Variable density thinning promotes variable structural responses 14 years after treatment in the Pacific Northwest

Young stands are commonly assumed to require centuries to develop into late-successional forest habitat. This viewpoint reflects the fact that young stands often lack many of the structural features that define late-successional habitat, and that these features derive from complex stand dynamics that are difficult to mimic with forest management. Variable density thinning (VDT) is a silvicultural strategy designed to accelerate development of late-successional habitat by applying a variety of harvest intensities within a stand. Previous reports indicate that VDT has had initial success increasing growth and regeneration. However, few studies have examined the effects of VDT at longer time scales. Here, we report 14-year growth response of residual trees in the thinned and unthinned VDT sub-treatments in five young mixed-conifer stands located on the Olympic Peninsula in western Washington. Our objectives were to investigate whether thinning has accelerated the recruitment of large trees (>80 cm dbh), recruitment of shade-tolerant species into the mid-story (40-65 cm), or development of longer crowns relative to the unthinned sub-treatment. In addition, we investigated whether the basal area distribution in the combined VDT sub-treatments has become more diverse compared to the unthinned sub-treatment. The response to thinning varied consistently across the diameter size class gradient. Thinning was ineffective at stimulating growth of upper canopy trees (65-80 cm). In this size class neither diameter growth nor crown length increased significantly compared to trees in unthinned patches. Further, only one stand has reached the restoration benchmark for large tree density. In contrast, thinning significantly increased diameter growth and crown length among trees in the mid-story (40-65 cm) and shade-tolerant species in the future mid-story (20-40 cm). Higher rates of recruitment into the mid-story were also observed from shade tolerant species growing in the thinned (34%) compared to unthinned (19%) patches, with two stands reaching the restoration benchmark for shade-tolerant mid-story density. Clear trends in basal area diversity and evenness have yet to develop in either the combined or unthinned sub-treatments. Collectively, our results demonstrate that VDT has partially accomplished its objectives. Although thinning has not yet accelerated recruitment of large trees, it has accelerated the advancement of shade-tolerant species into the mid-story and the development of deeper crowns among trees in smaller size classes. In addition, differing rates of diameter growth among smaller diameter trees in the various VDT sub-treatments suggest that increases in structural diversity may be developing more quickly than in untreated stands.