Douglas-Fir Encroachment into Mountain Grasslands in Southwestern Montana

Abstract

A study of plant succession in relation to disturbance history was conducted in Douglas-fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] forest and fescue (Festuca L. spp.) grassland communities along the eastern slope of the Continental Divide in Montana. The objective was to obtain ecological information needed for assessing management alternatives aimed at enhancing big game habitat and livestock forage. Fire history was reconstructed through analysis of fire scars and age classes of trees. Sizes and ages were inventoried in sapling stage, pole stage, and mature forest stands. Results indicate that prior to 1890 fires occurring every few decades favored grassland and confined tree growth to rocky or topographically moist sites. Since 1890 fires have been rare as a result of livestock grazing (which removes fine fuels), fire suppression, and cessation of ignitions by Native Americans. Lack of fire allowed extensive areas of Douglas-fir now of pole size to become established in former grasslands between 1890 and 1915. Widespread invasion of sapling size trees occurred between 1941 and 1955, when seed crops apparently coincided with unusually favorable moisture conditions. For management of these areas, we recommend use of prescribed fire in conjunction with timber harvesting to enhance declining forage productivity for big game and livestock. Previously, we reported that Douglas-fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] forests have expanded and thickened near the lower timberlines in southwestern Montana (Arno and Gruell 1983, Gruell 1983). In this paper we present information on the magnitude of Douglas-fir encroachment in mountain grasslands in a study area representative of this region. We examine the relationship of fire history to changes in age-class and size-class structure in both and old-growth forest stands, occurring on different forest habitat types (potential climax). Douglasfir is invading sites that were previously maintained as grasslands by periodic fires. In 1983, a prescribed fire demonstration area was established on the Galena Allotment, Deerlodge National Forest, to test prescriptions for managing east-side Douglas-fir forests and mountain grasslands. Objectives were to enhance wildlife habitat (primarily for elk, Cervus elaphus L., and mule deer, Odocoileus hemionus Rafinesque), improve forage for livestock, and reduce wildfire hazard. National Forest and research personnel had observed the spread of Douglas-fir into mountain grasslands on the Galena area and recognized that continued tree encroachment would reduce forage production. However, there was a lack of detailed information on the ecological background of this invasion. Nearby oldgrowth Douglas-fir forests were dense, decadent, and contained little forage. These stands contained dying remnants of quaking aspen (Populus tremuloides Michx.) groves and overmature seral shrubs, indicating that they had previously supported considerable browse. S. Arno is a research forester in the Prescribed Fire and Fire Effects Research Work Unit of the Intermountain Research Station, Forest Service, U.S. Department of Agriculture, located at the Intermountain Fire Sciences Laboratory, Missoula, Mont. 59807. At the time this study was conducted, G. Gruell was a research wildlife biologist in the same unit; he is currently regional fuels management officer, Intermountain Region, Forest Service, Ogden, Utah 84401. The authors are indebted to personnel of the Deerlodge National Forest for their assistance during this research effort, and to the following reviewers for their helpful comments: Stephen Barrett, Systems for Environmental Management, Missoula; Wendel Hann, Northern Region, USDA Forest Service; Richard Laven, Colorado State University; Walt Mueggler, Intermountain Research Station. USDA Forest Service; Earl Willard, University of Montana; and two anonymous referees. Manuscript accepted 29 August 1985. We conducted a brief study of the Galena area to collect quantitative information concerning plant succession relative to disturbance history. This information was needed as an ecological basis for assessing management alternatives, including tree harvesting and possible conversion back to open forest, savanna, or grassland. Such practices might also be applicable to other Douglas-fir and mountain grassland areas, which are widespread in the Northern Rockies.