The effect of large woody debris, direct seeding, and distance from the forest edge on species composition on novel terraces following dam removal on the Elwha River, WA.
The removal of two dams on the Elwha River, Washington, exposed over 300 hectares of reservoir sediments and created primary successional habitats that posed challenges to revegetation efforts. In order to meet Elwha restoration goals, coarse sediment deposits would require revegetation methods aimed at quickly restoring native vegetation while deterring exotic species invasions. I examined the effect of two restoration treatments—large woody debris translocations and native seed enhancements—on plant species composition on novel terraces in the former Lake Mills reservoir four years after dam removal. I sampled vegetation in seeded and unseeded treatment areas with and without large woody debris. I also examined species composition and seed dispersal mechanisms to determine whether distance limited native plant recruitment from the nearby forest edge. I used two-way analysis of variance, NMDS ordinations, and permutational multivariate analysis of variance to determine whether wood placements, seeding treatments, and distance from the forest influenced species composition on novel terraces. My results revealed that Shannon-Weiner diversity, species richness, and percent exotics increased on plots containing wood, compared with surrounding bare sediments, but plant establishment did not substantially increase on wood plots. Plots located in seeded treatment areas had higher species richness and plant abundance, with decreased exotic species recruitment. As distance from the forest edge increased, Shannon-Weiner diversity, species richness, and percent exotics on unseeded plots declined, but plant abundance did not change significantly with distance. In addition, a greater proportion of plants were wind-dispersed at greater distances, while plants dispersed by gravity and ballistic mechanisms were associated with closer distances to the forest edge. This study’s results help fill a knowledge gap regarding the efficacy of using translocated large woody debris and direct seeding to restore vegetation in primary successional habitats following dam removal and helps inform best practices regarding the use of these restoration methods at future dam removal projects
