Water potentials in native woody vegetation during and after a drought in Canterbury

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Abstract
The effect of the 1987-89 drought on the native vegetation of bush remnants on Banks Peninsula was examined using measurements of water potentials and growth, and ordination of species composition. Marked individuals of five species (Alectryon excelsus, Helichrysum aggregatum, Kunzea ericoides, Melicytus ramiflorus, and Podocarpus totara) were studied in detail in Ahuriri Valley. The drought was severe, whether judged by visual appearance (extensive wilting and leaf fall), water potentials (≤-7.1 MPa), or survival (three of the marked individuals died). Water potentials were initially very low, but most plants eventually recovered. One K. ericoides plant had all five samples below -7 MPa in December 1988 yet still survived. Growth was reduced, however, with narrower growth rings in A. excelsus and shorter leaves in P. totara during the drought. The first ordination axis of vegetation samples from the reserve matched the rainfall/moisture gradient, suggesting that water availability is important in determining species distribution in Ahuriri Valley. The results from the ordination agreed with the water potential data, but neither correlated well with visual ranking. The effect of the drought on M. ramiflorus varied widely in three nearby reserves. Of the five species studied, M. ramiflorus and P. totara were drought avoiders (maintaining high internal water potentials), while K. ericoides and H. aggregatum were drought tolerators (surviving very low internal water potentials), and A. excelsus was intermediate. Since P. totara and M. ramiflorus suffered most mortality (i.e., were most susceptible to drought), tolerance seemed to be a more effective strategy.