DROUGHT AND CLIPPING EFFECTS ON TILLER DEMOGRAPHY AND GROWTH OF 2 TUSSOCK GRASSES IN UTAH

Tiller demography and growth were determined for clipped and unclipped plants of crested wheatgrass (Agropyron desertorum) and bluebunch whe atgrass Pseudoroegneria spicata under drought, natural or irrigated co nditions from 1984 until 1986. Mild water stress during the 1984 growi ng season did not reduce herbage accumulation at the end of that seaso n on plants of both species. However, green leaf number, rate of leaf initiation, height and total green leaf area were all reduced on tille rs of both species when predawn leaf xylem pressure potentials fell be low - 2.5 MPa during two or more growth periods. In the 3rd year of re peated treatments, the lowest daughter tiller production and growth we re observed under the simultaneous influence of drought and clipping. Repeated late and severe grazing of these species under long-term drou ghts (2 or more years) could then be expected to rapidly reduce their persistence in the community.