Drought impacts on tropical forests: A multi-scale perspective

Maria Uriarte and Naomi B. Schwartz, Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY, Robert Muscarella, Plant Ecology and Genetics, Uppsala University, Uppsala, Sweden, Chris Smith-Martin, Ecology, Evolution and Environmental Biology, Columbia University, New York, NY, Jess Zimmerman, Department of Environmental Sciences, University of Puerto Rico, Río Piedras, PR

Background/Question/Methods
Models predict stronger droughts and greater rainfall seasonality across many regions in the tropics. Tropical forests account for the majority of terrestrial aboveground biomass and severe water stress can lead to changes in the structure, composition and function of these ecosystems, potentially creating a positive feedback to climate warming. Understanding the impacts of drought on forests requires a multi-scalar perspective that combines information on variation in responses among species and life-history stages, knowledge of drivers spatial and temporal variation in vegetation impacts, and models. Here we use observations of seedling and tree responses to drought, eco-physiological measurements, and remote sensing data to assess impacts of the 2015 drought on the Luquillo Experimental Forest, Puerto Rico. We ask: (1) what life-history stages and species are most vulnerable to drought and (2) what factors drive spatial heterogeneity in vegetation the severity of water stress.

Results/Conclusions
Sensitivity to drought-induced cavitation varied drastically across 11 dominant species. Although early-successional tree species were more vulnerable to drought, several dominant mid- and late-successional species were also highly vulnerable, suggesting that increasing drought frequency and severity could lead to shifts in species composition in this forest. Observational data, however, showed that topography, rather than variation among species in vulnerability, was the main driver of drought impacts on trees. Average tree growth was lower during the drought year and growth response to drought varied with topography: tree growth in valley-like microsites was more negatively affected by drought. Landsat-derived water stress metrics confirmed that drought effects were more severe on drier topographic positions, that is, steeper slopes and southwest-facing aspects. Water stress during drought was also higher in younger forests, which are dominated by species vulnerable to cavitation. Topographic heterogeneity in soil moisture and biotic interactions rather than temporal variation in rainfall were the main drivers of seedling survival. This environmental heterogeneity modulated impacts of rainfall. Negative conspecific effects were amplified during rainy years and at dry sites, demonstrating that environmental heterogeneity is not only the main driver of seedling survival in this forest but also plays a central role in buffering or exacerbating impacts of climate fluctuations on forest regeneration. Together, these studies suggest that incorporating drivers of spatial heterogeneity in drought-induced water stress in earth system models is essential. Understanding how this heterogeneity influences species distributions is also key to assess impacts of drought on tropical forest composition and function.