Salt marsh vascular plant distribution in relation to tidal elevation, Humboldt Bay, California

The relationship between tidal elevation and the distribution of salt marsh vascular plants was investigated in Humboldt Bay, California. Plant species composition and cover were assessed and elevation surveyed in 148 one-square-meter plots in July-August, 1985, within five salt marsh study sites around northern and central Humboldt Bay. Elevations were surveyed with reference to the National Geodetic Vertical Datum (NGVD) and converted to a mean lower low water (MLLW) scale based on National Ocean Service (NOS) calculations for the relationship between NGVD and MLLW. Twenty-two plant species were encountered in the sampling, including the three rare plant species: Orthocarpus castillejoides var. humboldtiensis, Cordylanthus maritimus ssp. palustris and Grindelia stricta ssp. blakei. Direct gradient analysis was used to illustrate the change in species composition with elevation. Using cluster analysis, the samples were grouped on the basis of floristic similarity into three major vegetation types: Salicornia virginica marsh, Spartina densiflora marsh and mixed marsh, of which the mixed marsh type was most floristically diverse. At the four study sites associated with North Humboldt Bay, salt marsh vegetation ranged from 5.7 to 8.4 ft. MLLW. Tidal characteristics were notably lower at the fifth study site, Elk River, with salt marsh vegetation occurring between 3.9 and 6.1 ft. MLLW. Through discriminant analysis, tidal elevation data for North Bay were assessed as 60.0 percent effective in predicting the class memberships identified by cluster analysis. While there was overlap in their elevation distribution, each of the salt marsh types was most common within a particular range of elevation. Salicornia marsh was most prevalent below 6.9 ft. MLLW, Spartina marsh between 6.9 and 7.3 ft. MLLW and mixed marsh above 7.3 ft. MLLW.