Using seed banks to discover how dams affect riparian vegetation diversity on the Elwha River

Dams fragment rivers and their associated riparian zones, altering hydrochorous seed dispersal and flood regimes, which can affect the diversity of riparian vegetation. Seed banks represent a way to link past and present flood-related seed dispersal to extant riparian vegetation. Understanding seed bank composition will be crucial for restoration of native riparian plant communities after the removal of Glines Canyon Dam and the Elwha Dam on the Elwha River, in Olympic National Park, Washington. To assess the relationship between the seed bank and extant riparian vegetation, I collected pooled seed bank samples and surveyed vegetation in 134 100 m² vegetation plots established in 5 transects per reach above, below, and between the two dams on the Elwha River. In extant vegetation, I observed a 15% drop in species diversity (measured as species richness) downstream from each of the two dams, resulting in a total 30% decline in native species diversity from above to below both dams (P=0.015). In sharp contrast, in the seed bank, native diversity actually increased (P<0.0001) downstream. Exotic species diversity increased downstream in both the soil seed bank (P < 0.0001) and extant riparian vegetation (P < 0.05). The consequences of a 30% reduction in native diversity in riparian zones are not well understood, but may be related to effective ecosystem functioning and resilience. The presence of a rich soil seed bank may play a crucial role in restoring native diversity to areas affected by the dams and dam removal. However, given the abundance of exotic species in the seed bank, it will be important to carefully monitor plant species invasions during the Elwha River Restoration. My research will provide baseline data for assessing changes that occur post dam-removal, which will help to establish the degree to which dams cause these patterns.