Can group selection with legacy retention change compositional trajectories in conventionally managed hardwoods?

摘要

Group selection has been proposed as a complementary system to conventional, uneven-aged management in temperate hardwoods (i.e. single-tree selection) to counteract declines in overstory diversity and growing importance of shade-tolerant species caused largely by past management activities. Mixed results from experimental group selection harvests in eastern North America and growing interest in retention forestry have led some scientists and managers to apply ideas from retention forestry into group selection systems. The Yellow Birch Legacy-Tree Project is a group selection experiment established in 2003, which incorporates single, dominant/co-dominant yellow birch (Betula alleghaniensis Britt.) legacy-trees into the centers of harvested canopy gaps in a northern hardwoods forest in the Upper Peninsula of Michigan. Group selection openings (n = 49) in three size classes based upon radii-small (11 m, n = 16), medium (16.5 m, n = 17), and large (22 m, n = 16)-were surveyed alongside single-tree selection reference sites (n = 20) 15 years post-harvest. Our goals were to (1) examine long-term trends in seedling and sapling abundance and diversity, (2) evaluate the initial and contemporary effects of gap size on regeneration, and (3) assess the overall efficacy of group-selection with legacy-tree retention in regenerating mid-tolerant and intolerant species, especially yellow birch. At 15 years post-harvest, we found that shade-tolerant sugar maple (Acer saccharum Marsh.) dominated regeneration layers across all treatments, but we observed modest increases in seedling and sapling diversity within openings that may alter long-term canopy composition over several cutting cycles. We found that gains in diversity and evenness in canopy gaps persisted through time and that large gaps (22 m radius) had the most species-rich and diverse regeneration compared to reference sites. Canopy gaps contained significantly higher densities of shade-intolerant and mid-tolerant seedlings and saplings compared to reference sites, namely red maple (Acer rubrum L.), black cherry (Prunus serotina Ehrh.), and yellow birch; however, most diversity occurred in shorter height classes. Nevertheless, legacy retention within group selection systems holds promise for maintaining ecological memory and structural complexity through time, but further treatments such as scarification may be necessary to shift compositional trajectories in favor of shade-intolerant and mid-tolerant species.