By altering flood, hydrochory, and sediment regimes, large dams have been shown to reduce downstream plant diversity and increase the number of nonnative species present. Dam removal could potentially reverse these effects, but there have been few studies documenting changes to plant communities and species richness following large dam removal. The Elwha River in Washington State, USA, the site of the largest dam removal to date, provides a unique opportunity to observe the effect of dams and dam removal on riparian vegetation. The objectives of our research were to determine 1) whether the Elwha dams are associated with lower downstream plant diversity and altered species composition, and 2) whether dam removal restores downstream diversity and composition. To address these questions, we surveyed vascular plant species composition and cover at ~75 100 m2plots on 15 cross-river transects located above, below, and between the two Elwha River dams. Environmental data, including ground cover, canopy cover, and soil depth were also recorded at each plot. On each transect, plots were randomly stratified across riparian landforms (bars, floodplains, back-channels, and terraces). Plots were sampled twice before (2005 and 2010), and three times after (2013, 2014, 2016) dam removal.
Results/Conclusions
Prior to dam removal, native species richness was 45-50% lower downstream from the dams (p < 0.01). This did not change in the two years following removal. Likewise, plant community composition was different downstream from dams compared to the upstream reference segment. This also did not change in the two years following removal. In 2016, four years after removal, we found a slight, non-significant increase in native species richness below the dams (p = 0.08), continuing an upward trend from previous years. The large influx of sediment into the system following dam removal may be affecting natural recovery of native species, and we predict that with more time, species richness will increase to reference levels. However, it is not known how long this will take, and long term monitoring will be essential to fully understand the effects of dam removal on riparian vegetation.