Emulating natural disturbance dynamics in northern hardwood forests: Long-term effects on species composition, forest structure, and yield

Ecological forestry practices are designed to emulate natural disturbance processes and restore species and structural features that may be deficient in conventionally managed stands. Recent application of ecological forestry treatments in managed uneven-aged northern hardwood forests have typically emphasized modified selection silviculture that creates small single-tree and multiple-tree gaps and retains some structural features of old-growth forest, such as large trees, snags, and logs. These treatments, however, currently do not emulate the effect of moderate-severity disturbances, which play an important role in shaping the structure and diversity of northern hardwoods, and there is little evidence available on the short- and long-term effects of these treatments on forest structure and yield. The objectives of this project were to (1) investigate the impacts of moderate-severity disturbances on forest structure (2) design a new multi-cohort management system that emulates these disturbances (3) incorporate ecological forestry algorithms into an existing forest model (CANOPY) and (4) evaluate the long-term effects of multi-cohort management and other ecological forestry practices on forest composition, structure, and yield.Moderate wind disturbances produced residual forest conditions that were much more heterogeneous than managed stands. As in standard selection silviculture, moderate windstorms removed trees from a wide range of size classes, but gap sizes and light regimes were much more variable than in managed stands. Based on these findings, the revised CANOPY model was then used to simulate 22 alternative silvicultural treatments over a span of 300 years. Treatments with 80 cm maximum diameters reduced cumulative yield by 24% relative to conventional harvests but were the only treatments to promote structural characteristics similar to old growth. Multi-cohort treatments resulted in little to no reduction in yield but did not differ in structure from treatments with the same maximum diameter. Permanent reserve trees did not greatly affect structure or yield until they were larger than the specified maximum diameter. Reserve tree densities of 7--22 trees/ha reduced cumulative yield by 7--20%. The retention of large diameter trees resulted in a 1% reduction in yield per tree, but if large trees were never harvested, the reduction increased to 2.5% per tree.