Influence of partial harvesting and site factors on the abundance and composition of natural regeneration in the Acadian Forest of Maine, USA

Understanding the factors regulating the composition and abundance of natural regeneration in forest ecosystems is critical to sustainable management worldwide. Using a long-term silvicultural experiment in Maine, we partitioned the variation in natural regeneration and examined the contribution of overstory and understory vegetation (biotic factors), substrate and soil attributes (abiotic factors), and silvicultural treatment. We hypothesized that silvicultural treatment (single-tree selection with 5, 10, and 20-year cutting cycles and diameter-limit cutting) would be less influential than abiotic and biotic factors in structuring regeneration pattern. Overall, treatment and site (biotic and abiotic) factors accounted for 26% of the variation in regeneration species composition, highlighting stochasticity of the regeneration process. In support of our hypothesis, regeneration pattern that could be explained was primarily associated with local site factors; overstory and understory vegetation uniquely accounted for 16% of the variation in species composition. Substrate and soil attributes along with silvicultural treatment accounted for 4% of variation, and uniquely accounted for 2% and 4%, respectively. In support of earlier reports, shifts toward hardwood dominance were apparent at early stages of cohort development and were associated with high harvest intensities. Our findings suggested that overstory and understory vegetation exhibit greater control over regeneration patterns than substrate and soil attributes, or silvicultural treatment, in partially harvested Acadian Forest mixedwood stands, and also indicated the potential for composition control through selective overstory removal.