Gap and Stand Dynamics of a Southern Appalachian Spruce-Fir Forest

Southern Appalachian spruce-fir forest dynamics were investigated by field studies and computer simulations. Mechanisms of old-growth stand dynamics were determined in a Great Smoky Mountains spruce-fir forest (elev. 1770-1830 m). The possible effects of anthropogenic disturbances on the mature forest were investigated with a gap model adapted to upper spruce-fir zone forests.

An analysis of stand dynamics involved the study of population interactions in the context of gap phase dynamics. Tree population structure and dispersion were quantified. Regeneration requirements and canopy-understory interactions were determined. Canopy turnover was modeled using species-by-species gap phase tree replacement probabilities. Finally, a spatial gap model was developed to simulate compositional and structural dynamics of a forest stand

The old-growth forest is comprised of stable, all-aged spruce and fir populations. Fir is more abundant than spruce, but spruce dominates the forest because of its relatively long life span and large size. Shade tolerant spruce and fir saplings, regenerated in advance, are highly successful invaders of canopy gaps created by tree fall. Fir captures a large proportion of gaps regardless of gap maker species. It is successful because of its high regeneration and growth rates. The canopy residence time of fir, however, is relatively short. Birch and other hardwoods are maintained by gap phase regeneration resulting in occasional gap capture.

Simulated balsam woolly aphid infestation results in a spruce dominated forest. In the absence of fir, stand biomass recovery is slow and total stand density is reduced. A moderately severe spruce growth decline in the presence of an undisturbed fir population results in a fir-birch forest. In association with a fir population decline the effects of spruce growth stress are diminished. The contemporaneous decline of spruce and fir populations results in a hardwood dominated forest with low total stand biomass.

The old-growth forest is close to equilibrium composition. As a consequence of small disturbance patch size, forest composition, biomass and structure are relatively constant over a small landscape area. This degree of equilibrium is unique among spruce-fir forests of eastern North America.