Age Structure and Successional Dynamics of a Colorado Subalpine Forest

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Abstract
Population age structures of old-growth Engelmann spruce [Picea engelmannii], subalpine fir [Abies lasiocarpa] and lodgepole pine [Pinus contorta] are described in the Colorado [USA] Front Range subalpine forest (2850 and 3500 m). Among the populations of these 3 spp., 5 types of age structures can be recognized; up to 4 different types of age structures were found in 1 sp. under different environmental or historical conditions and no species showed the same type of age structure throughout its entire range of occurrence. The age structures of populations are used to infer their climax or successional (self-replacing or not self-replacing) status. From the combination of climax or successional populations in a stand and the distributions of these combinations on environmental gradients, 3 types of forest are recognized: climax lodgepole; climax spruce-fir, which is pioneered after disturbance by lodgepole; and climax spruce-fir pioneered primarily by spruce. The changes from one forest type to another follow a unidimensional gradient which begins with climax lodgepole at low elevation, dry sites, followed by a forest mosaic of successional lodgepole and climax spruce-fir at both higher elevations and intermediate moisture sites and ends with climax spruce-fir in which lodgepole is not pioneer, at both high elevations and on the most mesic sites.