Phenotypic plasticity in the xylem traits of the chaparral shrub Ceanothus spinosus before and after wildfire

A wildfire in February of 2006, at Castro Crest in the Santa Monica Mountains of southern California, facilitated a comparative examination of xylem traits between post-fire resprouts and pre-fire adults of Ceanothus spinosus. The resprouts and adults shared a common populational gene pool and thus any differences in xylem traits were assumed to represent phenotypic plasticity in response to ontogeny and microclimate. We examined the efficiency in xylem water transport (xylem-area specific conductivity, Ks), and the relative safety of xylem from water stress-induced cavitation (water stress level that causes 50% loss in hydraulic conductivity due to air embolism, P50). In July 2006, the Ks of resprouts was significantly lower (0.60 m2 MPa-2 s-1, + 1 SE = 0.14, n = 6) than adults (1.90 m2 MPa-2 s-1, + 0 .39, P < 0.02) and safety from cavitation, P50, was significantly lower for resprouts (-1.4 MPa, + 0.4, n = 6) than adults (-3.6 MPa, + 0.5, P < 0.01). However, stem elongation rates were 10-fold greater for resprouts than adults. This plasticity in xylem traits may be facilitated by a luxurious supply of water to shoot tissues after wildfire, resulting from an improved root/shoot ratio. Furthermore, xylem plasticity may be adaptive for the rapid shoot elongation required for resprouts to compete with herbaceous annuals and perennials in the post-fire environment.