Regional (climate/soils) and local (aspect) physiography determine plant community composition. However, changes in initial floristic composition after a disturbance may be severe enough to alter the successional trajectory predicted by physiography. We addressed the question of which is more important, disturbance or physiography, in determining vegetation composition and the consequent successional trajectory. We evaluated understory vegetation of forest communities exposed to four disturbance types (control, single burn, diameter-limit cut, and first-removal shelterwood) 2–5 yr postdisturbance. Study sites were located within each disturbance type on northeast and southwest aspects within the Appalachian Plateau and the Ridge and Valley provinces. Vegetation composition was analyzed with nonmetric-multidimensional scaling, two-way nonparametric multivariate ANOVA, and indicator species analysis. The relationship between disturbance and key environmental variables, including canopy opening and soil fertility, was analyzed with generalized linear mixed models. There were 363 species in our study area. Composition differed by province and aspect. Composition also differed by disturbance but with a significant province interaction. Although physiography was more important, some species served as disturbance indicators that differed by disturbance type with two possible outcomes. First, expected successional trajectories (as defined by the regional and local environmental filters) may deviate toward recovery of native species (e.g., Epigaea repens) that benefit from low-level disturbance (as defined by fire as a filter). Second, successional trajectories after a relatively severe disturbance (as defined by shelterwood harvest as a filter) may deviate toward systems that are vulnerable to invasion by exotics or dominant native species.
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Vol. 83 • No. 1