Water Dynamics and Vegetation Structure: Soil texture has a large influence on soil water dynamics especially with respect to the depth of penetration and storage of water. We used field data from three locations each with a different soil texture, for which we have soil water data (1986-1990), to calculate available water by soil layer and to estimate parameters for a daily time step soil water model. The simulation model was used to calculate the fifty-year average behavior of soil water at each location. The two key results from this study were: 1) coarse textured soils had a larger deep water resource than fine textured soils and this was associated with a greater relative abundance of shrubs; and 2) the additional water available in coarse textured soils resulted in higher plant productivity. These results support the idea that woody plants have greater access to soil water resources stored deep in the soil than grasses.

Manipulation of Grass and Shrub Density: We selectively removed grasses, shrubs, both or neither from pots on which we subsequently monitored soil water content by depth (0-180cm). Removal of grasses resulted in increases in soil water content of the top 60 cm of the soil profile. A portion of this water eventually moved deeper in the soil were it was used by shrubs. Removal of shrubs had no effect on soil water storage indicating that grasses were able to use all of the available water. Further, removal of shrubs had no effect on grass productivity.

Stable Isotope Analysis of Water Use: The d¹⁸O abundance of potential sources can be compared to the d¹⁸O abundance of water extracted from plants to identify source of sources from which a plant obtains its water. We investigated the role of precipitation and ground water in providing growing season water resources to grasses, shrubs and trees. We further evaluated the significance of water resources by examining the d¹³C abundance as an indication of whether the plants had been subjected to water stress. Our results indicated that trees (Populus sargentii) used only ground water and that grasses (Bouteloua gracilis) used only soil water from precipitation. Results for the shrub, Atriplex canescens, indicated that it likely uses a mixture of precipitation and ground water sources. The general conclusion from this work also suggested that woody plants have greater access to and rely more heavily on deep soil water than do grasses.