Ring Growth and Dominance in a Spruce-Fir Association in Southern Wyoming

Abstract

The spruce-fir forests on the east slope of the Medicine Bow Range in southeastern Wyomintg exist from an elevation of about 8500 feet to timberline. Associated with the spruce (Picea engelmanni) and subalpine fir (Abies lasiocarpa) is lodgepole pine (Pinus contorta), which gradually diminishes and finally disappears as the elevation increases. Occasionally near timberline and on dryer sites will be found a straggling specimen of white bark pine (Pinus albicaulis), but spruce is by far the most abundant tree at timberline in this region. Griggs (1938) found spruce, fir, or white bark pine at timberline in the northern Rocky Mountains, depending largely upon slope and exposure. As the elevation decreases the proportion of fir increases and lodgepole pine becomes more abundant. Spruce finally disappears and the fir-pine mixture gives way to pure stands of lodgepole pine. The latter is a subclimax species here as well as throughout much of its range, where it forms pure stands where fire or other forces have destroyed the climax forest. Because of its extreme intolerance for shade, it is gradually replaced by fir and spruce, and if the normal succession is not interrupted, it is found in the climax forest only as mature and large individuals. In this respect it is similar to western white pine (Pinus monticola) in the cedar-hemlock-white fir association of northern Idaho (Hubermann 1935), and to Douglas fir (Pseudotsuga mucronata) in the cedar-hemlock formation of the Puget Sound region. Pollen analysis of bogs also shows lodgepole pine to have been the pioneer postglacial invader in both of these regions (Hansen 1938, 39). Pollen analysis of bogs in the Medicine Bow Range by Sears1 substantiates the evidence of the subclimax status of lodgepole pine in this area during prehistoric times. He finds evidence that lodgepole pine alternated with spruce-fir in this area in the past, suggesting the destruction of the climax forest several times by fire, invasion by pine, and subsequent replacement with spruce and fir. In some casce the zone usually occupied by lodgepole pine may support a sparse forest of Douglas fir and western yellow pine (Pinus ponderosa). Lodgepole pine occupies a position between the Douglas fir-yellow pine and spruce-fir forests with respect to its demands for moisture and temperature (Shantz & Zon 1924).