Environmental controls in Pacific Northwest intertidal marsh plant communities

Relationships between environmental variables and species distribution were studied in a brackish intertidal marsh formed by the Skagit River as it enters the Puget Sound bay system in Washington. Transects were established which covered the range of environmental variation in the marsh. A grid of environmental measuring stations provided information on soil texture, organic content of soil fines, macroorganic material in the soil, soil temperatures, interstitial soil water salinity, soil redox potential, and site elevation. Binary discriminant analysis, a nonparametric method using species presence–absence data, was used to construct standardized residual matrices. Principal component analysis of standardized residuals (Q mode) indicated that salinity and soil texture were strongly correlated with the first factor generated, elevation with the second, and soil redox potential with the third. The factors explained, respectively, 48, 21, and 14% of the variance in the residuals matrix. From R-mode analysis, eight community types were derived: three dominated by Carex lyngbyei, two by Scirpus americanus, one by Scirpus maritimus, and two which are highly diverse.