Response of vegetation of the Northern Great Plains to precipitation amount and grazing intensity.

Changes in basal cover of vegetation were predicted in response to variation in precipitation and grazing intensity. Multiple regression analysis was used with basal cover as a dependent variable and precipitation parameters as independent variables to develop predictive equations. Predicted cover values were used to develop three dimensional response surfaces which describe individual species responses to fluctuating precipitation and different grazing intensities. Results indicate that each species reacts to precipitation regimes and grazing pressure in a unique manner. Continual changes in basal cover can be expected in the plant community as the precipitation regime changes. Moderate grazing intensity, approximately 0.92 ha (2.3 acres) per AUM, appears to be most conducive for maintaining vegetative cover that is desirable for livestock production. However, stocking rate changes need to be anticipated and planned to coincide with available forage because of large fluctuations in cover due to varying precipitation. Climate appears to be the major factor controlling plant growth in the Great Plains. Whitman et al. (1943) stated that drought is the primary influence of vegetational change. Reed and Peterson (1961) Hurtt (1951) and Clark et al. (1943) stated that major trends in vegetation are primarily determined by fluctuations in climatic conditions, and that changes within these major trends are influenced by grazing intensity. Authors are former graduate student, Department of Animal and Range Sciences, Montana State University. Bozeman 597 17; research plant physiologist, USDA-ARS. Livestockand Range Research Station, Route. I Box 2021, MilesCity, Mont. 59301; and assistant professor. Department of Animal and Range Sciences. Montana State University, Bozeman 59717, respectively. The research is a contribution from the Western Region. USDA-ARS, Livestock and Range Research Station and the Montana Agricultural Experiment Station, Journal Series No. J-1445. Manuscript accepted January 2. 1985. JOURNAL OF RANGE MANAGEMENT 38(4), July 1985 There is a close relationship between time and amount of precipitation and plant growth (Albertson et al. 1953). One wet or dry year will affect production, but 2 or more wet or dry years are required to change the basal cover (Johnson 1981). Abrupt and fundamental changes in the amount and kind of living plant crown area may result from severe drought and last longer than anticipated (Reed and Peterson 1961). Both frequency and duration of drought are important in determining the severity of effects of climate on vegetation (Cook and Sims 1975, Herbel et al. 1972). Native vegetation has developed under conditions of wide climatic variation (Albertson et al. 1957). Marked changes in botanical composition occur, but plant populations are seldom completely destroyed by climatic fluctuation (Cook and Sims 1975). These changes are slow, whether for better or worse (Sarvis 1941). Native grass species will remain dominant in prairies unless they are severely grazed or buried by dust (Cook and Sims 1975, Schumacher 1974, Albertson et al. 1957). A more comprehensive understanding of the responses of range ecosystems to weather and grazing is desirable for good management (Reed and Peterson 1961). Drought cannot be controlled, but its impact can be moderated by proper livestock management. To do this, changes in plant communities need to be better defined so that effects of both drought and above average precipitation can be anticipated on the range. There are general ideas of the influence of precipitation and grazing intensity on rangeland plant communities, but only limited effort has been made to predict the responses within plant communities to fluctuating precipitation regimes and different grazing intensities. Models have been developed to predict forage production at different levels of precipitation (Smoliak 1956, Sneva and Hyder 1962, Abel et al. 1962) but similar endeavors that evaluate plant cover response to fluctuating precipitation