Historical Tidal-Terrestrial Transition Zone in South SF Bay

First there was the great salt marsh, with all its winding sloughs and creeks, covered with samphire grass [Salicornia] and tufts of Grindelia; next was a line of natural salt pan; next again was a strip of land of varying width, from a few hundred yards to one fourth mile, with a short wiry hard grass [Distichlis]… (Beardsley, in Cooper 1926)

The tidal-terrestrial transition zone (T-zone) occupies the gradient between the intertidal zone and terrestrial (i.e., levee faces, valleys, hillsides, alluvial fans, and bluffs) and/or fluvial (i.e., rivers and streams) environments. The T-zone provides a number of valuable ecosystem functions and services, and also serves as accommodation space for estuarine transgression and flood water dispersal/storage as sea level rises in the future.

The T-zone is also one of the most heavily impacted areas of the Bay ecosystem, and emerging plans call for the conservation and reconnection of a T-zone where tidal marshes and their terrestrial connections can be created or allowed to naturally evolve. Ecological restoration of the T-zone based on reference conditions and analog models is recognized as a conservation and restoration priority; however, there have been no quantitative regional assessments of the distribution or condition of the historical T-zone to date to inform planning efforts.

This study, funded by the California Coastal Program of the U.S. Fish and Wildlife Service, seeks to partially address this information gap by reconstructing the historical (ca. 1850) extent and character of the tidal-terrestrial T-zone in the South Bay. We found that the South Bay tidal-terrestrial interface was dominated (nearly 70%) by low-gradient seasonal wetlands grading into high marsh. Within this overall pattern, however, the T-zone was quite diverse. Seasonal wetlands included wet meadow, alkali meadow, and vernal pool complexes – each with distinct ecological characteristics and functions. Willow thickets, depressional wetlands (freshwater marsh), and alluvial grassland and riparian forests also contributed to the diversity of the T-zone. Relatively steeply-sloped grasslands bordering tidal marsh – the historical interface type most similar in composition to the levee faces that constitute most of the T-zone observed today in the South Bay – comprised less than 10% of the South Bay tidal-terrestrial interface. The T-zone included variation on the tidal side as well, with fresh-brackish tidal marshes associated with major creek mouths interspersed within a broad salt marsh context.

Our study also suggests that the width of the T-zone varied dramatically with physical setting, from several meters to thousands of meters. The zone was broadest in areas with significant fluvial influence and in the most gently-sloping portions of the region. The emerging scientific understanding of the character and function of the Bay’s T-zone developed through this effort can assist scientists and managers in identifying appropriate objectives for T-zone conservation and restoration in the context of projected changes in land use and climate.

Completed May 2013