Demographic Aspects of Coexistence in Engelmann Spruce and Subalpine Fir

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American Journal of Botany

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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 between 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.