Ecological Monographs, 77(2), 2007, pp. 179–201 Ó 2007 by the Ecological Society of America MULTISCALE CONTROLS ON WOODY PLANT DIVERSITY IN WESTERN OREGON RIPARIAN FORESTS DANIEL A. SARR 1 AND DAVID E. HIBBS Department of Forest Science, Oregon State University, Corvallis, Oregon 97331 USA Abstract. 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-m 2 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. Key words: gap disturbance; multiscale controls; plant diversity; riparian; woody plants. INTRODUCTION Riparian forests can be among the most floristically diverse terrestrial ecosystems (Gregory et al. 1991, Naiman et al. 1993, Mouw and Alaback 2003), but ecologists are only beginning to unravel the sources of this diversity. Riparian vegetation is shaped by factors acting at multiple scales, from local fluvial and gap disturbances to broad-scale gradients in climate and geology (Baker 1989, Bendix 1994, Fetherston et al. 1995, Pabst and Spies 1998, Dykaar and Wigington 2000, Johnson et al. 2000, Wimberly and Spies 2001, Dixon et al. 2002). An integrated understanding of these multiscale factors is lacking, yet fundamental to conserving or restoring riparian plant diversity. In nonriparian environments, a growing body of empirical and theoretical work has explored how local and broad-scale factors influence species diversity at different scales (Ricklefs 1987, Caley and Schluter 1997, Cornell 1999, Huston 1999, Sarr et al. 2005). Most field studies and theoretical treatments have focused on the effects of local controls such as soil resource or light availability, competition, and disturbance dynamics on floristic diversity (Grime 1973, Huston 1979, 1999, Tilman 1982, 1987, Stevens and Carson 2002). Broader scale studies have illustrated how historical or climatic factors may also act to constrain diversity (Ricklefs 1987, Kleidon and Mooney 2000). Still others have suggested that the primary controls on diversity vary with scale (Whittaker et al. 2001, Sarr et al. 2005). While scientists have taken increasing interest in the multiscale controls on riparian species composition (Baker 1989, Bendix 1994, Turner et al 2004, Sabo et al. 2005), these studies have not typically tested multiscale diversity theory. Manuscript received 20 January 2006; revised 14 August 2006; accepted 6 September 2006; final version received 29 September 2006. Corresponding Editor: N. C. Kenkel. 1 Present address: Klamath Network, National Park Service, 1250 Siskiyou Boulevard, Ashland, Oregon 97520- 5011 USA. E-mail: Dan_Sarr@nps.gov 179