Revegetation of Post-Dam-Removal Riparian Sediments in the Lower Elwha River, WA

Whisman, Marisa

With the deconstruction of Glines Canyon Dam nearly complete, Olympic National Park's Elwha Revegetation Crew has implemented the second season of a 7-year native riparian plant restoration effort. A critical component of restoring the lower Elwha River ecosystem is the establishment of late seral riparian forests to provide stream shading, woody debris, nutrient inputs and erosion control. As a restoration strategy, installing woody plants may prevent invasive weed colonization, facilitate native plant recruitment, and provide seed sources beyond intact forest edge. The greatest challenge facing native plant establishment in the dewatered Lake Mills reservoir is the survival of vegetation in post-dam-removal sediments. Fine sediments (alluvial silt and clay) lack porosity, are subject to inundation and desiccation during wet and dry periods, and create hypoxic growth conditions. Coarse sediments (gravel, cobbles, sand) are highly porous and prone to desiccation and wind erosion. Determining which plant species can survive in specific sediment textures may provide guidelines for successful plant establishment at Elwha River post-dam-removal plant restoration progresses. This study focuses on the survival and performance of six native woody plant species in three post-dam-removal sediment textures on the dewatered Lake Mills reservoir during the initial 2012 revegetation efforts. Selected species included ocean spray (Holodiscus discolor), Nootka rose (Rosa nutkana), thimbleberry (Rubus parviflorus), western redcedar (Thuja plicata), black cottonwood (Populus balsamifera ssp. trichocarpa), and Douglas-fir (Pseudotsuga menziesii). Total individuals surveyed equaled 860 (n=80-180 samples per species). Sediment moisture, particle size and nutrient content were also assessed. By late September plant morality reached a total of 66, with 82% of dead individuals being Douglas-fir. The lowest mortality (<1%) occurred for black cottonwood. Proportionally, plant mortality was highest in the coarse sediment. These and similar data collected over the next 5 years may be useful in determining woody plant species selection in other post-dam-removal restoration efforts.

Revegetation of Post-Dam-Removal Riparian Sediments in the Lower Elwha River, WA

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