This review examined studies that strongly suggest that livestock, as well as fire suppression, logging, and other anthropogenic activities have contributed to altered ponderosa pine and mixed conifer forests throughout the interior west of the United States. Not only have cattle and sheep helped to convert the original park-like forests into dense stands of less fire-tolerant tree species, but they have changed the physical environment by reducing fire frequencies, compacting soils, reducing water infiltration rates, and increasing erosion. As a result, many contemporary ponderosa pine and mixed conifer forests differ from those of presettlement times in density, composition, structure, and critical soil properties. These forests also appear to be less resilient to natural disturbances, such as fire and disease, and will probably be less resistant to future changes that are expected to result from expanding human populations and global climate change. The effects of livestock grazing and trampling are not homogenous across the western landscape. Nonetheless, the similarities of the changes occurring in grazed low- and mid-elevation forests throughout the interior west suggest that livestock grazing has had profound effects over a wide range of conditions. The studies discussed suggest that livestock have actively participated in the destabilization of ponderosa pine and mixed coniferous forests. The hot fires that swept through forests of central and eastern Washington and Oregon during the summers of 1994 and 1996 may have been, partially, a result of a century of livestock grazing.
Citations and enhanced abstracts for journals articles and documents focused on rangeland ecology and management. RSIS is a collaboration between Montana State University, University of Idaho, and University of Wyoming.