Drought and Clipping Effects on Tiller Demography and Growth of Two Tussock Grasses in Utah

Tiller demography and growth were determined for clipped and unclippedplants of crested wheatgrass (Agropyron desertorum) and bluebunch wheatgrass Pseudoroegneria spicata under drought, natural or irrigated conditions from 1984 until 1986. Mild water stress during the 1984 growing season did not reduce herbage accumulation at the end of that season on plants of both species. However, green leaf number, rate of leaf initiation, height and total green leaf area were all reduced on tillers of both species when predawn leaf xylem pressure potentials fell below — 2·5 MPa during two or more growth periods. In the 3rd year of repeated treatments, the lowest daughter tiller production and growth were observed under the simultaneous influence of drought and clipping. Repeated late and severe grazing of these species under long-term droughts (2 or more years) could then be expected to rapidly reduce their persistence in the community