Understory vegetation dynamics 15 years post-thinning in 50-year-old Douglas-fir and Douglas-fir/western hemlock stands in western Oregon, USA

• Thinning increased forb cover initially.
• Shrub cover increased after thinning.
• Continuous canopy layer had not developed after fifteen years.
• Effects of understory spraying were still evident after fifteen years at one site.
• Abundant natural regeneration of western hemlock was affecting vegetation development at one site.

Recent trends in forest management practices designed to restore or enhance late-successional structure for habitat values typically include provisions for the creation and maintenance of structural and compositional diversity within and among stands. The Pacific Northwest in particular has expanses of young, structurally simple forests that range from high stocking (unthinned and little understory development) to relatively lower stocking having been previously thinned or initially regenerated at low densities. We have remeasured two long-term study sites in Oregon, USA, for 15 years to examine structural development and understory persistence at a range of overstory canopy cover, spatial arrangement, and understory treatments (spraying and planting). Understory responses to thinning varied primarily by site and species group, given the range of pre-existing plant communities, and through time. In general, shrub cover increased after thinning, while forb cover increased initially after thinning and then declined. Though spraying effects were still visible 15 years after treatment driven by reductions in one dominant understory species, it had little effect on the long-term development of structure at these overstory densities. Indeed, the range of overstory densities and spatial arrangements in this study did not have any large effects because all of the thinning treatments resulted in increased light availability to the understory at least through fifteen years after thinning. Combining more extensive overstory removal and understory treatments likely can be used to stimulate heterogeneous structure and composition development in these forests, including the possible stimulation of persistent early-seral vegetation within older stands.