coast. Black mangroves (Aviaennia gevminans) occasionally appear southward of 

 Galveston and on offshore islands farther north. 



b. Western Region Marshes . Marsh vegetation in the western region is 

 less uniform than in the east. The vast majority of western marshes are found 

 in scattered bays and lagoons and in the mouths of some tributaries. The 

 coastal marshes of the southern Pacific are very distinct from those in the 

 northern Pacific. Along the southern Pacific, seasonally high levels of 

 salinity in coastal estuaries greatly limit the diversity of intertidal 

 vegetation. In parts of the larger bays such as the San Francisco Bay, 

 salinity concentrations reach or exceed sea strength during the dry summer 

 months when evaporation exceeds rainfall runoff and tidal exchange. Smaller 

 tributaries periodically run dry allowing seawater to intrude upstream. Under 

 these conditions Pacific cordgrass {Spartina fotiosa) , the western region 

 equivalent of smooth cordgrass, is the dominant flowering plant of the regu- 

 larly flooded part of the intertidal zone (Fig. 4). Only in the reaches of 

 tributaries where freshwater is a consistent influence is cordgrass displaced 

 by other species such as Alkali bulrush {Saivpus vohustus) . Broad reaches of 

 the irregularly flooded high marsh are dominated by pickleweed, a plant widely 

 distributed but of little importance on the east coast. The stresses of 

 salinity in the high marsh are even more severe than in regularly flooded 

 marshes. The pickleweed 's ability to tolerate salinity concentrations of more 

 than twice sea strength provides this species with a formidable advantage over 

 most other species. Saltgrass is common but seldom a dominant high marsh 

 species. 



Figure 4. Pacific cordgrass marsh (San Francisco, California). 



Freshwater is a much greater influence on the marshes of the northern 

 Pacific. In some areas such as the Olympic Peninsula of Washington, annual 

 precipitation is more than 250 centimeters. The specific composition of these 



19 



