Maintaining channel abandonment processes increases riparian plant diversity within fluvial corridors Simon Dufour, 1 * Maya Hayden, 2 John Stella, 3 John Battles 2 and Herve Piegay 4 1 Geography, CNRS UMR 6554 LETG Rennes COSTEL, Université Rennes 2, Rennes, France 2 Dept. of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA 3 Forest and Natural Resources Management, State University of New York, Syracuse, USA 4 CNRS UMR 5600 EVS, University of Lyon, Lyon, France ABSTRACT Within dynamic alluvial riverine corridors, abandoned channels form and experience hydrogeomorphic processes that are distinct from similar elevation surfaces along active channels. Compared with the relatively well-studied role of fluvial disturbance on floodplain plant communities along active channels, the drivers of plant diversity and community dynamics along terrestrializing abandoned channels have received much less attention. In this study, we addressed several related questions within the context of the Sacramento River (California, USA): (1) Do abandoned channels host different plant species compared with the surrounding floodplain? (2) How do plant communities vary among abandoned channels in relation to time since cutoff and disturbance regime? (3) Do understory plant species within an abandoned channel display distinct zonation along a disturbance gradient from the wetted edge laterally to upland areas? Our results show that although species richness is similar to floodplain sites, abandoned channels support a different species pool, notably due to presence of more wetland-associated species, and this contributes to increase biodiversity within the fluvial corridor. We found substantial shifts in species composition that occurred since channel abandonment, likely related to decreases in the strength of hydromorphic disturbance through time. Lastly, we found that lateral environmental gradients within abandoned channel sites were significant, although much weaker drivers of understory vegetation patterns than typically found along active channel banks. These results argue for a management approach that preserves and promotes natural processes of channel migration and sediment dynamics. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS abandoned channel; sedimentation; understory vegetation; flood disturbance; floodplain forest; channel cutoff; riparian vegetation Received 21 February 2014; Revised 16 July 2014; Accepted 29 July 2014 INTRODUCTION Process-based approaches to riparian management and restora- tion have gained prominence in recent decades with our increased understanding of the strong physical and biological linkages within these systems (Naiman et al., 2005). Variation in hydrogeomorphic conditions – particularly the disturbance regime – profoundly influences riparian vegetation commu- nities, both in the pioneer phase of colonization and in later stages as communities develop (Bendix and Stella, 2013). These physical drivers include flood intensity, frequency, timing and inundation duration, sedimentation, scour inten- sity and water availability (Menges and Waller, 1983; Hupp, 1992; Van Coller et al., 1997; Bendix and Hupp, 2000; Steiger et al., 2005). Studies beginning in the early to mid-20th century established that biological diversity at the valley-bottom scale is related to the presence of fluvial landforms such as channel banks, point bars and floodplain scrolls that exhibit strong gradients of topography and hydrology (Hefley, 1937; Johnson et al., 1976; Hupp and Osterkamp, 1985; Budke et al., 2008). Better quantification of the links between physical drivers and vegetation responses spurred the development of a process-based understanding of fluvial systems and approach to their restoration (Boon et al., 1992; Auble et al., 1994; Dufour and Piégay, 2009; Stella et al., 2013). Rather than establishing a fixed pattern of topography or vegetation distribution, a process-based approach seeks to restore the underlying physical regimes such as flooding, channel morphodynamics and sediment transport that generate and maintain diverse aquatic and floodplain habitats (Ward et al., 2001; Hughes et al., 2005). Lateral channel mobility is one of the main hydromorphological processes that influences riparian vegetation pattern and diversity, primarily because it shapes a complex mosaic of sediment deposits (with regard to depth, texture and organic material), topographic conditions (e.g. elevation above the water table) and terrestrial and aquatic habitats (Salo et al., 1986; Florsheim et al., 2008). Lateral channel mobility also drives the creation of *Correspondence to: Simon Dufour, CNRS LETG Rennes COSTEL, Geography, Place Recteur le Moal, Rennes, 35000, France. E-mail: simon.dufour@uhb.fr ECOHYDROLOGY Ecohydrol. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1546 Copyright © 2014 John Wiley & Sons, Ltd.