Date of Award

Spring 2024

Rights

Access is available to all users

Document Type

Thesis

Degree Name

Master of Science (MS) in Biology

Department

Biology

Abstract

The Elwha and Glines Canyon dams on the Elwha River, Washington, USA were removed between 2011 and 2014 after being in place for a century, and at the time were the largest dams ever removed. The resulting dewatering of Lake Aldwell, the reservoir of the Elwha dam, and Lake Mills, the reservoir of the Glines Canyon dam exposed over 270 hectares of land that had been submerged. While several studies have examined the revegetation of these drained reservoirs, none have gone beyond 5 years since dam removal. These studies found that sediment grain size and reservoir landform (valley walls, terraces of reservoir sediment, and active riparian zones) were important predictors of revegetation patterns. The goal of my study was to extend this timeframe to assess revegetation patterns in the first ten years following dam removal. First, I examined the natural revegetation of the former Aldwell and Mills Reservoirs in the absence of seeding or planting and I expected that sediment and landform would continue to be important for vegetation succession. I predicted that the finer sediment and earlier drawdown of the Aldwell Reservoir would result in overall greater plant diversity and cover, and distinct plant communities when compared to the Mills Reservoir in early years; however, differences would wane over time. In addition, I predicted that areas with coarser sediments that initially had lower plant diversity and cover, and distinct species composition compared to areas with finer sediments would become more similar to areas with fine sediments. To test these predictions, I used long-term vegetation monitoring transects established in 2012 and sampled in 2013, 2016, and 2022. I found that the Aldwell Reservoir maintained higher levels of native species richness than the Mills Reservoir; however, non-native richness became more similar with time. Differences in plant richness and cover among landforms within each reservoir also tended to wane over time; however, plant communities remained distinct among reservoirs and landforms. Second, I sought to determine whether active restoration was associated with higher native species diversity and lower non-native species diversity and abundance when compared to natural revegetation in drained reservoirs over longer time scales. I hypothesized that in sites with edaphic factors leading to slow colonization, seeding would continue to reduce species richness, increase native cover, and reduce non-native cover, as found in previous studies. In contrast, in sites with conditions favorable to rapid colonization, I hypothesized that after 10 years there would be little effect of seeding due to rapid natural revegetation. And finally, I hypothesized that planting woody species would increase diversity, and after a decade, increase cover as well, as these species take longer to establish. To test these hypotheses, I resampled plots established by the National Park Service in seeded, seeded and planted, and control areas without seeding or planting. I found that active restoration, especially the combination of planting and seeding, was most effective on coarse sediment surfaces with increases in native species richness and reductions in non-native richness. Species composition only varied between seeded and passively restored valley walls, and valley wall species richness was lower on seeded areas. Overall, my results show that long-term monitoring is needed to elucidate the successional processes that occur over large temporal scales and that while planting and seeding is most effective on coarse sediments, it may not be necessary where conditions are favorable to native plant establishment.

Download
Off-Campus Download

Share

COinS