Structural retention requirements for a key ecosystem engineer in conifer-dominated stands of a boreal managed landscape in eastern Canada

Abstract

The concept of retention forestry brings a new point of view for forest management as more emphasis is put on what is retained as opposed to what is removed during harvesting. Details of retention harvesting prescriptions vary among forest ecosystems reflecting differences in their disturbance dynamics, stand composition and condition, and landscape context. Although recent studies underlie the important role of trembling aspen (Populus tremuloides) in cavity networks of Canadian western temperate forests, limited information exists for conifer-dominated forests such as the black spruce (Picea mariana) forests of central Québec where the proportion of deciduous trees is generally <10%. We used the habitat requirements of an ecosystem engineer of eastern boreal forests, the Black-backed Woodpecker (Picoides arcticus), to determine harvesting prescriptions likely to preserve most of the biodiversity of this ecosystem. We describe the mean physical characteristics of trees selected for nesting, quantify nesting habitat selection at two scales (nest tree and nesting stand), and identify quantitative targets for forest managers to select types and amounts of retention structures that will offer optimal resources to cavity nesting birds and dead wood associated species in managed boreal landscapes. During the three years of this project, we found 18 Black-backed Woodpecker nests: 9 were in old coniferous, 6 in young cuts, and 1 nest in each of the other habitats (barren land, young coniferous and old cut). Selection of nest trees by Black-backed Woodpeckers was influenced primarily by tree species (jack pine; Pinus banksiana) and also by the dbh of trees, while their selection of nesting stand habitat was influenced positively by the mean density of recent-solid snags and negatively by the mean basal area of live trees. Our results show that forest stands with ⩾200 recently decayed snags/ha (⩾9cmdbh) exhibit higher predicted probability of selection for nesting by Black-backed Woodpeckers, and most of the cutblocks with a nest tree originate from harvesting with advance regeneration protection with at least that many snags. Moreover, cutblocks should contain suitable candidate nest trees, with retention of ⩽10 live and dead trees/ha with a dbh⩾27cm, and retention of jack pine snags in proportion of their availability before harvesting in black spruce-dominated forest stands. We recommend that, where forest stands present suitable structures, every stand harvested with retention harvesting should retain ⩾200 recent-solid snags/ha (⩾9cmdbh), which represents about 1 recent-solid snag each 7m (50m2). The medium and long-term persistence of dead wood associated species may not rely only on retention forestry, thus we recommend the protection of patches (100ha) of old forest stands (⩾120y) inside these agglomeration of cutblocks to ensure the dynamic of dead wood creation within the managed landscape.