Effects of experimental drought on post-hurricane recovery of an ever-wet tropical forest

The changing climate is increasing the frequency and intensity of hurricanes and drought, which cause tree crown damage and widespread tree mortality. Forests undergoing post-hurricane succession may be particularly vulnerable to drought because crown damage alters tree carbon allocation, which could be exacerbated by drought. Hurricane canopy damage also results in elevated light in the understory, leading to the recruitment of fast-growing early succession species that tend to be more vulnerable to drought. Yet, our understanding of the effects of drought on post-hurricane forest recovery is extremely limited. Hurricane María is the strongest hurricane to make direct landfall in Puerto Rico since 1928. We are leveraging the disturbance caused by Hurricane María in 2017 to gain important new knowledge on the compound effect of hurricanes and droughts by conducting a large-scale throughfall exclusion experiment. We have established three 20 x 20 m experimental plots and three 20 x 20 m control plots in the Luquillo Experimental Forest, in northeastern Puerto Rico. The forest is an ever-wet tropical forest at approximately 300 m above sea level and has a mean annual temperature of 24 °C and a mean annual precipitation of 3500 mm. We tagged and identified to species the trees in the six plots. We selected 62 target individuals from the five dominant tree species and one dominant palm on which we installed point dendrometers and are measuring predawn (PD_Ψ) and midday leaf water potential (MD_Ψ) and sap flow three times per year (during the driest, wettest, and highest solar irradiation period of the year). We have collected one year of pre-treatment data, have finished installing the throughfall exclusion roofs in the three experimental plots, and have begun post-treatment sampling of tree responses. We have also started collecting leaf samples and sapwood cores to extract nonstructural carbohydrates and histological sections for imaging of stored starch distribution and depletion. Our pre-treatment data show similar mean PD_Ψ (-0.2 to -0.4 MPa) and MD_Ψ (-0.5 to -0.6 MPa) among the three pre-treatment campaigns, meaning that there was no significant drought stress. Sap flow was higher during the highest solar irradiation (mean species-level pick whole tree sap flow 1,500 to 8,000 cm³ h^-1), whereas during the wettest and cloudiest time of year, there was ~ 50% reduction in sap flow (1,000 to 4,000 cm³ h^-1). On the same dominant species, we measured xylem vulnerability to embolism (P₅₀), leaf turgor loss point (TLP), and calculated stomatal safety margins (SSM = TLP-P₅₀). Species fell along a range of P₅₀ from drought-vulnerable (P₅₀ =  -1MPa; SSM = 0 MPa) to relatively drought-tolerant (P₅₀ = -3 MPa; SSM = 1 MPa). Given the differences in trait values among the dominant tree species, we expect very different species-level responses to the imposed drought that will likely change seasonally and throughout time as the experimental drought progresses.