Abstract
Predicting drought responses of individual trees in tropical forests remains challenging, in part because trees experience drought differently depending on their position in spatially heterogeneous environments. Specifically, topography and the competitive environment can influence the severity of water stress experienced by individual trees, leading to individual-level variation in drought impacts. A drought in 2015 in Puerto Rico provided the opportunity to assess how drought response varies with topography and neighborhood crowding in a tropical forest. In this study, we integrated 3 years of annual census data from the El Yunque Chronosequence plots with measurements of functional traits and LiDAR-derived metrics of microsite topography. We fit hierarchical Bayesian models to examine how drought, microtopography, and neighborhood crowding influence individual tree growth and survival, and the role functional traits play in mediating species’ responses to these drivers. We found that while growth was lower during the drought year, drought had no effect on survival, suggesting that these forests are fairly resilient to a single-year drought. However, growth response to drought, as well as average growth and survival, varied with topography: tree growth in valley-like microsites was more negatively affected by drought, and survival was lower on steeper slopes while growth was higher in valleys. Neighborhood crowding reduced growth and increased survival, but these effects did not vary between drought/non-drought years. Functional traits provided some insight into mechanisms by which drought and topography affected growth and survival. For example, trees with high specific leaf area grew more slowly on steeper slopes, and high wood density trees were less sensitive to drought. However, the relationships between functional traits and response to drought and topography were weak overall. Species sorting across microtopography may drive observed relationships between average performance, drought response, and topography. Our results suggest that understanding species’ responses to drought requires consideration of the microenvironments in which they grow. Complex interactions between regional climate, topography, and traits underlie individual and species variation in drought response.
Highlights
Tropical rainfall regimes are predicted to change in future climate scenarios, with many parts of the tropics becoming drier (Chadwick et al, 2015; Duffy et al, 2015)
We considered two functional traits that are associated with carbon metabolism and plant hydraulics: specific leaf area (SLA) and wood density (WD)
Wood density was not significantly associated with any parameters in the survival model, and we found no significant trait associations with the effects of drought, microtopography, crowding, or their interactions on survival
Summary
Tropical rainfall regimes are predicted to change in future climate scenarios, with many parts of the tropics becoming drier (Chadwick et al, 2015; Duffy et al, 2015). Moisture, and topography have typically focused on variation across sites or plots (Fekedulegn et al, 2003; Guarín and Taylor, 2005; Ashton et al, 2006; Engelbrecht et al, 2007; Comita and Engelbrecht, 2009). We expected that growth and survival would be lower on steeper slopes and ridges (i.e., areas with more convex curvature) Because these topographic positions tend to be drier, we expected the effects of drought on tree growth and mortality would be amplified on steeper slopes and more convex surfaces. We predicted that trees with acquisitive traits would be more sensitive to stressful conditions, i.e., drought, dry topographic position, and crowding
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