The Influence of Cutting Cycle and Stocking Level on the Structure and Quality of Managed Old-Growth Northern Hardwoods

The effects of 57 years of uneven-aged management on the structure and quality of old-growth northern hardwood forests were examined within a long-term Forest Service study in the Dukes Experimental Forest, Michigan. Treatments within the study included several differing stocking levels (30, 50, 70, and 90 ft2/acre) and cutting cycles (5, 10, 15, and 20 years) replicated across three blocks. Tree growth and quality, as well as stand structural attributes were examined within each treatment and compared to an unmanaged Research Natural Area (RNA). Comparisons of tree grade between treatments indicated that cutting cycle length and stocking level had no effect on tree quality. In contrast, average annual diameter growth was greatest in lower stocking treatments, and regression analyses of tree growth versus residual stocking level indicated that optimum levels of growth were achieved between residual basal areas of 50 and 70 ft2/acre. Comparisons of stand structure between treatment areas and the unmanaged RNA suggested that volumes of CWD in the high residual basal area treatments most closely approximated those found within unmanaged old-growth. Treatments with high residual basal areas and longer cutting cycles had significantly greater snag basal area and volume than other treatments and were closer in this respect to the RNA than other treatments. Analysis of diameter distributions over time indicated that cutting cycle had little effect on distribution shape over the course of this study. In contrast, treatments with lower residual basal areas developed from increasing q to concave or negative exponential distributions over time, while the 90 ft2/acre treatment displayed an increasing q distribution throughout the experiment. The findings of this study suggest that manipulation of stocking levels within northern hardwood stands is of greater importance than cutting cycle in controlling stand growth, as well as influencing the development of structural complexity