Managing for old-growth structure in northern hardwood forests

Recent research in the northeastern United States has focused on "structure-" or "disturbance-based" silviculture. The as yet untested hypothesis is that these approaches can sustain a broader array of biodiversity and ecosystem functions than conventional systems. I am testing this hypothesis using a system that promotes old-growth structural characteristics, termed "structural complexity enhancement" (SCE). This approach is compared against two conventional, uneven-aged systems (single-tree selection and group-selection) modified to enhance post-harvest structural retention. The study is replicated at two mature, northern hardwood forests in Vermont. Manipulations and controls were applied to 2-ha units. The uneven-aged treatments were replicated twice; the SCE treatment and controls were each replicated four times. Structural objectives include multi-layered canopies, elevated large snag and downed log densities, variable horizontal density, and reallocation of basal area to larger diameter classes. The latter objective is achieved by using an unconventional marking guide based on a rotated sigmoid target diameter distribution, applied as a nonconstant q-factor. The marking guide is also derived from a target basal area (34 m(2)/ha) and maximum diameter at breast height (90 cm) indicative of old-growth structure. Crown release was also used to promote growth in larger trees. Prescriptions for enhancing snag and the density of downed woody debris are based on stand potential. Forest structure data, including Leaf Area Index (LAI), detailed measurements of individual trees, and coarse woody debris (snags and downed logs) densities and volumes, have been collected over 2 years pretreatment and 2 years post-treatment. A before/after/control/impact approach was used to analyze these data. Fifty-year simulations of stand development were run in NE-TWIGS, comparing alternate treatments and no-treatment scenarios. Basal area retention, relative density, canopy closure, LAI retention, and coarse woody debris volumes and densities were significantly higher under the old-growth silvicultural system. Residual diameter distributions achieved the target rotated sigmoid form. There will be significant differences in stand development based on the simulation modeling. Late-successional structural and compositional characteristics will develop to a greater degree under SCE. Large tree (> 50 cm d.b.h.) recruitment will be impaired under the conventional treatments, whereas rates of large-tree development will be significantly accelerated under SCE.