Long-Term Influence of Retained Forest Aggregates on Conifer Regeneration in Harvest Units of the Pacific Northwest

Abstract

Aggregated retention, in which undisturbed patches of forest are retained within harvest units, is increasingly used in forests managed for multiple objectives. Retained aggregates can serve as refugia for disturbance-sensitive species and as sources for dispersal into adjacent harvested areas. However, it less clear how they shape natural regeneration processes, including the recruitment and growth of early- and late-seral tree species. I modeled the effects of distance from edge (0-70 m), edge type (1-ha aggregate vs. harvest-unit boundary), and edge exposure (a proxy for heat/moisture stress) on the recruitment and height growth of early- and late-seral conifers nearly two decades after aggregated-retention harvest at four sites in the Cascade Mountains of Oregon and Washington. Aggregates effectively mimicked the influences of harvest-unit (intact-forest) boundaries on recruitment and growth. For both seral groups, recruitment density declined similarly with distance from edge. Distance-related trends in growth were generally similar between edge types but varied among sites. Height tended to increase with distance but peaked closer to edge for late-seral advanced regeneration than for early-seral recruitment. Late-seral recruits—common in the two Washington sites—were shorter than survivors and unresponsive to distance from edge. Edge exposure had no consistent effect on recruitment or height growth. My results suggest that 1-ha aggregates are functionally equivalent to larger blocks of intact forests in their influences on conifer recruitment and growth. By distributing aggregates across harvest units, managers can accelerate rates of recruitment and promote heterogeneity in the seral composition and height structure of the regenerating forest. Additional research is needed on the sensitivity of regeneration processes to aggregate size, shape, and spatial distribution.