Differential susceptibility to xylem cavitation among three pairs of Ceanothus species in the Transverse Mountain Ranges of southern California

Chaparral shrub communities of California are often dominated by indigenous species of the genus Ceanorthus, which is further divided into two subgene ra: Cerastes and Ceanothus. Cerastes is comprised of species that are non-s prouters after fire and which have leaf morphological features suggesting s trong tolerance to drought. In contrast, species in subgenus Ceanothus gene rally sprout after fire and have less xeromorphic leaves. We examined three pairs of Cerastes/Ceanothus species that commonly grow together in the Tra nsverse Mountain Ranges of southern California (e.g., Santa Monica Mountain s and San Gabriel Mountains). Each species Fair was sequentially replaced a long an elevational gradient, with C. megacarpus/C. spinosus occurring at a low elevation, coastal rite; C. cuneatus/C. oliganthus at a site intermedi ate in elevation and distance from the ocean, and C, crassifolius/C. leucod ermis st a high elevation, inland site. In general, the length of the summe r drought period decreases and the amount of rainfall increases with increa sing elevation and distance from the ocean In all pairwise comparisons, the non-sprouters in subgenus Cerastes (C. megacarpus; C. cuneatus; C. crassif olius) Here found to be more resistant to xylem embolism caused by water st ress than species in subgenus Ceanothus (C. spinosus; C. oliganthus: C. leu codermis). The level of water stress causing 50% loss in hydraulic conducti vity due to gas embolism varied between -7.1 to -10.8 MPa for subgenus Cera stes but -4.6 to -7.3 MPa for subgenus Ceanothus. As species pairs were seq uentially replaced from low elevation to higher elevation, susceptibility t o embolism also progressively increased. These results are consistent with the xeromorphic differences between subgenera and geographic distribution o f the species.