Date of Award

2012

Document Type

Thesis

Degree Name

Master of Science (MS) in Biology

Department

Biology

Abstract

"Dams alter the dynamics inherent to river systems by displacing natural hydrologic and sediment regimes, which can fundamentally alter riparian ecosystem function. However, with better understanding of how dams negatively impact river systems, and as many dams approach the end of their lifespan, dam removal is being used to facilitate ecosystem restoration. Whereas researchers have successfully illustrated the negative impacts dams have on biological communities, the long-term ecological implications of dam removal are not well understood. At present, two dams are being removed along the Elwha River (Washington, USA), providing a valuable window for ecological studies concerning the effects of dams, and their removal, on biotic communities. In this study I described plant community dynamics along the twice-dammed Elwha River for use as a baseline in assessing the long-term effects of dam removal on this river system. I determined the relationships between understory and overstory riparian plant communities and how they vary across geomorphic landforms relative to the dams over a five-year period (2005 to 2010). I also evaluated the relative utility of under- and overstory species as indicators of plant community type, reach location and geomorphic landform. Vegetation and environmental surveys were conducted in 2005 and 2010 on 100-m² plots located along 15 perpendicular transects on river reaches above, below and between the Glines Canyon and Elwha dams. I used multivariate analyses to define plant communities along transects by assessing species composition within each plot (via frequency and abundance of species), and characterized their distribution. I used a general linear models approach to assess compositional change in plant communities along river reaches over the five-year interval to determine the stability of understory and overstory plant communities. Finally, I used an indicator species analysis to examine the distributions of individual plant species. I found that plant community composition along the Elwha River was heavily influenced by the distribution of geomorphic landforms. Physical factors (e.g. soil depth, substrate size, ground cover) were strongly correlated with longitudinal location and geomorphic position. River reaches delineated by the dams had markedly different plant communities. The reach between both dams had the fewer early successional communities associated with younger landforms, perhaps due to sediment starvation; this suggests the dams have played a role in plant community distribution. In reaches above and below the dams there were greater differences between understory and overstory community composition as compared to the middle reach. Understory communities were less stable, meaning they had greater species compositional changes over time, compared to overstory communities, which were more stable. These data suggest the dams may have attenuated natural disturbance events in the middle reach. Overstory species were the more useful for indicating the overall plant community, however, understory species were more reliable indicators of reach location understory than the overstory. These data provide a useful baseline for post-removal comparisons evaluating the long-term effects of dam removal on the Elwha River. My results concur with others that have suggested that reaches downstream of the dams will be most affected post- removal by the influx of sediments from the former reservoirs. I predict that, in addition to the reestablishment of younger landforms, dam removal will result in an increase of early-sere, disturbance-tolerant communities in downstream reaches. Also I anticipate that the stability of the understory and overstory communities will become more reminiscent of natural conditions (more stable overstory than understory) along all reaches. I also suggest that understory species not be neglected from indicator analyses, as they can be accurate, even exclusive, indicators for factors such as plant community type, geomorphic landform and reach location"--Document.

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