Effects of annual weather variation on peak herbaceous yield date in sagebrush steppe

climate variability impacts forage yield in semiarid rangelands, but it also affects the timing of peak yields. Knowledge of peak standing crop or yield dates would be useful when planning fieldwork for various research or management activities and for developing more accurate models linking herbage production to climatic variables. In this study, herbaceous yield was measured every 2 weeks (April-August) over an 8-year period in a Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) community in southeastern Oregon. Date of peak yield (Julian day) was calcu-lated for morphological groups (tall perennial bunchgrasses, perennial forbs, annual forbs), Sandberg bluegrass (Poa secunda J. Pressl), and total herbaceous yield. Linear and multiple regression analyses were used to correlate date of peak yield of herbaceous morphological groups with spring precipitation, reference evapotranspiration (RET), and air tem-perature. Peak yield dates for the herbaceous response variables were strongly correlated to annual climatic variation, commonly a combination of early growing season precipitation (March to 16 May) and March through May RET. Depend-ing on morphological group, peak yield date varied by 3 to 7 weeks during the growing season. The best regression coefficients for peak yield date of perennial bunchgrasses, perennial forbs, Sandberg bluegrass, annual forbs, and total herbaceous vegetation alone or with various combinations of precipitation, temperature, and RET were 0.97, 0.82, 0.86, 0.90, and 0.98, respectively. For planning fieldwork, estimates of peak yield date can be used to accurately sample for yields of herbaceous morphological groups and community production potentials.