Demographic Aspects of Coexistence in Engelmann Spruce and Subalpine Fir

The mechanisms for the maintenance of coexistence of Engelmann spruce and subalpine fir in subalpine forests of the Colorado Front Range were examined by comparing age, size, and spatial distributions of spruce and fir in two adjacent, previously logged sites of differing moisture availability. Adult tree ages were calculated from stem cores, while seedling ages were calculated from a multiple regression equation based on diameter, height, and number of branch whorls. Tree size was measured by height and diameter; spatial distributions were described by Morisita's index of dispersion. Cumulative age and size distributions were significantly different in the two species, with greater longevity and a larger overall size in spruce than fir. Both species showed a significant linear relationship between size and age, while fir showed a faster height growth rate than spruce. The linear relationship beween age and size was much closer in seedlings than in adults. Seedling spatial distribution was highly clumped in both species, but mature trees showed little or no clumping. Because both species are mainly wind dispersed, the greater clumping in spruce than in fir seedlings suggests that spruce have more specific establishment requirements than fir. Colonization patterns indicated that spruce seedlings were primarily found in forest gaps or associated with fir canopy trees, while fir seedlings were more commonly found in the forest, associated with either spruce or fir canopy trees. Tree density, growth rates, and mortality rates were higher in the wet site, with spruce showing the largest between site differences. These data suggest a new hypothesis for coexistence stating that Engelmann spruce and subalpine fir are maintained as codominants because the greater longevity and size of spruce is balanced by the faster height growth and more flexible seedling establishment requirements of fir. THE QUESTION of what factors permit species to coexist has long engaged both plant and animal ecologists (Hutchinson, 1959; Grubb, 1977; Werner, 1979; Brown, 1981). Accepted coexistence theory has been derived from Gause's (1934) principle of competitive exclusion; that is, species must show niche differentiation in order to coexist. More recently, Pickett (1980) suggested that coexistence of plant species occurs either because species occupy different niches or because disturbance prevents competitive equilibrium, while Aarssen (1983) attributed coexistence of plant species to niche differentiation or reciprocal selection that maintains a balance of competitive abilities. In the present study, Engelmann spruce-subalpine fir forests were selected to examine these questions of coexistence because I Received for publication 1 October 1984; revision accepted 25 June 1985. !thank M. Farris, M. Grant, Y. Linhart, and T. Veblen for many helpful comments on this manuscript. N. Moss, D. McClearn, and L. Engkelking assisted in the field work. This work was supported by a J. S. Noyes Foundation Fellowship and grants from the K. Lichty and G. Alexander Funds. 2 Current address: Department of Biology, St. Olaf College, Northfield, MN 55057. they are relatively simple communities in which spruce and fir coexist, often as the only two tree species present (Marr, 1961), and because they comprise a large part of the forested area in the central Rocky Mountains. According to Alexander (1974), the subalpine forests are the largest and most valuable timber resource in Colorado and Wyoming. Although it is generally agreed that Engelmann spruce and subalpine fir coexist and form the predominant climax community in the subalpine zone of the central and northern Rocky Mountains (Daubenmire, 1943; Stahelin, 1943; Marr, 1961; Romme and Knight, 1981), the mechanisms permitting their coexistence are not clear. Four different hypotheses have been presented specifically for Engelmann spruce-subalpine fir communities: 1) Peet (1981) suggested that coexistence of spruce and fir is maintained by disturbance because, in the absence of disturbance, the greater reproductive success of fir in the understory would lead to a steady increase in the dominance of fir and the virtual elimination of spruce after about 1,000 years; 2) in Alberta, Day (1972) found that subalpine fir cannot compete with Engelmann spruce x white spruce hybrids