Planting, seeding, and sediment impact restoration success following dam removal

Abstract

Large dam removals effectively restore geomorphic and hydrologic processes, but little is known about subsequent vegetation recovery and the role of active restoration. We studied whether planting and seeding affected vegetation recovery in a dewatered reservoir, and whether effects differed on landforms composed of fine or coarse‐textured sediments. The dewatered reservoir is behind the former Glines Canyon Dam, which was removed from the Elwha River in Washington State (U.S.A.) in 2011–2014. We measured bare ground in 2013–2017, and species richness, relative frequency of non‐native species, woody stem density, and species composition in 2016. Sediment texture affected all response variables. Several years after seeding, bare ground was reduced on coarse sediments and non‐native species frequency was reduced on both textures. Planting increased species richness. Seeding and planting altered species composition on both textures, with stronger effects on coarse sediments. High densities of riparian woody plants naturally established on fine sediments, reaching a mean of 34,000 stems/ha 5 years after dam removal began; density was only a fraction of this (5,000 stems/ha) on coarse sediments. Future dam removals should incorporate sediment texture into their revegetation plans. Seeding and planting can be valuable strategies for minimizing non‐native species frequency, increasing species richness, and establishing desirable target species after dam removal, particularly on landforms composed of coarse sediments.