Detecting geomorphic responses following invasive vegetation removal: Wickaninnish Dunes, Pacific Rim National Park Reserve, British Columbia, Canada.
Date
2012-04-26
Authors
Eamer, Jordan Blair Reglin
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Abstract
This thesis presents results from a large-scale dynamic restoration program implemented by Parks Canada Agency (PCA) to remove invasive marram grasses (Ammophila spp.) from a foredune-transgressive dune complex in Pacific Rim National Park, British Columbia, Canada. The program goal is to restore habitat for endangered Pink sandverbena (Abronia umbellate var breviflora) as required by the Canadian Species at Risk Act (SARA). Three sites were restored by PCA via mechanical removal of invasive marram grasses (Ammophila spp.) in September 2009. This study documents geomorphic and sediment mass exchange responses at one of these sites as derived from detailed Digital Elevation Model (DEM) surveys of a 10 320 m2 study area that spans three discrete geomorphic units (beach, foredune, and transgressive dune complex). Subsequent approximately bi-monthly total station surveys for the first year post-restoration are compared to a pre-restoration baseline Light Detection and Ranging (LiDAR) survey (August 2009) to quantify and describe morphodynamic responses and volumetric changes. Two different methodologies were utilized for post processing of volumetric change DEMs in order to filter out non-statistically significant change. The first filter used software developed for fluvial geomorphology and was tested using the student’s t distribution. This approach, while novel in the field of coastal geomorphology, was less complex than the second which was based on spatial statistical procedures popular in the ecological sciences. This filter was based on local Moran’s Ii, which was used to generate 1.5m and 5m distance thresholds of statistically
significant geomorphic change. These thresholds were specified to simulate the outer limit of saltating grains and the dimensions of landform development, respectively. Results show that the beach receives appreciable sediment supply via bar welding and berm development in the winter, much of which is transported to the foredune and transgressive dune complex units in the spring. This promotes rapid redevelopment of incipient dunes in the backshore, rebuilding of the seaward slope of the foredune following wave scarping, and localized extension of depositional lobes in the transgressive dune complex fed by sediment from the beach and foredune stoss (only shown in local Moran’s Ii results). The results of this study suggest that the foredune-transgressive dune complex at Wickaninnish Dunes has experienced enhanced aeolian activity and positive sediment volume changes over the first year following mechanical restoration. In addition, comparison of the two methodologies show that spatial statistics were found to provide both more realistic calculated volumes at a smaller threshold distance (e.g., – 0.012m3 m-2 in the foredune after devegetation; only +0.015m3 m-2 in the transgressive dune complex in the year following restoration) and better highlighting of important spatial processes at a larger threshold distance (e.g., foredune stoss erosion; feature highlighting) than the volumetric change calculations based on a simpler statistical threshold.
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Keywords
Physical Geography, Restoration, Coastal, Dune, Spatial Analysis