Common and Uncommon Understory Species Differentially Respond to Restoration Treatments in Ponderosa Pine/Douglas-Fir Forests, Montana

Restoration treatments have been widely advocated to address declining conditions in Pinus ponderosa forests throughout the western United States. However, few studies have examined treatment effects on individual plant species or whether responses differ for common species and uncommon species (those with low abundance in the community)—information that may be critical in managing for long-term biodiversity. We investigated understory species responses to restoration treatments in ponderosa pine/Douglas-fir forests using a randomized block experimental design with three blocks and four treatments (control, burn-only, thin-only, and thin-burn). Understory vegetation was sampled before treatment and for three consecutive years after treatment. We used richness and an index of uniqueness to compare responses of common and uncommon native understory species among treatments, and indicator species analysis to identify individual species that responded to each treatment. Treatments that included thinning had significantly more unique species assemblages than the control. The thin-only treatment increased common native species richness, whereas all active treatments significantly increased uncommon native species richness over the control, especially the thin-burn. Generally, life-forms did not explain the responses of individual species, though in the final sampling year several graminoids were exclusively indicative of treatments that included thinning. Very few species had reduced abundance in the thinning and burning treatments by the final sample year, whereas many uncommon and short-lived species benefited from active treatments, especially the combined thin-burn treatment. Active restoration treatments in these forests may foster plant diversity by minimally impacting common species while significantly benefiting disturbance-dependent native species.