Tree Age and Dominance Patterns in Great Basin Pinyon-Juniper Woodlands

Abstract

Prior studies of pinyon-juniper woodlands at a few locations have indicated considerable historical expansion of the trees and loss of understory. Whether these changes are a widespread phenomenon and related to pervasive, rather than local, influences was the question asked by this research. An objective sampling of 18 randomly selected mountain ranges in the Great Basin was undertaken. Tree age and dominance in the pinyon-juniper woodlands showed definite geographical, elevational, and historical trends. The oldest, most tree-dominated woodlands were located in areas of intermediate topography where disturbances may have been less frequent. Populations of both tree species [Pinus monophylla (Torr.) and Juniperus osteosperma (Torr. and Frem.)] were progressively younger and less dominant in both upslope and downslope directions from the intermediate elevations. Tree densities have also historically increased within the oldest woodlands. Pinyon density has increased faster than that of juniper. Approximately 40 percent of the sampled plots had their trees establishing during the last 150 years. These changes generally coincide with introduction of heavy livestock grazing, tree utilization by the mining industry, and fire suppression that followed settlement of the region. Associated climatic trends were also investigated. The relative importance of these influences on the changes in tree age and dominance cannot be determined without further research. The loss of understory, coincident with increasing tree dominance, has reduced forage production and made the woodlands progressively less susceptible to fire. Barring some major environmental change or management action, this forage reduction and decreased frequency of burning will continue until trees dominate much more area. Pinyon-juniper woodland is an important range type that has apparently increased in area and density over the last century (Plummer 1958, Christensen and Johnson 1964, Murdock and Welsh 1971, West et al. 1975). In the Great Basin, the type now occupies an area of nearly 18 million acres (Tueller et al. 1979). Although changes in tree dominance in these woodlands have been widely discussed, the few existing studies (Cottam and Stewart 1940, Woodbury 1947, Blackburn and Tueller 1970, Barney and Frischknecht 1974, West et al. 1975), have provided only limited documentation of these changes. Many combinations of natural and man-influenced causes have been proposed (West et al. The authors are former graduate research assistant, professor and former graduate fellow, respectively, Department of Range Science, Utah State University, Logan 84322. Tausch’s current address is 401 Manzanita Avenue, Carte Madera, California 94925. Nabi’s current address is Department of Range Science, Colorado State University, Fort Collins 80523. This research was performed under a cooperative agreement with the Intermountain Forest and Range Experiment Station, U.S. Forest Service. McIntire-Stennis Forestry Research Act funding was also provided from the Utah Agricultural Experiment Station, of which this is Journal Paper No. 2328. Nabi’s contributions were financially supported by the Ministry of Education of the Libyan Arab Republic. We are indebted to Mr. Robert Bayn for drafting the figures and to many others too numerous to mention individually for assistance in the collection and processing of these data. Manuscript received November 16, 1979. JOURNAL OF RANGE MANAGEMENT 34(4), July 1981 1975, Burkhardt and Tisdale 1976). Ecologists and land managers have questioned whether the reported thickening of trees in the original stands and invasion of former grasslands and shrubland are a widespread phenomenon controlled by a few wide-reaching factors common to the region or of only local importance. In addition, the rates of change, including the secondary succession resulting from tree re-establishment in areas denuded by tree cutting for mine props and charcoal, has not been totally determined (Lanner 1977, Budy and Young 1979). The following is a report of our attempts to provide an overview of the spatial and elevational variation of tree age and dominance in the Great Basin pinyon-juniper woodlands by describing existing conditions, historical changes, and apparent rates of change.