COS 223-2 - Effects of Seeding and Planting on Reservoir Revegetation ten years after Dam Removal on the Elwha River, Washington, USA.

Abstract: Dam removal is becoming more frequent as a form of river restoration, and in many cases, the drained reservoirs are actively planted and seeded. However, more information is needed to determine the long-term effects of these practices. Two large dams were removed along the Elwha River (Washington, USA) from 2011-2014, exposing ca. 278 ha of unvegetated sediment. Natural revegetation proceeded rapidly, and the National Park Service seeded and planted on exposed valley walls and reservoir sediment terraces in portions of the reservoirs. Our goal was to determine how planting and seeding affected native and nonnative plant species richness and plant community composition compared to natural revegetation. In July 2022, ten years after dam removal, we sampled plant species composition and cover in 33 100-m² vegetation plots in the former Lake Mills reservoir. The plots included three treatments: control, seeded, and plots that were both planted with woody species and seeded. The treatments were stratified across valley wall and terrace landforms, with terraces formed by reservoir sediment that remained after dam removal. We found that seeding reduced native species richness on the valley wall landforms, while the other treatment had no effect. The planting and seeding, and seeding treatments reduced nonnative species richness on both landforms. Nonnative species cover was not significantly affected by planting or seeding. These preliminary results suggest that active revegetation by planting and seeding can help reduce the diversity of nonnative species, but don’t necessarily affect native species richness where there is rapid natural revegetation.