OOS 45-2 - Inter- and intra-specific variation among tree populations in their vulnerability to drought across a rainfall gradient in Puerto Rico

Global climate change has led to shifting rainfall patterns and increases in the duration and frequency of drought events impact the growth, mortality, and regeneration of tree species and ultimately determine their distribution ranges. Within a species, variation in hydraulic traits has important consequences for population-level vulnerability to climate change and determines how individual populations respond to drought. In turn, inter- and intra-specific variation of hydraulic traits linked to drought resistance among tree populations will ultimately determine community-level vulnerability to drought and forest composition. However, this topic is poorly understood in particular for tropical forests. Here we have examined how hydraulic traits linked to drought avoidance (high leaf and stem capacitance) and drought tolerance (low leaf turgor loss point, high resistance to embolisms and hydraulic failure), vary along a pronounced rainfall gradient (1,000–4,000 mm mean annual rainfall) across the tropical Caribbean Island of Puerto Rico to address the following questions. How do these traits vary at: 1) the regional level (across the island), 2) forest-level (interspecific), and 3) species-level (intraspecific) across a pronounced rainfall gradient? At six sites across Puerto Rico with distinct mean annual rainfall, we measured hydraulic traits associated with drought avoidance and drought tolerance.

We measured a total 283 trees belonging to 20 different species. Across a declining rainfall gradient in the six forest plots, we found an overall decrease in stem capacitance (R2=0.14, p<0.001), P50,leaf (R2=0.22, p<0.001) and P50,stem (R2=0.17, p<0.001), increase in HSMleaf (R2=0.19, p<0.001) and HSMstem (R2=0.12, p<0.001), and no difference in Cft,leaf and Ψtlp. We also found evidence of intraspecific hydraulic variation, with individuals in the drier sites having more negative P50 values and higher HSMs. We conclude that: 1) There is an association of higher drought tolerance (higher embolism resistance and larger HSMs) with a decrease in rainfall and of higher drought avoidance (higher stem capacitance) with an increase in rainfall across Puerto Rico; 2) at the plot level, forests in the drier locations have traits associated with greater drought tolerance and forests at the wetter locations have traits associated with higher drought avoidance; 3) intraspecific variation in embolism resistance and HSMs does exist in some species, in particular, the species that grow across forests with the biggest range in rainfall.