Composition, structure, and dynamics of forest and meadow communities in the Three Sisters Wilderness Area/Biosphere Reserve, 1981-2009

The Three Sisters Wilderness Area encompasses nearly 100,000 ha of largely undisturbed landscape straddling the crest of the central Cascade Range in Oregon. The landscape is dominated by coniferous forests, but there are also a diversity of non-forested habitats, including meadows, alpine areas, lava flows, and wetlands. In 1976, the Three Sisters was designated a UNESCO Biosphere Reserve, representing the northern half of the Sierra-Cascade Province (it was later withdrawn from the program, together with 17 other U.S. reserves). Integral to the establishment of a Biosphere Reserve is the collection of baseline data on the composition, structure, and distribution of its major plant communities. At the time this study was initiated (1981), basic ecological research in the Three Sisters Wilderness was limited. The goals of this study were two-fold: (1) to characterize the diversity of forested and non-forested plant communities, and their relationships to the physical environment; and (2) to assess historical and ongoing changes in the vegetation across natural forest-meadow boundaries (or ecotones). Two studies were undertaken to address this second goal: a reconstruction of the historical patterns of tree invasion of meadows as related to grazing history, climatic variation, and site environment; and a resampling and analysis of more recent (1983-2009) changes in vegetation across these forest-meadow boundaries.