Multiscale controls on woody plant diversity in western oregon riparian forests

Riparian forests are known to be floristically diverse and influenced by multiscale phenomena, yet few studies have explicitly compared how these factors contribute to various aspects of riparian plant diversity. We analyzed woody riparian plant species and environmental data from four western Oregon watersheds distributed across a wide climate‐driven productivity gradient at three scales (40‐m² sample plots [alpha diversity], 1‐ha plots, and species pools from 16 1‐ha plots in each watershed) to compare three hypotheses of control on species diversity: (1) local control, (2) direct climatic control, and (3) indirect climatic control. We used a process model (3‐PG) to model gross primary productivity (GPP) as a functional climate index across our study area. We performed multiple linear regression to determine the best predictors of alpha (sample‐plot scale) diversity, compositional change within riparian forests (beta diversity), and hectare‐scale diversity and used path analysis to explore hypothesized causal linkages between climate and other factors and species diversity. We also analyzed a companion data set of gap and forest environments from a subset of the same sites to determine the influence of disturbance on species diversity across the gradient. We found evidence for strong spatial patterning in woody plant richness consistent with indirect climatic control on woody plant richness. Climate (GPP) showed negative relationships with alpha diversity and hectare richness of trees, shrubs, and woody plant species and was the most commonly selected explanatory variable in regression analyses. GPP and Rubus spectabilis cover increased from the least to most productive climates while understory light and moisture heterogeneity across the riparian area decreased. These environmental changes coincided with declines in alpha, beta, and hectare‐scale diversity. Disturbance gaps yielded higher richness at most sites, but differences in species richness between gap and forest sample plots did not increase at high levels of GPP, as hypothesized. This study points toward an integrated conceptual model whereby regional and landscape scale controls such as climate and watershed position complement and interact with local controls (i.e., vegetation structure, environmental gradients) to jointly govern woody plant diversity in riparian forests.