Xylem Embolism in Seedlings and Resprouts of Adenostoma fasciculatum after Fire

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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

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