A Comparison of Structure and Composition between Oldgrowth and Managed Maple-Dominated Forests in Central Ontario

In an effort to emulate natural disturbances typical of the region, Central Ontario’s uneven-aged hardwood forests are predominantly managed through partial harvesting, applied as single tree selection on crown land. Various studies have indicated however, that this silvicultural approach is not ideal for the maintenance of structural heterogeneity and compositional diversity characteristic of unmanaged forests. We quantified tree species composition and diversity, as well as various stand structural attributes following multiple stand harvest entries in 11 mature post-harvest stands and compared them with 11 unmanaged stands, distributed in both Haliburton Forest and Wildlife Reserve and Algonquin Provincial Park. Tree community compositional differences were largely site specific and likely influenced by a combination of history, topography and site quality. Haliburton Reserve had the highest level of tree diversity regardless of treatment, especially in unmanaged stands. By contrast, stands in Algonquin Park showed lower diversity and varied little between the two treatments. In Algonquin Park, STS management appeared to reduce the relative basal area of sugar maple, with the opposite occurring in Haliburton forest. Effects on other dominant canopy species such as eastern hemlock and yellow birch also varied according to location. The only shared compositional effect between locations was a reduction in American beech basal area and a lack thereof for yellow birch following multiple STS cycles. Diameter distributions were similar between treatments; however, STS stands had a reduced representation of large diameter stems, only significant when comparing stems that were ≥40, 60 and70 cm diameter at breast height. Structural differences, particularly for coarse woody debris, showed strong treatment effects and were not site specific. Overall, STS stands had a 50% decrease in volume of WD and a 58% decrease in basal area of snags compared to unmanaged stands. When restricting analyses to larger diameter stems of snags (≥ 30 and ≥50 cm diameter), those decreases become even more pronounced (62% and 73%, respectively). Reductions in downed woody debris occurred for all sizes and decay classes but were especially marked for large stems, especially ones in advanced decay. Finally, partial harvesting in this study was found to reduce heterogeneity of structural attributes at the ‘within-stand’ level relative to unmanaged forests. Our results suggest that partial harvesting may result in the regeneration of fewer tree species, but strongly reduces structural elements important for biodiversity