STRUCTURAL CHARACTERISTICS OF OLD-GROWTH HEMLOCK-HARDWOOD FORESTS IN RELATION TO AGE

We characterized the structure of 25 old-growth hemlock-hardwood forests in northern Wisconsin and adjacent Michigan in order to examine our working hypotheses that differences in their structure are related to stand age (i.e., stage of development) and that changes in stand structure continue after old-growth status is achieved. Estimates of stand age, i.e., age of oldest tree cored, based on 10 cores taken from hemlocks of a range of diameters in each stand, ranged from 177 to 374. By investigating the patterns of live tree structure, coarse woody debris (CWD), tip-up mounds, and canopy gaps in relation to stand age, we were able to infer changes that occur during stand development. Along the gradient of old-growth stand development, some changes in structural features, including total volume of CWD, snag (standing dead tree) basal area, and total area and average size of canopy gaps were continuous, linear increases over time. In contrast, changes in live tree and snag density, density of large trees, volume of well-decayed hemlock logs, and diameter-age relationships occurred after a threshold stand age of 275-300 yr was reached. Area and density of tip-up mounds and density of large seedlings and saplings were not correlated with stand age. Old-growth hemlock-hardwood stands at the upper end of the age continuum (> 275-300 yr) have accumulated both gradual and threshold structural changes, acquiring most of the following characteristics: (1) a strong correlation between age and diameter of trees, (2) low densities of live trees distributed across all size classes, (3) trees >70 cm dbh (diameter at breast height), (4) dead wood >120-150 m(3)/ha, with logs >80 m(3)/ha, (5) hemlock logs present in all decay classes, and (6) canopy gaps occupying >10% of the stand, with the average gap size > 50 m(2), some gaps > 200 m(2), and no more than 30% of the gaps <10 m(2). The two oldest stands may be at or near equilibrium, where maximum volumes of CWD are maintained through a balance of tree growth and mortality, and wood decay; and oldest tree age approaches 400+ yr, the maximum longevity typical for hemlock.