Xylem Embolism in Seedlings and Resprouts of Adenostoma fasciculatum after Fire

Abstract

Mediterranean-type ecosystems experience prolonged summer drought. By the timeof the first summer after a wildfire the root systems of seedlings aregenerally not developed enough to tap deep moisture resources, so seedlingsoften surpass their limits of tolerance to water stress, and mortality ishigh. Selective pressures therefore are possibly greater during the firstsummer drought after wildfire than at any other stage. Seedlings ofAdenostoma fasciculatum Hook. & Arn. (chamise), oneof the most common types of chaparral shrubs in southern California, whichestablished after a wildfire in November 1993, had water potentials as low as–10 MPa at the end of summer 1994. Such low water potentials in woodyplants are known to cause air emboli formation in xylem vessels and tracheids,potentially blocking water transport. By contrast, water potentials of theresprouts of chamise did not fall below –4.5 MPa. A comparative study ofchamise seedlings and resprouts was initiated to ascertain whether the degreeof xylem embolism was correlated with observed differences in minimum waterpotential and differential mortality (seedlings 79%, resprouts <1%) of these two life stages. A newly developed technique was used thatallows measurement of individuals with low levels of hydraulic conductance.Embolism (calculated as percentage loss in hydraulic conductance due to airemboli) was measured on whole shoots of seedlings and similar-sized branchletsof resprouts collected at a field site in the Santa Monica Mountains.Seedlings with extremely low water potentials (–9.8 ± 1.7 MPa,s.e., n = 10) were highly embolised (embolism= 82.8 ± 2.0%). Resprouts, with an average waterpotential of –3.8 ± 1.8 MPa, exhibited quite moderate levels ofembolism (26.0 ± 4.6%). These values suggest that xylem embolismmay have contributed to the high seedling mortality rate. Seedlings known tohave very low water potentials showed a significant decrease in levels ofembolism (63.2 ± 6.1%, s.e., n = 4)after a rainfall event of 31 mm. Investigations suggested that this was due topositive stem pressures in seedlings during or soon after the rainfall eventcausing refilling. The timing of post-fire rainfall events may be a criticalfactor in the survival and establishment of chamise seedlings.