Relatively mesic environments within arid landscapes—i.e. drought refugia—may be increasingly important habitats for species persistence in the face of climate change. Most methods for detecting potential climate refugia are focused on a “top-down” characterization of the abiotic components of the environment and are insufficient to assess how variation in species sensitivity may off-set benefits of reduced exposure. Comparative trait-based approaches offer an opportunity to detect drought refugia from the “bottom-up” by characterizing both the exposure and sensitivity of plants to drought. Semi-arid southern California and the nearby California Channel Islands provide a model system for examining variation in drought sensitivity across environments with contrasting exposure to seasonal and inter-annual droughts. Measurements of plant water relations and xylem cavitation resistance were used to compare the risk of drought-induced hydraulic failure in island and mainland plant communities. Measurements were performed on 10 closely-related taxonomic pairs from matched island and mainland field sites. In addition, remote sensing was used to compare drought-induced declines in vegetation health across island and mainland environments during recent drought events.
Results/Conclusions
Island plants experienced more favorable dry-season water relations compared to mainland relatives, but stem hydraulic traits were not consistently different between the two groups. This combination of factors results in greater hydraulic safety for island plants and indicates localized buffering from seasonal and inter-annual droughts. However, this buffering may be reaching its limits during the recent protracted drought affecting southern California. The reduced hydraulic vulnerability of island plants may explain how the California Channel Islands have functioned as a regional drought refugia, with important implications for management and conservation of these unique island ecosystems, as well as drought refugia in other arid systems.