Thursday, August 9, 2007 - 2:50 PMOOS 44-5: Structure, composition, and recruitment of woody floodplain vegetation along the Elwha River
Patrick B. Shafroth, US Geological Survey
Understanding physical and biological changes following dam removal requires pre- and post-dam monitoring along rivers with dams removed and along reference reaches not previously containing dams. We present baseline data focused on floodplain vegetation along the Elwha River, Washington, where two large dams are slated to be removed in 2009. We established five cross-valley transects in each of three reaches of the Elwha R.: a reference reach upstream of both dams, a reach between the dams, and a reach downstream of both dams. Along each transect, we sampled the composition, structure, and age of the woody vegetation within 138 randomly selected tree plots and 275 nested shrub plots. In addition, we collected point and line intercept data along each transect, and aged trees within 72 of the quadrats. We surveyed the transect topography, quadrat elevations, and estimated surficial soil particle size. Substantial geomorphic change has occurred recently within the braided reference reach, resulting in a relatively large number of young Salix, Alnus, and Populus patches, as well as a relatively large area of unvegetated bars. The reach between the dams is more stable with relatively few young vegetation patches or bars. The reach below both dams shows moderate channel and floodplain dynamics. We encountered 13 tree species and approximately 30 shrub species along the transects. Young bar surfaces (5-20 yrs) were dominated by Salix spp., Alnus rubra, and some Populus trichocarpa. Young floodplains (~80yrs or less) were generally dominated by Alnus rubra and sometimes by Populus trichocarpa. High floodplains and terraces were often dominated by Acer macrophyllum with an understory of Acer circinatum and seedlings of Abies grandis. Patches of Pseudotsuga menziesii occurred on relatively young and older floodplains. We present hypotheses and predictions regarding post dam removal vegetation responses based, in part, on our baseline data.