increased light on seedling performance (recruitment, growth and survival) and tree survival will determine which species regenerate successfully, potentially altering forest species composition. This project is important because tropical forests play a vital role in global carbon and water cycles and are the most biologically diverse ecosystem on the planet. A deeper understanding of the range of responses observed among plant species will provide insight into future forest responses to anthropogenic change including changing rainfall regimes and rising atmospheric carbon dioxide concentrations and temperatures.

The researchers will survey an existing network of tropical tree seedling plots within eight forests that a span a pronounced rainfall gradient in central in Panama during the 2016 El Niño event. Censuses of seedlings will measure their recruitment, growth and survival across two seedling size classes and (1) determine which forest sites and tree species are most affected by an extreme El Niño event, (2) examine the relative roles of reduced soil water availability and increased light on seedling performance, (3) evaluate the predictive value of functional traits for seedling responses to the event, (4) measure the size distribution of El Niño related seedling mortality and (5) provide a baseline for modeling the effects of extreme El Niño events on tropical forests through assessing minimum soil and leaf water potentials and their variation across forests and species. The project is among the most comprehensive studies of the effects of an El Niño event on tropical forest regeneration to date and will provide insight into variation of responses within and among species, seedling size classes, and forests. The research will improve projections for the consequences of global climate change for these forests, as well as for vegetation-climate feedbacks, which will be important for forest management under current and future climate conditions. This project will facilitate future research efforts both in Panama and in other tropical regions by providing research infrastructure and data from a relatively new set of plots in Panama and by gathering comparative performance data for a wide range of species, all of which will be made openly available online.