Water transport properties in leaves of the chaparral shrub, Heteromeles arbutifolia, in a post-fire environment

Background/Question/Methods Leaf hydraulic conductance (K_leaf) is a measure of how efficiently water is transported through a leaf. In theory, K_leaf should decrease with increasing water stress and should be higher for post-fire resprouts than pre-fire adults. These predictions were tested for a fire-prone member of the chaparral shrub community of California, Heteromeles arbutifolia (toyon; Rosaceae). Branchlets were excised before dawn, rehydrated for 2 hr, and subsequently dehydrated for increasing lengths of time before determination of K_leaf by the evaporative flux method, using a Scholander-Hammel pressure chamber was used to estimate leaf water potential (Ψ). As Ψ decreased, K_leaf declined, allowing the construction of a vulnerability curve to water stress. Standard pressure-volume-curves were also constructed to determine the Ψ at turgor loss and the osmotic potential at full hydration. Stomatal conductance (g_s) of post-fire resprouts and pre-fire adults were also compared to help interpret observed differences in K_leaf. Results/Conclusions For H. arbutifolia, leaves of resprouts had higher values for g_s, Ψ at turgor loss point and osmotic water potential at full hydration than adults (P < 0.003). Consistent with these patterns, K_leaf was significantly higher for resprouts than adults and leaf hydraulic failure occurred at a higher Ψ for resprouts than adults (-5 and -7 MPa respectively). Our results suggest that a luxurious supply of water to resprout leaves, characteristic of the post-fire environment, is associated with greater water transport efficiency but reduced safety against water stress-induced loss of conductance. This association may be an adaptation to facilitate rapid canopy expansion and reestablishment after wildfire, albeit, at the expense of tolerance of water stress.