A Structural Alternative to Chronosequence Analysis for Uneven-Aged Northern Hardwood Forests

Structural changes over time in forests of shade-tolerant species are difficult to study because these stands are often broadly uneven-aged and not suitable for traditional chronosequence analysis. Yet because of past disturbances of variable intensity and frequency, there is often substantial structural variation among stands of a given site quality class. In this study, diameter distributions from 70 primary northern hardwood stands were analyzed to determine if certain structural indices might allow placement of stands in a developmental sequence. Modal stand diameter and the fraction of total overstory crown area in large trees (> 46 cm dbh) were among the most useful measures for distinguishing stages of development. Modal diameter ranged from 10 cm to 62 cm for stands on good sites. Correlations between diameter and age were highly significant (P < 0.0001) for the three principal species on all three site quality classes, suggesting that stands with a greater proportion of large trees are generally in a later stage of develovment. Stands with < 45% of the crown area in large trees have unimodal size distributions. As modal stand diameter increases, the shave of the size distribution changes from positively skewed to nearly 'symmetric. As the proportion-of crown area in large trees exceeds 45% and the ratio of crown area in Craig G. Lorimer is Professor of Silviculture and Forest Ecology, Department of Forestry, University of Wisconsin-Madison, Madison WI 53706. Lee E. Frelich is Research Associate, Department of Forest Resources, University of Minnesota, St. Paul, MN 55108. Journal of Sustainable Forestry, Vol. 6(3/4) 1998 @ 1998 by Thc Haworth Press, Inc. All rights reserved. 347 348 JOURNAL OF SUSTAINABLE FORESTRY large to mature (26-45 cm dbh) trees exceeds 1.5, the form of the size distribution changes to multimodal, irregular, or descending monotonic. [Ariicle copies available for a feejiom The Haworrh Docun~ent Delivery Service: 1-800-342-9678. E-mail addr~ss: ge!info@hawor~h.com]