Regional Gradient Analysis and Spatial Pattern of Woody Plant Communities of Oregon Forests

Abstract

Knowledge of regional-scale patterns of ecological community structure, and of factors that control them, is largely conceptual. Regional- and local-scale factors associated with regional variation in community composition have not been quantified. We analyzed data on woody plant species abundance from 2443 field plots across natural and seminatural forests and woodlands of Oregon to identify and quantify environmental, biotic, and disturbance factors associated with regional gradients of woody species composition; to examine how these factors change with scale (geographic extent) and location; and to characterize and map geographic patterns of species and environmental gradients. Environmental correlates of species gradients, species diversity patterns, and the spatial patterning of woody plant communities varied with geographic extent and location. Total variation explained (TVE) by canonical correspondence analyses (CCAs) was 9–15% at three hierarchical geographic extents: the entire state, two half-states, and five subregions. Our high level of unexplained species variation is typical of vegetation gradient analyses, which has been attributed to landscape effects, stochastic processes, and unpredictable historical events. In addition, we found that TVE in canonical correspondence analysis is confounded by sample size. Large numbers of plots and species, as in our study, are associated with lower TVEs, and we propose a mechanism for this phenomenon. Climate contributed most to TVE (46–60%) at all locations and extents, followed by geology (11–19%), disturbance (6–12%), and topography (4–8%). Seasonal variability and extremes in climate were more important in explaining species gradients than were mean annual climatic conditions. In addition, species gradients were more strongly associated with climatic conditions during the growing season than in winter. The dominant gradient at the state scale was from the lower elevation, moderate, maritime climate along the coast to the higher elevation, drier, continental climate of eastern Oregon. The second canonical axis followed a gradient from the warm, dry, growing seasons of the western interior valleys and eastern Cascade Range to the cooler, wetter mountainous areas. Geologic variables were most strongly correlated with axis 3, and measures of local site and disturbance with axis 4. For most of the state, our findings on the associations of disturbance factors with species gradients were inconclusive due to confounding of land ownership patterns, disturbance histories, and elevation in our sample. Near the coast, where gradients were not confounded, clear-cutting and stand age accounted for only 2 and 1% of TVE, respectively, in partial CCA. Ordinations of our long, regional gradients were influenced more by species presence than by abundance, and few woody species have been totally eliminated from sites by clear-cutting. Within Oregon and for the range of geographic extents we examined, variation in the environmental correlates of species gradients was more strongly associated with geographic location than with geographic extent, although topographic factors explained slightly more variation at smaller geographic extents. The greatest subregional contrast in vegetation character was between eastern and northwestern Oregon, and the Klamath subregion was intermediate. In the drier climate of eastern Oregon, community structure varied at a finer spatial scale, and climatic and topographic moisture were more strongly associated with species gradients than in the moister areas of western Oregon. Topographic effects were weakest and climatic effects strongest near the coast, where climate is moderate. Alpha and gamma diversity were greater in western Oregon, but beta diversity was greater in eastern Oregon and greater for shrubs than for trees. Our findings supported a conceptual model of multiscaled controls on vegetation distribution, and the related notion that local community structure is the result of both regional- and local-scale processes. Despite strong ecological contrasts within the region, we were able to synthesize species–environment relations at the regional level. This suggests that apparent conflicts among local vegetation studies can be explained by real ecological differences among places.