Increasing frequency and severity of drought has been linked to elevated tree mortality in forests from boreal to tropical zones. These processes have been poorly explored in tropical dry forests, where species are adapted to seasonal water deficits. We combined a seven-year mortality record from forest inventory plots with transect data to investigate whether a severe El Niño event in 2015 resulted in elevated mortality in regenerating dry forests in northwestern Costa Rica. Our objectives were to determine whether certain species and size classes were more vulnerable, and which functional traits underpin responses to drought.
Annual rainfall ranged from 1113 to 2820 mm the first six years of the study, but was severely reduced to 627mm in 2015 during the ENSO event. From 2009-2014, mortality rates ranged from 0.5 to 3.3% per year, but increased significantly to 7.7% in 2015. For 24 species represented by at least 10 individuals, mortality rates during the drought ranged from 1.4 to 50%. Logistic regression showed that probability of mortality varied significantly among species and crown cover class, but there was no preferential mortality as a function of tree size. For the 31 most abundant species, we calculated Spearman rank correlation coefficients between mortality rates during the drought and wood density, vessel diameter and density, foliar nitrogen, specific leaf area, maximum height, and turgor loss point. The only trait that was correlated with mortality rate during the drought was turgor loss point, albeit weakly. These results underscore that tropical dry forest trees species vary tremendously in their susceptibility to drought, and highlight the need for more research on the specific physiological mechanisms that underlie tree responses to drought.