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1 January 2011 Seed Rain and Revegetation of Exposed Substrates Following Dam Removal on the Elwha River
James T. Michel, James M. Helfield, David U. Hooper
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Scheduled to begin in 2011, the removal of two dams on the Elwha River, Washington, will be one of the largest dam removal and river restoration projects undertaken in the United States. One challenge associated with this project is to understand how exposure and downstream deposition of sediments presently detained behind the dams will influence patterns of revegetation and invasive species colonization following dam removal. We conducted two greenhouse experiments assessing the potential effects of reservoir sediments on germination success and growth of (1) propagules dispersed naturally via seed rain, and (2) seeds of selected native and invasive species. Observed summer seed rain density was relatively low (<125 seeds m-2). This suggests slow recolonization in the initial years following dam removal, although these results may have been influenced by sampling methods and timing. In the selected species experiment, four out of five tested species exhibited reduced germination and growth on fine reservoir sediments, while the invasive forb Cirsium arvense was unaffected. Though reduced compared to more typical alluvial sediments, germination and growth of the natives Artemisia suksdorfii and Rubus parviflorus were comparable to that of C. arvense on reservoir sediments. These native species may be useful for revegetating exposed sediments. However, depending on such factors as source population sizes, seed production and dispersal rates, growth rates, water availability and competition during establishment, control of C. arvense and other exotics in the years following dam removal may be necessary to prevent a relative increase in invasive species populations on the new post-dam substrates.

© 2011 by the Northwest Scientific Association.
James T. Michel, James M. Helfield, and David U. Hooper "Seed Rain and Revegetation of Exposed Substrates Following Dam Removal on the Elwha River," Northwest Science 85(1), 15-29, (1 January 2011).
Received: 17 October 2010; Accepted: 1 December 2010; Published: 1 January 2011
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