Resistance of stem xylem to water stress-induced cavitation and embolism among chaparral shrub species in California has been extensively studied, providing the opportunity to examine broad patterns in cavitation resistance. We used previously published as well as unpublished vulnerability to cavitation curve data from 16 chaparral shrub species of southern California to examine the variability of cavitation resistance across sites, regions, and seasons. Additionally, these data provided a unique opportunity to address a recent methodological debate within the field of plant hydraulics. We found that different methods, specifically a centrifuge method and a dehydration method, produced similar results (P = 0.184). Vulnerability to cavitation varied seasonally, with species exhibiting greater susceptibility to water-stress induced cavitation during the wet season (P = 0.003). Cavitation resistance did not differ among sites that were less than 10 km apart even though these sites differed in their coastal exposure, precipitation, and temperatures (P = 0.476). However, across larger geographic distances and with increased climatic divergence, cavitation resistance significantly varied (P = 0.005), with populations from a higher rainfall mountain range exhibiting greater susceptibility to cavitation. These data suggest that species may be particularly susceptible to the onset of early summer drought before xylem has hardened. Variation in cavitation resistance may be limited locally, but broadly dispersed species may diverge in cavitation resistance across their range. Maintaining populations that vary in cavitation resistance may be an important component of species conservation planning in an era of increased climatic variability.
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Vol. 61 • No. 4