Recent (1975-2004) vegetation change in the San Francisco Estuary, California, tidal marshes

The establishment and monitoring of vegetation plots provide unique information on the spatiotemporal dynamics of plant distributions. In 1975, U.S. Geological Survey (USGS) scientists Atwater and Hedel established vegetation transects at six tidal marshes spanning the salinity gradient of the San Francisco Estuary, California, to establish a baseline against which future vegetation changes could be measured. Since 1975, estuarine salinity has increased because of water diversion, whereas sea level has risen at rates that exceed late-Holocene background levels. During the summers of 2003-04, we reoccupied these transects and measured plant composition to better understand the response of marsh vegetation to these hydrologic changes. Our results indicate that during the past 30 years, Sarcocornia pacifica, the most salt-tolerant plant species, increased in cover, whereas Schoenoplectus californicus, the least salt-tolerant plant species, decreased in cover. We hypothesize that increasing estuarine salinity resulted in these shifts. Additionally, Distichlis spicata has experienced large-scale replacement, predominantly by Schoenoplectus americanus. Because Distichlis spicata was found to be associated most strongly with high relative marsh elevations, we hypothesize that the decline in this species is a consequence of accelerated sea level rise. The vegetation changes observed in San Francisco Estuary marshes reported here are consistent throughout the estuary and have been observed in other studies and, therefore, serve as a guide to better understand the consequences of anthropogenically driven environmental change to estuarine wetlands.