Forest aggregates influence conifer recruitment and height growth in a long-term variable-retention experiment

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The practice of aggregated retention is becoming increasingly common in forests managed for multiple objectives (e.g., timber production and maintenance of biodiversity). Undisturbed patches of forest (aggregates) can serve as refugia for disturbance-sensitive species and as dispersal sources for recolonization of adjacent harvested areas. However, few studies have characterized the contributions of aggregates to the natural recruitment or height growth of regenerating trees. In this study, we modeled the density and height of early- and late-seral conifers, including post-harvest recruitment and advanced regeneration (survivors), two decades after aggregated-retention harvest at four sites in the Cascade Mountains of Oregon and Washington. Predictors included distance from edge (0–70 m), edge type (1-ha aggregate vs. intact forest/harvest-unit boundary), and edge exposure (a proxy for heat/moisture stress). For both early- and late-seral species, we observed similar declines in density with distance from aggregate and intact-forest edges. Trends in height growth showed greater variation among sites and seral groups. Height tended to increase with distance from edge, peaking sooner for late-seral advanced regeneration than for early-seral recruitment. In contrast, heights of late-seral recruitment were distinctly shorter and unaffected by distance from edge. Edge exposure did not have a consistent effect on recruitment density or height growth of either seral group. Our results indicate 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 density and height structure of the regenerating forest. Additional research is needed on the sensitivity of regeneration processes to aggregate size, shape, and spatial distribution.