Dynamics of soil water use by two cool-season Agropyron bunchgrasses during the warm season depletion of soil water reserves were monitored for two years in experimental plots in the field. Agropyron desertorum, an introduced, competitive species from Eurasia, extracted more water from the deeper ( > 50 cm) soil layers than the native, less competitive Agropyron spicatum. Agropyron desertorum both extracts this water earlier and to lower soil water potentials than Agropyron spicatum. From the

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... ter extraction dynamics of the grasses in monocultures and in their two-way (50:50) mixtures with a shrub they commonly co-occur with, Artemisia tridentata, partitioning of the soil water resource between the grasses and the shrub was inferred. This indicated that Artemisia tridentata and Agropyron desertorum partitioned the soil water resource fairly evenly, while considerable quantities of water in the deeper soil layers under Agropyron spicatum seemed to be available to the shrub without direct competition. The implications of this difference in water resource partitioning for competition of the grasses with Artemisia tridentata are discussed. Predawn and midday xylem pressure potentials were not different between the two grasses in spite of different fluxes through the plants. Agropyron desertorum initiated new adventitious roots in fall and early spring while Agropyron spicatum did so only during spring. Observations from a root observation chamber indicated essentially parallel pattern of lateral root elongation during the depletion phase through top 200 cm of the profile. In both species the number of active tips, and the rate of elongation of active tips, decreased as the soil dried out. Root tips at all depths were inactive by the middle of September. Agropyron desertorum maintained root elongation at 50-110 cm for two weeks longer than A. spicatum.