PNW 98 Session - Full Abstracts

Full Abstract

ANNA MOCKLER
Laura Casey
Mason Bowles
Jon Hansen
Nick Gillen

King County Department of Development & Environmental Services, 900 Oakesdale Avenue SW, Renton, WA 98055-1219

Results of Monitoring Wetland Mitigation Success in King County, WA

To evaluate wetland mitigation success, 30 mitigation sites in King County were monitored at least three years after installation using the following standards to evaluate success: 80% emergent cover; 45%-75% shrub and tree cover; and 80% survival rates. By these standards, seven sites were successful, 23 were not. Based on this sample group, net loss of wetlands, especially Class I and II wetlands, is occurring in King County. Monitoring forms were not designed to note the following common ingredients of failure: inappropriate design, including slopes steeper than 20% and excessive sun for specified plants; insufficient hydrology as a result of design or construction oversights; compacted or consolidated soil; and lack of maintenance. Field observations of undisturbed wetlands and mitigation sites in Puget Sound lowlands support Cowardin et al. (1979) - semipermanently to permanently flooded wetlands are dominated by emergents; seasonally to semipermanently flooded wetlands are dominated by shrubs; while wetlands that are temporarily to seasonally flooded are forested. Most mitigation sites have hydrology in this latter class, where succession typically progresses from FAC dominants (alder, cottonwood) to FACW and OBL dominants (cedar, salmonberry, skunk cabbage). It is proposed that mitigations designed for this class of wetlands mimic the recovery process where openings are created in wetland forests. Such mitigations might be densely planted to seral species like alder, willow, cottonwood, and Douglas fir, with an understory of robust, sun-tolerant natives like thimbleberry and lupine. After three years of growth, the site would be underplanted to a more diverse assemblage including cedar, hemlock, yew, and appropriate shade-dependent shrubs, forbs, and ferns. Additional proposals to increase mitigation success include requirements that soils on all sites be deconsolidated and amended with organic matter to 6" depth for emergents, and to at least 12" for woody vegetation; that elevations and soil preparation be inspected before plants are installed; and that maintenance requirements be enforced. Mitigation guidelines should be re-written, and monitoring forms re-designed to capture observations made by wetland scientists.