Species diversity is hypothesized to increase and then decline with both site productivity and disturbance intensity. In the case of forests, modern intensive management practices have the potential to simplify forest structure and composition. However, it has been difficult to determine whether, or to what extent, such simplification has occurred. This study exploits the systematic sample design and consistent plot size used by the Forest Inventory and Analysis (FIA) program across all landowners in Washington and Oregon, USA to evaluate this question. Field data on trees, snags, understory plants, and down wood was collected on forested plots between 2001-2007. Plot-level data were analyzed by owner category and management history as an indicator of management intensity, in addition to wilderness. Differences in climate, ecoregion, and successional stage were controlled among owner groups to test for differences in tree species richness, evenness, and species-accumulation curves.
Species richness differed by owner group and ecoregion. West of the Cascades, richness was lowest on private industry and wilderness lands, and highest on nonindustrial private lands. In contrast, east of the Cascades, richness was lowest on nonindustrial private, moderate on industrial private, and highest on wilderness lands. Species accumulation curves suggested similar patterns of diversity across larger spatial scales, although richness increased faster with increasing number of plots on wilderness than on other land types. Most of the variation in species composition was related to climate and successional stage, but management intensity does appear to have contributed to a small, but detectable, decrease in species diversity in western Washington and Oregon forests.