Effects of forest harvesting, logging debris, and herbicides on the composition, diversity and assembly of a western Washington, USA plant community

We examined plant community organization over the first five growing seasons after clearcut harvesting with retention of two levels of logging debris (light and heavy) and application of four vegetation control treatments (non-sprayed control, aminopyralid (A), triclopyr (T), and A + T). Our study site was 44 km northwest of Olympia, WA., USA, and before forest harvesting it was dominated by Pseudotsuga menziesii. We used a randomized split plot experimental design replicated in 6 blocks (each debris-treatment main plot had four herbicide-treatment split plots). We estimated percent canopy cover by species before forest harvesting and in post-harvest seasons 1-3 and 5 on 100 m2 plots centered in each split plot. We analyzed species composition and diversity and report the response of 10 species groups and several major species to the treatments over five seasons. We used ANOVA to examine annual treatment effects on abundance of major species and species groups as well as ordination and graphical methods to examine succession. Abundance of ruderal species, especially exotics and graminoids, was lower but abundance of native woody shrubs and vines was higher in heavy debris than in light debris. The vines developed higher cover in heavy debris where they used debris as a scaffold to gain a competitive advantage over other species. Heavy debris controlled Cytisus scoparius better than the herbicide treatments. Triclopyr reduced woody dicot, vine and native herb covers, while aminopyralid reduced these groups and Cytisus scoparius, but aminopyralid had less effect on total canopy cover. The combination herbicide treatment reduced woody dicots, vines and Cytisus scoparius, and had the biggest impact on total canopy cover. By year 5 there was little difference in total canopy cover among the herbicide treatments; however, for some species, both debris and herbicide treatment effects were still apparent. The ordination indicated that, by the fifth season, the floristic characteristics distinguishing the debris treatments were still distinct, but not for the herbicide treatments. We conclude that heavy debris is a viable treatment alternative to prevent aggressive exotic species from competing with planted conifers and the native plant community on edaphically dry western Washington sites.