Wednesday, August 8, 2007

PS 49-128: Xylem fatigue in post-fire resprouts and pre-fire adults of Quercus berberidifolia in the Santa Monica Mountains of California

Stephen D. Davis, Pepperdine University and Melinda Belisle, Pepperdine Univeristy.

Quercus berberidifolia is a drought tolerant, chaparral shrub that resprouts after wildfire. Adult shrubs are well adapted to drought conditions, however little is known about the water stress tolerance of newly emerging post-fire resprouts. There may be distinctive physiological and morphological differences. Curiously, species of adult oaks have been reported to experience high xylem embolism (blockage of water transport) under minimal water stress (-0.5 MPa). It has been speculated that this is due to vessel fatigue by environmental stress. The mechanism is unknown. We hypothesized that the xylem of resprouts would be more susceptible to water stress-induced embolism than adults and that xylem fatigue would result from exposure to severe water stress. To test these hypotheses, we determined xylem vulnerability to embolism by plotting the loss in stem hydraulic conductivity with increasing water stress and artificially induced water stress and vessel fatigue using centrifugal force. We also induced water stress by drying stems on a laboratory bench and estimating xylem fatigue. Results indicated that resprouts and adults were equally susceptible to water stress-induced embolisms and resprouts experience xylem fatigue after dehydration to -2.0 and -3.0 MPa, whereas adults did not experience fatigue. We conclude that water stress is a likely environmental cause of vessel fatigue in Quercus berberidifolia and that newly emerging resprouts are more vulnerable to xylem fatigue than hardened adults.

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