Ontogenetic differences in xylem traits between resprouts and adults of two species of chaparral shrubs, Ceanothus spinosus and Rhamnus ilicifolia

Background/Question/Methods

Post-fire resprouts of chaparral shrubs undergo rapid shoot elongation presumably to compete for light with fast growing herbaceous annuals and perennials.  The necessity for fast growth may result in physiological adjustments in xylem traits to support rapid shoot elongation.  In the summer of 2006, following a fire in February of that year, a study was initiated in the Santa Monica Mountains to examine differences in xylem traits between resprouts and adults of two co-occurring chaparral species in the family Rhamnaceae, Ceanothus spinosus and Rhamnus ilicifolia. Xylem traits in stems were examined by constructing vulnerability curves to water stress-induced cavitation using a centrifuge method.  Water potential at 50% loss of hydraulic conductivity (Ψ₅₀) was used as an index of water stress tolerance of stem xylem.  We also measured wood density, predawn water potentials (Ψ_pd) and midday water potentials (Ψ_md) over two growing seasons. 

Results/Conclusions

In C. spinosus, the water potential at 50% loss in hydraulic conductivity (Ψ₅₀) occurred at -1.4 MPa for resprouts but -3.6 MPa for adults (p < 0.05).  R. ilicifolia displayed a similar pattern but overall were more resistant to cavitation (Ψ₅₀ was -3.7 MPa for resprouts but -5.5 MPa for adults, p < 0.05).  Both resistance to cavitation and wood density increased from year one to year two in the species examined (p < 0.05) and seasonal water potentials were consistently higher for resprouts in comparison to adults. Seasonal water potentials for resprout C. spinosus approached – 4 MPa during the first summer drought after fire, which from vulnerability curves, predicted > 90% cavitation of stem xylem. This was consistent with the observation of 33% resprout failure in C. spinosus. In contrast, the lowest water potential for resprouts of R. ilicifolia was only -3 MPa, predicting only 30% cavitation, consistent with the observation of 0% resprout failure. We conclude that resprouts shift their xylem traits to promote rapid shoot elongation at the expense of susceptibility to water stress-induced cavitation. In some species, such as C. spinosus, this may lead to resprout failure due to severe water stress.