Plasticity in cavitation resistance and mechanical strength of stem xylem in Heteromeles arbutifolia, adults versus post-fire resprouts

Background/Question/Methods   Many species of chaparral shrubs of California undergo vigorous resprout success after wildfire. We hypothesized that the water transport properties of resprouts, as well as their biomechanical strength would differ from adults. In addition, we hypothesized that irrigated and shaded resprouts would display reduced cavitation resistance and biomechanical strength in comparison to controls and adults. We tested these hypotheses by using a centrifuge method to compare the vulnerability of xylem to water stress induced cavitation. We used 50% loss in hydraulic conductivity due to water stress (PLC50) as our estimate of vulnerability to cavitation. A Universal Materials Testing Machine (Instron) was used to measure the modulus of rupture (MOR) and modulus of elasticity (MOE) in resprout and adult stems. Our experimental subject was Heteromeles arbutifolia (Toyon) that burned in the Malibu wildfire of 21 October 2007.

Results/Conclusions   Resprout stems (n = 6) were more susceptible to cavitation than adults (resprout PLC50 = -4.2 MPa, +0.60; adults PLC50 = -5.9 MPa, +0.42 P < 0.05) but mechanical strength of resprouts (n = 12) was significantly greater than adults (resprout MOR = 357 N/mm2, +32.3 ; adults MOR = 206 N/mm2, +15.5, P < 0.0005). Irrigated resprouts (n =6) had higher susceptibility to cavitation than the shaded resprouts and controls (irrigated PLC50= -3.66 MPa, +1.49; shade PLC50= -4.6 MPa, +1.87; control PLC50=-5.01 MPa, +2.71, P<0.005), but lower mechanical strength (irrigated MOR = 170.0 N/mm2, +60.1, , shaded MOR=179.4 N/mm2, +73.2; control MOR = 220.5 N/mm2, +83.3, P < 0.005). The comparison of xylem vulnerability to cavitation between adult and resprouts converged 1.5 years after fire but required about 2 years to converge in shaded and irrigation treatments. We conclude that stems from post-fire resprouts of H. arbutifolia are very different in their water transport properties and mechanical strength than stems from adults and that these differences can be modified by artificial irrigation and shading. This pattern probably reflects xylem plasticity that maximizes post-fire stem elongation rates in a competitive post-fire environment.