- Knowledge of regional-scale patterns of plant community structure and controlling factors is largely qualitative and based on numerous local studies. Data from a subsample of 10,000 field plots were used to quantify and map compositional gradients of woody plant communities across Oregon forests. Canonical correspondence analyses explained 9-14% of the total species variation at three geographic scales. Climate contributed most to total variance explained (44-58%), followed by geology (6-16%), disturbance (7-13%), and topography (4- 8%). The dominant compositional gradient was associated with climate, from the lower elevation, moderate, maritime climate of the coast to the higher elevation, drier, continental climate of the interior. The second canonical axis followed a gradient from the warm, dry growing seasons of the interior valleys and eastern Cascades, to cooler, wetter mountainous areas. Only those parent materials that present extreme growing conditions, primarily ultramafic soils, were significant at the regional level. Ecological relationships differed substantially among subregions within the state. Except for topography, which contributed more to explained variation at finer geographic scales, ecological differences among subregions overshadowed effects of geographic extent. Topographic effects were stronger and beta diversity greater in eastern Oregon, where moisture is more limiting for plant growth, and weakest near the coast where climate is more favorable. The secondary importance of topography, disturbance, and substrate can be attributed to their influence on relative abundances of species within a local area, rather than on species presence or absence within broader regions. Community structure varied at a finer spatial scale in eastern than in western Oregon, and species turnover along gradients was greater for shrubs than trees. Amount of unexplained variation in the species data was high but not atypical of gradient analyses. Yet spatial structure in the species data that was uncorrelated with explanatory factors suggests potential to improve the canonical correspondence analysis models, particularly in eastern Oregon. Study findings have implications for considering biological diversity in regional conservation strategies, in planning for global climate change, and in the design of regional inventory and monitoring programs.
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