The xylem pressure inducing 50% loss of hydraulic conductivity due to embolism (P₅₀) is widely used for comparisons of xylem vulnerability among species and across aridity gradients. However, despite its utility as an index of resistance to catastrophic xylem failure under extreme drought, P₅₀ may have no special physiological relevance in the context of stomatal regulation of daily minimum xylem pressure and avoidance of hydraulic failure under non-extreme conditions. Moreover, few studies of hydraulic architecture have accounted for the buffering influence of tissue hydraulic capacitance on daily fluctuations in xylem pressure in intact plants. We evaluated three measures of stem xylem hydraulic safety margins among 104 coniferous and angiosperm species representing a range of woody growth forms and habitat types. Additionally, we assessed relationships between xylem capacitance, species-specific set-points for daily minimum stem water potential, and hydraulic safety margins in a subset of species for which relevant data were available.

Results/Conclusions

The three types of hydraulic safety margin defined increased with decreasing species-specific set-points for daily minimum stem water potential, suggesting a diminishing role of stem capacitance in slowing fluctuations in xylem pressure as stem water potential became more negative. The trends in hydraulic safety were similar among coniferous and angiosperm species native to diverse habitat types. Our results suggest that here is a continuum of relative reliance on different mechanisms that confer hydraulic safety under dynamic conditions. Species with low capacitance and denser wood experience greater daily maximum xylem tension and appear to rely primarily on xylem structural features to avoid embolism, whereas in species with high capacitance and low wood density avoidance of embolism appears to be achieved primarily via reliance on transient release of stored water to constrain transpiration-induced fluctuations in xylem tension.