Species coexistence in a situation of limiting water resources implies either avoiding interactions (niche segregation) or allowing some interaction (niche overlap). For example, the coexistence of different functional types regarding water resources enables plant communities to occupy a larger amount of physical space, exploring more resources (McConnaughay & Bazzaz, 1992). The exploitation of spatially and/or temporally distinct water resources by plants allows the coexistence of different species and life forms in environments where water is scarce (Noy-Meir, 1973; Reynolds etal., 2004).
Heterogeneity in hydrological conditions across topographic gradients may result in niche differentiation as has been observed in many plant communities (e.g. Dawson, 1990). Even in the absence of any obvious topographic variation, species segregation along a niche gradient of soil drying has been shown to occur (Silvertown et al., 1999). In water-limited environments successful competitors have root systems that are able to rapidly proliferate in resource-rich volumes of soil, depleting the resources before competing plants do (Passioura, 1982; Kroon et al., 2003). For example, Eissenstat and Caldwell (1988) showed that Agropyron desertorum, an invader bunchgrass of the Great Basin in the United States, exhibited root growth
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