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Linking watershed land-use change and wetland vegetation response in a coastal California watershed.
Byrd, Kristin1, 1 University of California, Berkeley, Berkeley, CA
ABSTRACT- The designation of a coastal marsh as a reserve or park does not protect it from land use activities outside its boundary. Elkhorn Slough, inland of Monterey Bay, supports one of the largest coastal marshes in California. While the slough contains an ecological reserve and a NOAA research reserve, agriculture in the watershed has increased dramatically since 1970, especially on steep slopes adjacent to pickleweed (Salicornia virginica) dominated salt marshes. This land use change has greatly increased sedimentation and formation of alluvial fans that have filled marshes and mudflats. Alluvial fans have historically been a small component of Northern CA salt marsh ecosystems and supported a highly diverse plant assemblage, but their now-greater prevalence here can potentially drive plant succession and community dynamics along the margin of the slough. Aerial photos were used to conduct an historic post-classification change detection analysis on 15 alluvial fans formed in coastal salt marshes. Photos from 1971, 1980, 1992, and 2001 were classified based on a modified NOAA Coastal Change Analysis Program land-cover classification system adjusted for fine-scale imagery. The decadal record of vegetation change indicated an almost 50% reduction in pickleweed cover within the study areas, and an encroachment of arroyo willow (Salix lasiolepis) into the marshes along with an invasion of cattails (Typha spp.) in some sites. Significant differences in elevation, soil texture, salinity, and nitrate concentration between pickleweed salt marshes and alluvial fan plant communities provide an explanation for the vegetation shift. Plant surveys indicated that some sites support high species diversity with composition similar to historic (1926) accounts of local alluvial fan plant communities, though the present-day fans are more dominated by non-native species such as poison hemlock (Conium maculatum). While some fans can provide refuge for plant diversity within Elkhorn Slough marshes, the formation of most fans leads to a shift in wetland type from salt marsh to monotypic freshwater marsh or riparian willow stand. This project explains larger-scale effects on wetland composition and structure and will guide wetland management activities that occur in highly impacted watersheds.
Key words: salt marsh ecosystem, remote sensing, coastal watershed land use, historic change-detection