Population structure in old-growth and managed northern hardwoods: An examination of the balanced diameter distribution concept

Silvicultural guidelines for uneven-aged management have traditionally been based on the regulation of 'balanced' stand structures that allow for periodic yields at regular intervals while providing adequate growing space for multiple cohorts. To evaluate certain assumptions behind the 'balanced' diameter distribution concept, we investigated differences in the size structure, canopy recruitment, and growing space allocation among 31 old-growth, managed uneven-aged (selection), and even-aged northern hardwood stands in north-central Wisconsin and adjacent western Upper Michigan. The frequency of stands meeting criteria for balanced uneven-aged structure was substantially affected by whether diameter distributions were plotted for all crown classes combined (traditional approach) or whether the distributions included only trees with crowns exposed overhead to skylight. While all 10 old-growth stands and 11 of the 15 selection stands met the traditional criteria for balanced structure, only 50% and 27%, respectively, met the more stringent criteria for balanced structure that included evidence of substantial gap sapling recruitment. Both negative exponential and rotated sigmoid diameter distributions were found among individual old-growth and selection stands. However, all old-growth stands classified as balanced (based on exposed canopy trees) had rotated sigmoid diameter distributions for all species pooled and for sugar maple only. Negative exponential distributions were also observed in some 'balanced' stands, but only among the selection stands. Aggregate crown area distributions in old-growth and selection stands with reasonably smooth reverse-J diameter distributions provided support for a modified interpretation of the equal area allocation hypothesis. While the aggregate exposed crown area occupied by each size class tended to be lower for small trees than for medium and large trees, total crown area distribution for trees with partially or fully exposed crowns did approximate a nearly uniform distribution.