Long-term response of understory vegetation to stand density in Picea-Tsuga forests

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Abstract
The 17-year response of understory vegetation to forest thinning experiments was examined in two study sites on the central Oregon coast to determine the role of overstory species composition and stand density on forest succession. At 6 months, no significant difference in shrub, herb, and moss species composition or abundance was detected between the two study sites. Seventeen years after treatment, however, the Piceasitchensis (Bong.) Carr. site had much less vegetation cover and diversity than the Tsugaheterophylla (Raf.) Sarg. site. Overall response of most vascular species to thinning was insignificant because of their high variability in the medium and heavy treatments. Carpet-forming mosses were consistent in increasing their percent ground cover following thinning in the Picea site, and were the only species group to increase cover relative to initial conditions. Tsuga saplings were significantly more abundant under Picea canopies than under Tsuga canopies, probably because of the increased openness of Picea canopies. The increased abundance of saplings at both sites, particularly following heavy thinning, led to a two-layered overstory canopy and relatively less shrub and herb cover and diversity. Except for ferns, most plants that persisted had animal-dispersed seed. Thinning had little effect on the loss of understory species relative to the controls. Increases in diversity with thinning occurred primarily as the result of increased invasion by shade-tolerant species. Regardless of thinning treatment, only one-third or less of the original plant cover was maintained on study plots at stand age 30 years. Early thinnings without subsequent treatments are unlikely to maintain stable herb and shrub populations in forest types where the understory would otherwise be eliminated during the intermediate stages of stand development.