Long-term response of forest bird communities to retention forestry in northern temperate coniferous forests

摘要

Retention forestry aims to maintain structural and functional continuity in managed stands. While short-term benefits to forest dwellers are well documented, few studies have followed patterns over decades to assess whether benefits continue. Initiated in 1992, the Date Creek Research Forest in northwest BC includes four replicates of four retention levels: 0% (clearcut), 40%, 70% and 100% (unharvested control). These similar to 20-hectare stands, with retention designed to mimic typical insect and wind disturbance, are now entering the canopy closure seral stage. We followed breeding bird community dynamics over 25 years (pre-harvest and 24 years post-harvest). Given relatively consistent landscape conditions over time, we focused on stand-level changes in bird community composition and species abundance. Community similarity to controls decreased as retention decreased; communities changed over time, with those in clearcuts changing most. Patterns in species abundance varied with habitat preference. Conifer-forest species declined immediately after harvest, with some responding linearly to retention level while others responded similarly to 40%, 70% and 100% treatments; Pacific Slope Flycatcher, Pacific Wren and Brown Creeper remained significantly less abundant in some treatments by year 24. Open-habitat and generalist species increased most in clearcut and somewhat in 40% retention stands, with generalists taking advantage of the increased heterogeneity immediately and open-habitat species colonizing by year 10; some of these species, particularly shrub specialists, remained at high abundance in harvested stands in year 24. Mixed-forest species reached their highest abundance at 40% retention in year 2 and 10, and dropped to control levels by year 24. Although conifer-forest, open-habitat and generalist species treated retention stands as intermediate between clearcuts and controls, mixed-forest species perceived them as a different, preferred habitat for the first decade. We conclude first that most forest birds do not discriminate between 70% retention stands and unharvested forest, confirming strategies in the region to maintain biodiversity in landscape corridors, second that aggregated 40% patch retention partially or fully mitigates impacts for conifer-forest birds, and provides new habitat for mixed-forest and open-habitat species, and third that bird communities are still changing after 24 years as these stands enter canopy closure.