Post-fire mortality and water relations of three congeneric shrub species under extreme water stress - A tradeoff with fecundity?

Mortality and water relations of three cooccurring Hakea species were studied over the first year following a wildfire in scrub-heath in southwestern Australia. Hakea ruscifolia regenerated from both resprouting adults and seedlings, whereas H. smilacifolia and H. polyanthema regenerated only from seedlings. We monitored seedling and resprout survival relative to pre-fire numbers and water relations (shoot water potential, transpiration, pressure-volume parameters) in order to determine the relationship between plant mortality and water relations over the critical first summer drought. Seedlings of H. polyanthema (few initial seedlings per parent) showed little mortality (8%), achieved through both drought avoidance (low transpiration, high predawn water potential and relative water content (RWC), substantial osmotic adjustment) and drought tolerance later in the season. Seedlings of H. smilacifolia and H. ruscifolia (high seedling/parent ratios) showed little drought avoidance and high mortality (54-68%). The remaining seedlings spent 3-4 months in a wilted condition (up to 3.6 MPa and 45% RWC below the turgor loss point in H. ruscifolia) indicating marked drought tolerance of the survivors. In contrast to its seedlings, H. ruscifolia resprouts were successful drought avoiders and experienced no mortality. The high level of survival and drought resistance of H. polyanthema was consistent with its large seedlings (via large seeds) and low initial fecundity. The study highlights the importance of the interaction between ecophysiology-morphology and demography in determining the recruitment strategies of plants.