Ecohydrological characteristics of a newly identified coastal raised bog on the western Olympic Peninsula, Washington State, USA

Abstract

In western North America, ombrotrophic bogs are known to occur as far south as coastal regions of British Columbia. A recent discovery of a peatland with a raised peat surface on the western Olympic Peninsula in Washington State (Crowberry Bog), USA, suggested that the distribution range of this ecosystem type extends further south along the coast. To confirm if the site was an ombrotrophic peatland, we analysed its topography, hydrologic regime, water chemistry and vegetation. LiDAR data indicated that the peatland is elevated nearly 3 m above the surrounding landscape. Water table variations in the plateau were strongly associated with seasonal and daily precipitation events, indicating ombrotrophy. The hydraulic gradient on the plateau is downward through most of the year, demonstrating that precipitation is percolating vertically into deeper peat layers. In the rand, the hydraulic gradients are horizontal over much of the year, indicating that the plateau is draining through the rand to the lagg. Calcium, magnesium and potassium occur in very low concentrations, and the only ions in higher concentration are sodium and chloride, suggesting inputs of precipitation influenced by Pacific Ocean sea spray. Distinct vegetation composition and structure are associated with the plateau, rand and lagg zones of the site. These multiple lines of evidence indicate that Crowberry Bog is an ombrotrophic peatland, the first of its type identified in the conterminous western USA and the most southerly occurrence of its type in western North America.