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 "invasion" 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.