Patterns of Community Dynamics in Colorado Engelmann Spruce‐Subalpine Fir Forests

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Abstract
A five—stand chronosequence spanning >500 yr is used to characterize changes in age structure, overstory mortality, recruitment, and understory growth in developing Colorado Engelmann spruce (Picea engelmannii)—subalpine fir (Abies lasiocarpa) forests. Stand development follows a postdisturbance sequence of colonization, spruce exclusion, spruce reinitiation, and second—generation forest. This model of spruce—fir forest development reflects a range of disturbance intensities from large conflagrations to small—scale tree deaths. Catastrophic disturbance initiates stand development, and canopy gap replacements occur at predictable times during stand development as the life—spans of the two species are expressed. Previous, spruce—fir forest have been described as either equilibrium or nonequilibrium systems. In the equilibrium coexistence model, spruce and fir constitute a stable, climax community through balanced life history strategies. Nonequilibrium coexistence relies on periodic catastrophic disturbance to disrupt competitive exclusion of spruce by fir. Neither of these models, taken alone, adequately explains the range of stand behavior observed in Colorado spruce—fir forests. Instead, stand development appears to involve processes from both models. The stands we investigated are undergoing postfire development and, therefore, support the nonequilibrium model, but a successful spruce cohort late in stand development insures the continued presence of both species, as predicted by the equilibrium model.