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

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Abstract
Chaparral shrub communities of California are often dominated by indigenous species of the genus Ceanothus, which is further divided into two subgenera: Cerastes and Ceanothus. Cerastes is comprised of species that are non-sprouters after fire, and which have leaf morphological features suggesting strong tolerance to drought. In contrast, species in subgenus Ceanothus generally sprout after fire and have less xeromorphic leaves. We examined three pairs of Cerastes/Ceanothus species that commonly grow together in the Transverse Mountain Ranges of southern California (e.g., Santa Monica Mountains and San Gabriel Mountains). Each species pair was sequentially replaced along an elevational gradient, with C. megacarpus/C. spinosus occurring at a low elevation, coastal site; C. cuneatus/C. oliganthus at a site intermediate in elevation and distance from the ocean; and C. crassifolius/C. leucodermis at a high elevation, inland site. In general, the length of the summer drought period decreases and the amount of rainfall increases with increasing elevation and distance from the ocean. In all pairwise comparisons, the non-sprouters in subgenus Cerastes (C. megacarpus; C. cuneatus; C. crassifolius) were found to be more resistant to xylem embolism caused by water stress than species in subgenus Ceanothus (C. spinosus; C. oliganthus; C. leucodermis). The level of water stress causing 50% loss in hydraulic conductivity due to gas embolism varied between -7.1 to -10.8 MPa for subgenus Cerastes but -4.6 to -7.3 MPa for subgenus Ceanothus. As species pairs were sequentially replaced from low elevation to higher elevation, susceptibility to embolism also progressively increased. These results are consistent with the xeromorphic differences between subgenera and geographic distribution of the species.