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g. fynbos) than where the native vegetation is deciduous SRT1720 manufacturer (e.g. grasslands, savanna) and the invaders are evergreen (Dye and Jarmain 2004). Challenges for Research The variety of species The 28 invasive taxa in South Africa mapped by Kotz�� et al. (2010) cover a wide range of growth forms with varying physiology, phenology, rooting depths, LAIs, specific leaf areas and other key traits that affect their potential water-use (Table?1). Even so, they are a subset of the major and emerging invading species in South Africa and elsewhere in the world. However, this review has found that the impacts of the different species do vary in ways that are consistent with their key traits (Fig.?3, Calder 1991, 2005; Le Maitre 2004) so this information can be used for extrapolating the results for known species to unknown species in the interim and to prioritize measurements on other species. Figure?3. Effects of combinations of major plant traits that have been found to influence the impacts of plant invasions on water resources relative to natural vegetation (after Calder 1991, 2005; Le Maitre 2004). Plant traits are inter-related but can be grouped ... Stand density and age The hydrological impacts vary depending on the size (age) (Fig.?3) and density of the invasions with the effects of increasing size and density (canopy cover) being seen in the typical sigmoidal change in streamflow associated with increasing tree or stand check details age (Bosch and von Gadow 1990; Dye 1996a; Le Maitre and Versfeld 1997; Scott et al. 2000a; Zhao et al. 2012). Initially a plantation comprises small saplings, with a low (UNC2881 and streamflow reductions are not detectable but, as the trees grow, the reductions become evident with the maximum reductions being reached and sustained following canopy closure, which is well before biomass peaks (Le Maitre and Versfeld 1997). The same structural changes occur as invasions progress (Moody and Mack 1988), so these relationships need to be investigated more fully to allow the effects of size, density and canopy cover to be explicitly included in estimates of the impacts. The same changes occur when native woody plants encroach (i.e. density and canopy cover increases) and insights can be gained from research into the hydrological impacts and controlling factors (e.g. Huxman et al. 2005; Wilcox et al. 2008). Water availability Invasions occur in two different situations: (i) upland or dryland areas where the available moisture is limited to the rainfall which is retained in the rooting profile and (ii) riparian zones, floodplains or other areas where groundwater is available within the rooting depth. In this second situation, the potential and actual water-use generally is not primarily limited by water availability but by the climatic conditions and the growth form, root depth, phenology and physiology of the plants.
