DROUGHT TOLERANCE AND XYLEM EMBOLISM IN COOCCURRING SPECIES OF COASTAL SAGE AND CHAPARRAL

Some species of coastal sage and chaparral shrubs of California are extremely tolerant of tissue dehydration, surviving water potentials as low as -9 MPa during dry summer months. Such low water potentials (high tensions on xylem water) are known to cause severe embolism formation in the xylem vessels of woody plants, blocking water transport and potentially causing shoot dieback. Thus drought hardy species of coastal sage and chaparral are either extremely resistant to water stress-induced embolism or they become severely embolized during summer drought. We compared the seasonal changes in xylem water potential and xylem embolism (percent loss in hydraulic conductivity of stem segments due to air emboli) between co-occurring Salvia mellifera (coastal sage) and Ceanothus megacarpus (chaparral) growing in the Santa Monica Mountains of southern California. We also determined the relative sensitivity of each species to water stress-induced embolism by artificially dehydrating branches and measuring percent loss in hydraulic conductivity of xylem tissues at a given water potential. We found that both species experienced the same minimum in seasonal water potentials (-8 MPa) but the xylem of S. mellifera lost 78% in hydraulic conductivity whereas the xylem of C. megacarpus lost only 17% in hydraulic conductivity. These values for a natural plant community were within 10% of those predicted by our artificial dehydration curves. Our estimate of susceptibility to water stress-induced embolism indicated that 50% loss in hydraulic conductivity would occur at -4.5 MPa for S. mellifera but at -11 MPa for C. megacarpus. Irrigation of S. mellifera for one summer reduced loss in conductivity from 78 to 38% and increased leaf areas 10-fold, indicating that xylem embolism and leaf drop were drought induced. Our results show that xylem tissues of S. mellifera are more sensitive to water stress and tissue dehydration than those of co-occurring C. megacarpus. The observed ability of S. mellifera to inhabit drier sites than C. megacarpus may result from drought deciduousness in summer and high growth rates in spring that facilitate rapid construction of new xylem and leaf tissues. It may be that facultative drought deciduousness in coastal sage is tightly coupled to drought-induced embolism of xylem tissues.