Forest management practices interact with multiple sources of variability to influence regeneration trends in northern hardwood forests. There is uncertainty whether low-intensity selection harvesting techniques will result in adequate and desirable tree regeneration. Our research is part of a long-term study that tests the hypothesis that a silvicultural approach called “structural complexity enhancement” (SCE) can promote accelerated development of late-successional forest structure and functions. Our objective is to understand the regeneration dynamics following three uneven-aged forestry treatments modified to increase postharvest structural retention: single-tree selection, group selection, and SCE. In terms of regeneration densities and composition, how do light availability, competition, substrate, and herbivory interact with treatment effects? To explore these relationships, manipulations and controls were replicated across 2-hectare treatment units at two sites in Vermont, USA. Forest inventory data were collected pre-harvest and annually for 13 years post-harvest. We used a linear mixed effects model with repeated measures to evaluate the effects of treatment on seedling and sapling diversity (Shannon-Weiner H’). To evaluate the effects of treatment on seedling and sapling abundances over time, a generalized linear mixed effects model was used with a negative binomial distribution. Multivariate analyses evaluated the relative predictive strength of treatment versus alternative sources of ecological variability.
Results/Conclusions
Thirteen-years post-harvest, SCE has the highest number of seedlings per hectare (39,000/ha), followed by single-tree selection (27,600/ha), control (24,500/ha), and group selection (16,000/ha). For every year post-harvest, the group selection treatment shows the highest relative abundance of American beech (Fagus grandifolia) for both seedlings and saplings. However, in all of the treatments prolific beech sprouting dominates the understory in patches. Total seedling densities are significantly different immediately post-harvest, converging towards similar means after thirteen years. Total sapling densities are similar immediately post-harvest; however, after thirteen years the three harvested treatments have significantly more saplings than the control. Sugar maple saplings are less prevalent than American beech in all of the treatments across all years, changing minimally between the treatments. Seedling diversity is not significantly different between the control and the SCE treatment. SCE results in overall higher seedling diversity, compared to single-tree selection and group selection. Multivariate analyses suggest that while treatment had a dominant effect, other controls such as deer browse were strongly influential in driving regeneration responses. Results indicate variants of uneven-aged systems that retain or enhance stand structural complexity, including old-growth characteristics, generally show resilience to regeneration limitations depending on site conditions.