Climate change is causing shifting rainfall patterns and increases in the duration and frequency of drought events impacting the growth, mortality, and regeneration of tree species and ultimately determining their distribution ranges. Among (interspecific) and within (intraspecific) species, variation in hydraulic traits has important consequences for population-level vulnerability to climate change and determines how individual populations respond to drought.
We examined how hydraulic traits linked to drought avoidance (high leaf and stem capacitance) and tolerance (low leaf turgor loss point, high resistance to embolisms, and higher hydraulic safety margins), vary along a pronounced rainfall gradient (1,000–4,000 mm-y) across Puerto Rico to address the following questions. How do these traits vary at the regional level (across the island), site-level (interspecific), and species-level (intraspecific)?
At six sites with distinct rainfall (1040, 1660, 1900, 2170, 3070, 3900 mm-y), we measured hydraulic traits associated with drought avoidance—leaf (Cft,leaf) and stem (Cft,stem) capacitance —and drought tolerance—turgor loss point (Ψtlp), embolisms resistance in leaves (P50,leaf) and stems (P50,stem), and hydraulic safety margins in leaves (HSMleaf) and stems (HSMstem). We measured a total of 283 trees belonging to 20 different species. We measured as many of the 20 species as possible at each of the six sites resulting in 4 to 13 species at each location.
Across a declining rainfall gradient, we found an overall decrease in Cft,stem (R2 = 0.14), P50,leaf (R2 = 0.22) and P50,stem (R2 = 0.17), increase in HSMleaf (R2 = 0.19) and HSMstem (R2 = 0.12), and no difference in Cft,leaf and Ψtlp. At the forest-level, we found variation among plots for all traits. Sites with higher rainfall had higher Cft,stem, lower P50,leaf and P50,stem and higher HSMleaf and HSMstem while there was no clear pattern for Cft,leaf and Ψtlp. For five species, we found evidence of intraspecific hydraulic variation, individuals in the drier sites had more negative P50 values and higher HSMs. For example, Tabebuia heterophylla, had P50,leaf = -3 MPa in the wetter forests and P50,leaf = -5 MPa in the drier forest.
There is an association of higher drought tolerance with a decrease in rainfall and of higher drought avoidance with an increase in rainfall. At the site-level, forests in the drier locations have traits associated with greater drought tolerance and forests at the wetter locations have greater drought avoidance. 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.
plant ecophysiology, tree hydraulic traits,