Abstract
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 P 50 and P 100 from vulnerability curves and Ψ x 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.
Highlights
The contribution of riparian habitats to ecosystem function and ecosystem services has been well documented (Malanson, 1993; Naiman and Décamps, 1997; Naiman et al, 2005)
M. angustifolia and B. stellatifolium occurred at all sites, R. melanophloeos occurred at all sites except Bosboukloof, and B. neriifolia occurred only on the Eerste River at Swartbrug and Witbrug
The large differences between Ψmax at the high and medium sites suggests that water availability did decrease between those two sites, at least for two of the four species examined, B. stellatifolium and M. angustifolia
Summary
The contribution of riparian habitats to ecosystem function and ecosystem services has been well documented (Malanson, 1993; Naiman and Décamps, 1997; Naiman et al, 2005). In temperate ecosystems (Gasith and Resh, 1999; Salinas et al, 2000). Intense disturbance associated with peak flows during rainfall events in winter months is coupled with water deficits in summer months. As stream power diminishes and disturbance intensity decreases, water availability decreases during the period of greatest evaporative demand. This is in contrast to temperate areas with summer rainfall where water availability remains highest during periods of greatest evaporative demand. The combination of summer drought and winter disturbance creates a narrow recruitment niche making restoration of native species to degraded riparian ecosystems difficult (Salinas et al, 2000; Petit and Froend, 2001)
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