Print PageSan Francisco Bay-Delta estuary is the largest estuary on the Pacific Coast. While vegetation follows a low diversity to high diversity gradient (salt water to freshwater), brackish tidal wetlands in the middle of the salinity gradient exhibit a strongly disjunct similarity relationship to other wetlands. We collected vegetation data from six tidal wetlands along this estuarine salinity gradient using ten randomly placed 0.1 ha plots within each wetland. Within each plot, all species were determined by surveys and 10 random 8 m2 subplots were used for cover analysis. At all wetlands, tidal frequency was calculated, soil salinity assessed at multiple times during the growing season, and sedimentation/accretion rates were estimated. Vegetation data were analyzed using similarity indices (Jaccard, Sorenson, Czekanowski) and multivariate methods (non-metric multidimensional scaling).
Results/Conclusions
Soil salinity increases greatly in the less frequently inundated marsh plain during rainless summer months, and brackish wetlands display a within-wetland disjunction in salinity patterns that correlates with the vegetation relationships. We found that the pattern of seasonal variability in soil salinity strongly influences vegetation composition, and that marsh plains of tidal brackish marshes are very similar to salt marshes, while channel side vegetation is similar to freshwater tidal wetlands. Accelerating sea-level rise, shifts in freshwater runoff, and warmer summer temperatures will have an immediate impact on the brackish wetland systems in this climate. Our data suggest salinity will modify brackish wetlands toward salt marshes at a faster rate than sea level rise will affect vegetation composition. We conceptually model these changes using our vegetation, salinity, tidal and sedimentation data to indicate potential future shifts.
