Drought induced xylem embolism in four riparian trees from the Western Cape Province: Insights and implications for planning and evaluation of restoration

In Mediterranean climates, rainfall is restricted to periods of low evaporative demand, leaving plants to survive the summer drought. The purpose of this study was to determine the contribution of drought tolerance to the distribution of riparian species. These physiological insights will assist in developing target species lists for restoration of riparian ecosystems in the Western Cape, currently heavily degraded due to land clearing and invasive aliens. We estimated P50 and P100 from vulnerability curves and Ψx in four species across a range of summer water availability estimated by streamflow. As expected, decreasing streamflow resulted in lower predawn and mid-day xylem water potential, and species identified in previous studies as having broad distributions, such as Brabejum stellatifolium and Metrosideros angustifolia, sustained greater decreases in mid-day xylem water potential and were less vulnerable to cavitation than Rapanea melanophloeos or Brachyleana neriifolia, species with more restricted distributions. These results provide preliminary evidence that a consideration of drought tolerance might be useful in refining lists of target species for active restoration and evaluation of restoration success across projects in streams and rivers with different fluvial regimes.