Vegetation Development over 25 Years without Grazing on Sagebrush-Dominated Rangeland in Southeastern Idaho

Abstract

Data from permanent vegetation transects, established on the Idaho National Engineering Laboratory Site in 1950, were analyzed to determine what changes had taken place in the vegetation complex over the past 25 years in the absence of grazing by domestic livestock. Cover of shrubs and perennial grasses has nearly doubled. Shrub cover in 1975 was 154% greater than in 1950; this change was almost entirely due to increases in cover of big sagebrush between 1957 and 1965. Cover of perennial grasses increased exponentially over the 25-year period, from 0.28% in 1950 to 5.8% in 1975. This was paralleled by significant increases in density and distribution of the four most important grasses on the study area. The 20-fold increase in perennial grass cover has not been at the expense of the shrub overstory. There was no obvious correlation between trends for perennial grass cover and precipitation patterns. Rather, the exponential growth is believed to reflect the availability of seeds as formerly depleted populations increase in size. No evidence of seral replacement, as predicted by classical succession, was found. The data seem more consistent with the “initial f’loristics/relative stability” concepts of vegetation development. A widely held generalization about recovery of overgrazed rangelands following protection from grazing is that provided by classical successional theory: Vegetation development will be directional and predictable, leading to the re-establishment of a Authors are professors, Department of Biology, Idaho State University, Pocatello 83209. This research was supported by the Division of Biomedical and Environmental Research, U.S. Department of Energy. The 1975 data were collected by Richard and Karen Jeppson. R. Wilkosz, G. Marlette, and D. Humphrey assisted with data analyses. We thank N. West, R. Harniss, and N. French for providing portions of previous data and O.D. Markham for his support and suggestions. Manuscript received June 4, 1979. JOURNAL OF RANGE MANAGEMENT 34(l), January 1981 climax community (Stoddart et al. 1975; Heady 1975). If such a view is valid, then it becomes imperative that range scientists and managers know (a) the expected direction and how individual species will respond, (b) the expected time required for recovery, and (c) the nature of the climax community. If, on the other hand, the classical view is not valid, it is imperative that range scientists and managers seek appropriate alternative models and understand their implications for management and the interpretation of results of experiments as well as managerial decisions. Past studies of vegetal dynamics on exclosures or other protected areas do not provide a clear choice regarding the validity of the classical model. McClean and Tisdale (1972) estimated that from 20 to 40 years were required for rough fescue and ponderosa pine ranges to recover to excellent condition under full rest. The developmental trends were generally similar across three sites in each range type, and the authors were able to generalize about the individual responses of a number of shrub, forb, and grass species. Thus, the trends appeared to be both directional and predictable. In contrast, Rice and Westoby (1978) found that vegetal changes on protected areas in semidesert shrub vegetation of northern Utah were neither consistent across sites nor predictable, and they concluded that the classical concept of range succession was not useful in interpreting the results of excluding grazers from those ranges. Similar results were reported for Texas ranges by Smeins et al. (1976), who found that vegetation change over 25 years had been primarily an adjustment in relative dominance of species rather than species replacement. They concluded that woody species, once established, tend to increase to a point of stabilization under complete protection, and they stated, “It seems probable that after each round of disturbance, natural or manmade, community development tends toward a relatively stable community, but fidel-