Water acquisition is a major limiting factor for plant growth and survival. This is especially true for the chaparral species in southern California that experienced the worst seasonal drought in recorded history, in 2007. Although these chaparral species are well adapted to periodic drought and wildfire, increased frequency and intensity of these events may push some species beyond their threshold for survival. This study examined xylem hydraulic traits of two chaparral species with populations in both the San Gabriel and Santa Monica Mountains: Rhamnus californica and Ceanothus crassifolius. R. californica is an obligate sprouter after wildfire; its seeds are killed by fire thus post-fire recruitment is exclusively by sprouting and C. crassifolius is a non-sprouter after fire; the adults are killed by fire thus post-fire recruitment is exclusively by seed germination. The San Gabriel Mountains receive twice the annual precipitation of the Santa Monica Mountains thus it was predicted that the Santa Monica Mountain chaparral populations would be more drought tolerant. It was also predicted that the difference would correlate to each species life history type for post fire recruitment.
Results/Conclusions
To detect possible ecotypic variation, we constructed vulnerability curves to water stress induced cavitation using a centrifuge method. We estimated cavitation resistance in each population as the water potential at which a stem loses 50% of its hydraulic conductivity (Ψ50). The Santa Monica Mountain populations were found to be more resistant to cavitation than the San Gabriel Mountain populations. A small but significant shift in cavitation resistance was found in R. californica (-1.4 MPa and -1.1 MPa, respectively; P < 0.05) and a larger shift was observed for C. crassifolius (-9.5 MPa and -6.6 MPa, respectively; P < 0.05). An estimate of xylem vessel resistance to implosion (t/b)h2 for R. Californica was not significantly different between populations (P > 0.05). The xylem density of the R. californica population in the Santa Monica Mountains was higher than the San Gabriel Mountain population (4.9 g/ml and 3.9 g/ml, respectively; P < 0.05). Analysis for C. crassifolius will be completed by July 2008. It appears that the non-sprouting trait in the Rhamnaceae family displays greater ecotypic variation than in obligate sprouters.
