Understanding reduced plant diversity below dams on the Elwha River, Washington

Background/Question/Methods

Riparian zones are highly diverse ecosystems important for ecosystem function, however they are greatly impacted by human activities such as dams.   Despite the presence of over 75,000 large dams in the United States, there is little ability to predict the response of riparian plant communities to dam construction or removal.  One might expect that dams would lower diversity in downstream reaches by reducing sediment redistribution, flood disturbance, and the downstream spread of propagules.  Would such changes be reversed when dams are removed? The removal in 2011-2013 of two large dams on the Elwha River, which lies mostly within Olympic National Park, Washington, provides an unprecedented opportunity to study the long-term impacts of dams and their removal on riparian vegetation, which is the goal of our project.  Here we determine how geomorphology interacts with reach position and sampling time to affect vegetation diversity.  In 2005 and 2010 we surveyed vegetation, geomorphic position (bar, floodplain, terrace), and environmental variables in 63 100-m² plots distributed along 15 cross-river transects located above, below, and between the two Elwha dams. We analyzed patterns of native and exotic species richness related to reach, geomorphology, and year using general linear models. 

Results/Conclusions

From 2005 to 2010, there was a 26% increase in species richness in the lower reach; however there were no significant changes in the other reaches.  In both 2005 and 2010 there was a significant decrease in native species richness (26% and 30% respectively) from the furthest upstream to furthest downstream reach.  Exotic species richness was significantly higher in the downstream reach compared to the upstream reach both years.  In 2005, we found that in the lowest reach, there were significantly more exotic species on bars than floodplains, which may partly explain the increase in exotic species in the lower reach. Reach and geomorphology did not interact to influence native species richness values.  We are exploring whether these results changed in 2010.  These data will provide an important baseline for monitoring long-term responses of riparian vegetation to dam removal.