Population Structure of Forests Over a Large Area

Forest structure was examined along a wide transect extending from Oklahoma to North Carolina and covering a wide variety of forest types and species. A composite curve produced by all the species included in the sample showed an overall concave form of the semilogarithmic density-diameter curve, with a rotated sigmoid shape at the mid-diameter size-class range. The degree to which individual species exhibited the concave or convex shape was directly related to shade-tolerance capacities of species. Stratification of regional forest stands by diameter class (age) showed the rotated sigmoid shape to exist at the all-species level and for some individual species within the oldest stands. In eastern deciduous forests, rotated sigmoid density-diameter distributions will occur on a regional basis in response to canopy-understory interactions as stand average diameter increases and as the relative importance of shade-tolerant tree species increases. Sigmoid density-diameter distributions in contrast to linear or slightly concave/convex distributions do not necessarily indicate a deviation from the structural development patterns of undisturbed forest stands. However, effects from forest harvesting on regional forest structure cannot be totally discounted as a source of structural variation from a negative exponential pattern. Forest Sci. 27:701-710.