entrances have been stabilized by dredging 

 and jetty construction. Fewer species of 

 salt marsh plants tend to be found in such 

 areas (Table M). Cordgrass ( Spartina 

 foliosa ) is usually absent from wetlands 

 which are frequently closed to tidal 

 circulation, and it has become eliminated 

 from Los Penasquitos Lagoon since Purer 's 

 1939-40 observations. A 1980 transplant 

 of 10 cordgrass sprigs to the pickleweed- 

 dominated lower marsh of Los Penasquitos 

 Lagoon resulted in one survivor through 

 the well- circulated 1980 growing season. 

 However, it too died during 1981 when the 

 lagoon was closed much of the year 

 (Williams and Zedler, unpub. data). 

 Eilers (1980) found that cordgrass 

 remained behind a dike at Bolsa Chica 

 marsh, even though regular tidal 

 circulation had been lacking for almost 80 

 years. It had, however, migrated to 

 elevations which would normally be too low 

 for this species. Soils where cordgrass 

 occurred were moist (65 to 89?) and their 

 salinity varied from brackish to 

 hypersaline during Eilers' 1977-78 study 

 period, suggesting that seepage was 

 occurring despite the closure of this 

 marsh to tidal circulation. Hence it may 

 not be access to tides that is required 

 for cordgrass's persistence, but 

 protection from extremes which usually 

 occur after closure, i.e. long periods of 

 high salinity and/or drought and 

 competition with pickleweed. 



Monotypic stands of pickleweed 

 ( Salicornia virginica ) are often found in 

 marshes with reduced tidal circulation. 

 The occurrence of pickleweed in marshes of 

 wide-ranging salinities and moisture 

 conditions and its broad range at Tijuana 

 Estuary (Figure 9) both document that this 

 species has a broad tolerance to 

 environmental conditions. The laboratory 

 experiments of Mahall and Park ( 1976b, c) 

 also showed that pickleweed from San 

 Francisco Bay tolerated a wider range of 

 salinities than cordgrass. 



Reduced tidal circulation may also 

 affect species composition of the upper 

 marsh by reducing soil moisture. Although 

 experimental evidence is lacking, the 



elimination of Frankenia grandifolia (but 

 not saltgrass [ Distichlis spicata ] ) from 

 the upper marsh at Los Penasquitos Lagoon 

 during dry periods has been, documented 

 (92% frequency and 1429 g/m in August 

 1977, compared to 38? frequency and 7 g/m 

 in 1981) (Zedler et al., unpub. data). 

 Salinities were brackish during much of 

 the three-year period, but soils were very 

 dry in 1981. Although Frankenia declined 

 substantially in the upper- marsh, it 

 remained robust (x=347 g/m in August 

 1981) in the adjacent lower marsh, 

 suggesting that plant disease or grazing 

 were not the cause. Competition with 

 saltgrass could have contributed to its 

 demise, but the biomass of saltgrass also 

 declined. Only alkali weed ( Cressa 

 truxillensi^ ) increased in Augjust biomass 

 (from 7 g/m in 1977 to 15 g/m in 1981). 



Natural Flooding 



Flooding and reductions in soil 

 salinity have dramatic effects on local 

 species distributions, thereby altering 

 marsh community composition. Cordgrass 

 appears to be especially sensitive to 

 freshwater input; it showed substantially 

 greater ability to become established 

 outside its usual boundaries following the 

 1980 flood at Tijuana Estuary (Zedler, 

 unpub. ms .b) . The observations which 

 follow suggest strongly that flood events 

 play an important role in allowing 

 halophytes to invade new habitats. 



At Tijuana Estuary, there are two 

 abandoned sewage lagoons which received 

 effluent from the city of Imperial Beach 

 up until the early 1960's. The periphery 

 of the lagoons is ringed with vegetation 

 dominated by pickleweed ( Salicornia 

 virginica ) , but 33 discrete patches of 

 cordgrass were located and their diameters 

 measured in 1979. A recensus after the 

 1980 flood revealed 103 patches, most of 

 which were very small and of recent origin 

 (Figure 28). Those which were near old 

 patches may well have established 

 vegetatively, but a few of the new patches 

 were several meters away from the nearest 

 vegetative source. Excavations of two new 



35 



