Xylem hydraulic safety margins in woody plants: coordination of stomatal control of xylem tension with hydraulic capacitance

P>1. The xylem pressure inducing 50% loss of hydraulic conductivity due to embolism (P(50)) 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, P(50) 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. 2. We used data from 104 coniferous and angiosperm species representing a range of woody growth forms and habitat types to evaluate trends in three alternative xylem hydraulic safety margins based on features of their stem xylem vulnerability curves and regulation of daily minimum stem water potential (Psi(stem min)) under non-extreme conditions: (i) Psi(stem min) - P(50), (ii) Psi(stem min) - P(e), the difference between Psi(stem min) and the threshold xylem pressure at which loss of conductivity begins to increase rapidly (P(e)) and (iii) P(e) - P(50), an estimate of the steepness of the vulnerability curve between P(e) and P(50). 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. 3. The three types of hydraulic safety margin defined increased with decreasing species-specific set-points for Psi(stem min), suggesting a diminishing role of stem capacitance in slowing fluctuations in xylem pressure as Psi(stem min) became more negative. The trends in hydraulic safety were similar among coniferous and angiosperm species native to diverse habitat types. 4. 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.