Restoration of old-growth forest structure in the northeastern United States: Experimental evaluation of silvicultural options

Restoration of late-successional forest habitats and functions is an important element of ecosystem management in the northern hardwood region of North America. This study tests the hypothesis that uneven-aged silvicultural practices can be modified to accelerate rates of late-successional forest development in northern hardwood-conifer forests. An approach, termed “Structural Complexity Enhancement (SCE), is compared against conventional uneven-aged systems modified to increase post-harvest structural retention.  The study is replicated at two study areas in Vermont, U.S.A..  Manipulations and controls were applied to 2 ha units and replicated four times per treatment. 

Structural objectives include multi-layered canopies, elevated large snag and downed coarse woody debris (CWD) densities, variable horizontal density, and re-allocation of basal area to larger diameter classes.  The later objective is achieved using an unconventional marking guide based on a rotated sigmoid target diameter distribution. The marking guide is also derived from a target basal area (34 m²/ha.) and maximum diameter at breast height (90 cm) indicative of old-growth structure. Accelerated growth in larger trees is also promoted through crown release. Prescriptions for enhancing snag and downed woody debris densities are based on stand potential. On some units downed CWD is created by pulling trees over to create pits and exposed root wads. Vegetation, wildlife (birds, small mammals, amphibians, and soil invertebrates), soils, and economic data have been collected over two years pretreatment and four years post-treatment. There will be significant differences in stand development based on simulation modeling using NE-TWIGS and FVS. Late-successional structural characteristics will develop to a greater degree under SCE. Large tree recruitment rates were related primarily to the form of residual diameter distributions and, possibly, to maximum diameter limits.  This system is economically profitable under the right site and market conditions. Potential applications include old-growth restoration, riparian restoration, and low intensity commercial forest management.