Age and Size Structure of Subalpine Forests in the Colorado Front Range

Abstract

Age and size data on over 2200 trees were used to reconstruct developmental patterns and regeneration dynamics of four successional and two climax stands of subalpine forests in the Colorado Front Range. The dominant tree species of these forests are Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), limber pine (Pinus flexilis), and lodgepole pine (Pinus contorta). Due to relatively weak relationship between age and size for all four species, patterns of stand development could not have been reliably inferred from size data alone. Following wildfire on the most xeric sites, limber pine is the principal pioneer species and dominates seedling establishment for 50 to 100 or more years. At most sites, however, Engelmann spruce and subalpine fir eventually establish and gradually replace limber pine. On less xeric sites, depending on availability of seed, either Engelmann spruce or lodgepole pine, alone or together, may act as pioneer species. Establishment of subalpine fir may be coincident or may br several decades later. Eventually it and Engelmann spruce replace lodgepole pine as the canopy dominants. The age structures of the stands investigated are consistent with the view that variation in establishment is at least as important as mortality in shaping age frequency distributions in successional stands. In climax Engelmann spruce-subalpine fir stands both species have all-aged populations; the typically greater abundance of young subalpine fir appears to be compensated by the much greater longevity of Engelmann spruce.