Water transport properties and xylem mechanical strength in stems and roots of post-fire resprouts compared to pre-fire adults of Heteromeles arbutifolia

Background/Question/Methods Chaparral vegetation of southern California is resilient to periodic wildfire. One mechanism of resilience is vigorous sprouting from a basal root crown. Because resprouting shoots have a luxurious supply of water (large intact root with few stems and leaves), we hypothesized that the water transport properties as well as the mechanical strength of post-fire stems would be very different than typical adults stems. In contrast, we reasoned that roots of resprouts and adults would be similar. We tested this hypothesis by using a centrifuge method to compare the vulnerability of stem and root 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 October 21, 2007. Results/Conclusions Our results showed resprout stems to be much more susceptible to cavitation than adult stems (resprout PLC50 = -4.2 MPa, +0.60, n = 6; adults PLC50 = -5.9 MPa, +0.42 P < 0.05). In contrast, there was no significant difference between resprout and adult roots (resprout PLC50 = -1.1 MPa, +6.1, n = 5; adults PLC50 = -1.7 MPa, +0.24 P > 0.05, roots being more variable than stems). The mechanical strength of stems from resprouts was significantly greater than for stems from adults (resprout MOR = 357 N/mm2, +32.3, n = 12; adults MOR = 206 N/mm2, +15.5 P < 0.0005. Resprout MOE = 26918 N/mm2, +3182, n = 12; adults MOE = 9495 N/mm2, +874 P < 0.0001). We conclude that stems from post-fire resprouts of H. arbutifolia are very different in their water transport properties than stems from adults. This is not the case for roots. We also concluded that the mechanical strength of resprout stems is very different than that of adult stems. This pattern probably reflects adaptations that maximize post-fire stem elongation rates and utilization of luxurious supplies of water by stems in the post fire environment.