Predicting effects of selection cutting on avian diversity and abundance in shade-tolerant hardwood forests of central Ontario

Abstract

Emulating natural disturbance (END) patterns to conserve biodiversity is often a goal of forest management but the impact of forest cutting is still controversial. We examined effects on avian assemblages of partially-cut forest stands of 1–10, 11–20 and > 45 years (cut classes) post-harvest differing in the % basal area retained as wildlife species’ habitat in 85 shade-tolerant hardwood stands in Algonquin Park, Ontario. Permutational multivariate analysis of variance indicated significant compositional differences among cut classes, after controlling for year and township (spatial variation in surficial geology and topography), both of which were also significant. Pairwise comparisons indicated significant compositional differences only between young and old stands. The difference in beta diversity (homogenization/differentiation) among cut classes was marginally significant (P = 0.066). Pairwise differences were significant between old versus mid-aged cut classes, marginally significant for young versus old cuts, and non-significant for young versus mid-aged cut classes. Six species and cavity users as a group were more abundant in > 45 years post-harvest stands than in the other two cut classes. Six species had higher abundances in younger cuts than > 45 years post-harvest cuts. Red-eyed vireo (Vireo olivaceus) showed a nonlinear response and was least abundant in 11–20 year cuts. Blackburnian warblers Setophaga fusca, black-throated green warblers S. virens and pileated woodpeckers Dryocopus pileatus increased with the density of supercanopy trees (≥ 60 cm diameter at breast height - dbh) and in the case of pileated woodpeckers more large snags (≥ 40 cm dbh). Brown creeper Certhia americana abundance increased with greater basal area of trees ≥ 25 cm dbh and ovenbird numbers were significantly related to overall basal area. Chestnut-sided warbler S. castanea abundance decreased with an increase in overall basal area and was higher in the 1–10 year cuts and American redstarts S. ruticilla decreased with an increase in basal area. Our modelling suggests that effects of selection cutting systems on the avian community may be reduced as tolerant hardwood stands become regulated and have higher residual basal areas. Current guidelines in Ontario for the retention of large, scattered conifer trees appear to meet the requirements for most, but not all, species; however, guidelines for supercanopy trees are too low. These results indicate that the END paradigm for single-tree selection cutting in tolerant hardwoods may be partially effective but requires modifications, such as retaining higher densities of supercanopy trees that would be beneficial for birds.