The importance of xylem constraints in the distribution of conifer species

Vulnerability of stem xylem to cavitation was measured in 10 species of con ifers using high pressure air to induce xylem embolism, Mean values of air pressure required to induce a 50% loss in hydraulic conductivity (phi(50)) varied enormously between species, ranging from a maximum of 14.2 +/- 0.6 M Pa (corresponding to a xylem water potential of -14.2 MPa) in the semi-arid species Actinostrobus acuminatus to a minimum of 2.3 +/- 0.2 MPa in the ra inforest species Dacrycarpus dacrydioides. Mean phi(50) was significantly c orrelated with the mean rainfall of the driest quarter within the distribut ion of each species. The value of cp,, was also compared with leaf drought tolerance data for these species in order to determine whether xylem dysfun ction during drought dictated drought response at the leaf level. Previous data describing the maximum depletion of internal CO2 concentration (c(i)) in the leaves of these species during artificial drought was strongly corre lated with phi(50) suggesting a primary role of xylem in effecting leaf dro ught response. The possibility of a trade-off between xylem conductivity an d xylem vulnerability was tested in a sub-sample of four species, but no ev idence of an inverse relationship between cp,, and either stem-area specifi c (K-a) or leaf-area specific conductivity (K-1) was found.