Linking water uptake with rooting patterns in grassland species.

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004-2005) in which we measured the isotopic signature of plant xylem water from seven species (including C(3) forbs and shrubs and C(4) grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (delta(18)O) values were compared to soil water delta(18)O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C(3) species used proportionally more water from deeper portions of the soil profile relative to the C(4) grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.