Nest-site selection by old boreal forest cavity excavators as a basis for structural retention guidelines in spatially-aggregated harvests

Abstract

Cavity excavators (woodpeckers, chickadees, nuthatches) are vulnerable to loss of old forest associated with traditional harvest practices. In the boreal plains of Canada, old forest excavators are retained in the short term in small (<100ha) and large (1000sha) harvests with residual trees and patches. Information on habitat selection can be used to guide “what, where, and how much” structure to retain and ensure excavators, and the secondary species that use their cavities, are conserved in harvests in the short term. We examined multi-scale nest-site use and selection by excavators in intact old upland forest and spatially-aggregated harvests with structural retention and developed guidelines for the types and spatial arrangement of trees, patches, and stands retained during forest planning (i.e. planned residuals) and harvesting (i.e. operator residuals). In both intact and harvest landscapes, live large (>35cm) and tall (>25m) trembling aspen (Populus tremuloides) with multiple (∼20) conks of a heartrot fungus (Phellinus tremulae) were the preferred cavity trees for yellow-bellied sapsucker (Sphyrapicus varius), hairy woodpecker (Picoides villosus), and pileated woodpecker (Dryocopus pileatus). Short (∼10m), broken-top aspen snags of medium (>20cm) and large (>35cm) diameter were preferred by black-capped chickadees (Poecile atricapillus) and northern flickers (Colaptes auratus), respectively. In the harvests, sapsuckers, hairy woodpeckers, and flickers selected cavity trees in clumps with other medium- and large-diameter aspen with conks. Black-capped chickadees selected trees with no live large conifers within ∼10m. Sapsuckers selected old aspen and mixedwood stands in intact landscapes. In harvest landscapes, sapsuckers selected trees in or near planned patches greater than 5ha and composed of mature and old aspen and mixedwood stands. Sapsuckers tended to nest in areas with ∼50% residual forest cover whereas flickers selected open areas of cutblocks away from planned patches. We recommend harvest operators retain several live, large (>35cmdbh) aspen with numerous (∼20) fungal conks in clumps that are adjacent to or near (<60m) large (>5ha) patches. Planned patches should have one to two thirds of their area derived from mature and old aspen and mixedwood stands and should be at least 5ha, with most greater than 15ha. These prescriptions will facilitate retention of the cavity-using assemblage of old upland forest in aggregated harvests in the short term. However, regional strategies for old forest conservation may be necessary to conserve cavity users in managed forest landscapes over the long term.