River flow modifications often disrupt riparian forest communities by modifying demographic processes of dominant species. In 2001, a portion of the Merced River was fitted with an engineered channel scaled to the modern flow regime. Revegetation efforts were limited to the floodplain above bank-full elevations. In 2008, we measured post-restoration seedling and sapling recruitment as well as survival and growth of planted trees in plots situated at point bars, cut banks, and riffle banks. Plots extend from within the active channel out onto the floodplain. Using a detailed survey of the active channel, a 2-D model is used to simulate recent flows and to estimate basic flow parameters such as extent, depth, velocity, and shear stress.
Results/Conclusions
Nearly all seedlings and saplings are located on channel banks below bank-full elevation. We find fewer recent recruits on point bars, an apparent result of herbaceous competition facilitated by the absence of a suitable scouring flood in that year. Patterns of tree community structure and composition are related to modeled hydrologic parameters. River extent during periods of seed release and dispersal can create important differences in seed supply. Extended inundation increases seedling mortality through drowning, while velocity and shear stress influence patterns of scour, burial, and abrasion. Sustainable riparian forests depend on continued recruitment, which is affected by modified flows through diverse and sometimes opposing mechanisms.
