spread over the year should produce less 

 saline soils, which probably affect plant 

 growth rates, if not species composition. 



The composition of marshes to the 

 north of Point Conception differs in the 

 absence of southern species ( Salicornia 

 bigelovii , S. subterminalis , Monanthochloe 

 littoralis , Batis maritima ) . Pickleweed 

 ( Salicornia virginica ) and cordgrass 

 ( Spartina foliosa ) are important in the 

 upper and lower elevations, respectively, 

 except that cordgrass is absent from 

 Elkhorn Slough, Tomales Bay and Bodega Bay 

 (Macdonald 1977b). Disturbances have 

 played important roles in the history of 

 these wetlands as well, notably salt works 

 and urban development surrounding San 

 Francisco Bay, logging and urban 

 development at Humboldt Bay, and dredging 

 to enhance navigation at Elkhorn Slough. 



South of Tijuana Estuary are the 

 wetlands of Baja California, which are 

 just now facing the prospect of intensive 

 development. The marshes of Bahia de San 

 Quintin (30°31'N, 116°0'W) are the most 

 well studied (Neuenschwander et al. 1979). 

 These marshes are of special interest 

 because so little disturbance has 

 occurred. Thorsted (1972) did not measure 

 soil salinities in the intertidal marsh, 

 but the low rainfall (5 to 10 cm/yr) 

 strongly suggests hypersaline soils even 

 with the moderating influence of frequent 

 fog cover. Seven marshes within the Bahia 

 were sampled for frequency and occurrence 

 in the same manner as Vogl's (1966) study 

 of Upper Newport Bay. I calculated 

 percent similarities (Whittaker 1975) of 

 about 70% between the lower, middle and 

 upper marsh zones for these two widely 

 separated (nearly 500 km [310 mi]) bays. 

 The high similarity of species lists for 

 Bahia de San Quintin, Tijuana Estuary, 

 Mission Bay, Upper Newport Bay, Anaheim 

 Bay, and Mugu Lagoon suggests that the 

 northern Baja California marshes belong to 

 the southern California regional group. 



2.5 EFFECTS OF DISTURBANCE ON MARSH 

 COMMUNITY STRUCTURE 



Disturbance takes many forms in 

 southern California wetlands. Some 

 disturbances are natural; most are 

 man-caused. Some are chronic; others are 

 catastrophic. The first response of 

 vegetation to disturbance will normally be 

 change in the functional attributes of the 

 plants, i.e. their rate of growth may 

 increase or decrease. Changes in species 

 composition of the vegetation, i.e. the 

 invasion of new species or elimination of 

 previously occurring species, generally 

 take longer. This discussion will 

 emphasize changes in species composition 

 which result from the catastrophic 

 disturbances of altered tidal circulation 

 and of flooding. Changes which result 

 from chronic disturbances, such as 

 fertilization from adjacent agricultural 

 runoff, are much more difficult to 

 document. However, the effects of reduced 

 tidal circulation, natural flooding, and 

 artificially augmented flooding are 

 becoming clearj and most of the species 

 compositional changes can be explained on 

 the basis of the changing soil salinities 

 which are associated with these 

 disturbances. 



Reduced Tidal Circulation 



As discussed in Chapter 1, closure of 

 a wetland can result either in more 

 brackish or more saline soils. Wetlands 

 which undergo an annual cycle of closure, 

 or which remain closed for several years 

 at a time most certainly experience a 

 broader range of soil salinities and soil 

 moisture than marshes which are 

 continually under the influence of tides. 

 Given that halophytes have upper limits 

 for salinity and drought stress, one might 

 expect that sensitive species would be 

 eliminated from habitats or from entire 

 wetlands where tidal circulation is 

 unpredictable. Furthermore, one would 

 expect widely tolerant species to dominate 

 under these disturbance conditions. 



The smaller wetlands more frequently 

 lose their tidal connection, unless 



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