Abstract:
1. 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, P5o 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 (Ystem min) under non-extreme conditions: (i)Ystem min - P50, (ii)
Y stem min - Pe, the difference between Ystem min and the threshold xylem pressure at which loss
of conductivity begins to increase rapidly (Pe) and (iii) Pe - P50 an estimate of the steepness of
the vulnerability curve between Pe and P50. 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 Y stem min, suggesting a diminishing role of stem capacitance in slowing fluctuations
in xylem pressure as Ystem 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.
