Shifting Forests: Historical Grazing and Forest Invasion in Southwestern Montana

Forest distribution often changes as a result of land use disturbances. In southwestern Montana we suspected, based on visual evidence and on knowledge of the region's historical land use, that livestock grazing caused changes in forest tree distributions, specifically tree invasions into grasslands and shrublands. To test that theory we undertook this study to look at the historical changes in grazing and in the distribution of the forest. The study's conclusions could assist in a better analysis of management options, allowing both for conservation of environmental resources and for sustained grazing. In this study, we reconstructed the history of livestock grazing for the region, documented climate changes, measured the extent and the timing of forest changes, and then tested for associations among the data. Coniferous trees in southwestern Montana have migrated into lowerelevation grasslands and shrublands over which domestic livestock have grazed for the last eighty to one hundred thirty years (see figure 1). These changes in tree distribution have become exaggerated with time, wR;::t ? Rh.t : .