No 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 nonnative species frequency, increasing species richness, and establishing desirable target species after dam removal, particularly on landforms composed of coarse sediments.

No abstract

Early seral vegetation was studied on a former lake bottom after the removal of the 64-m-tall Glines Canyon Dam on the Elwha River. In 2015, vegetation cover of all vascular plant species was determined in 63 plots located on sites that emerged in 2011-2012. The sites had been planted and/or seeded, or were permitted to revegetate spontaneously. The plots were further classified by substrate texture: coarse sediments on the valley bottom and fine ones on the valley slopes. Plots were located randomly along random transects perpendicular to the former lake shore that extended into coarse sediment terraces perched above the floodplain. Additionally, 32 plots were sampled in surrounding native forests near these transects. Data were analyzed by detrended correspondence analysis and by canonical correspondence analysis. Substrate texture, that is whether fine or coarse, appeared to explain most of the variability in vegetation. The distance to forest and successional age, that is time since the site had been drained, were also significant explanatory variables, while assisted restoration by planting and seeding appeared to be insignificant to date. Spontaneous succession on fine sediments led to a species composition approaching that of adjacent natural forests. Invasive species were much less abundant than expected. Spontaneous restoration of vegetation on fine sediments in drained lake bottoms can rapidly produce a desirable vegetation composition and structure. On coarse sediments, active restoration may be useful to accelerate the development of native vegetation communities.