Vulnerability to xylem cavitation and the distribution of Sonoran desert vegetation

We studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Psi(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina. Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the vulnerability curve of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Psi(x) and the Psi(erit) representing the pressure at complete loss of transport. We also measured minimum in situ Psi(x)(Psi(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Psi(xmin) than riparian species. Values of Psi(erit) were correlated with minimum Psi(x). Safety margins (Psi(xmin)-Psi(erit)) tended to increase with decreasing Psi(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Psi(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in parr from a weak bur significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats.