clones failed to reveal rhizomal 

 connections outside the clone. Hence, at 

 least some of the new cordgrass patches 

 originated as seedlings. Prior to the 

 1980 flood there had been no record of 

 Spartina foliosa establishment by seed 

 despite several years' observation of 

 other species' seedlings at Tijuana 

 Estuary. 



In addition, the original patches 

 expanded in diameter significantly more in 

 1980 (x = 2.4 m) than during the 1979 

 growing season (x=1.1 m) (Zedler, unpubl. 

 ms.b). This expansion appeared to be 

 entirely vegetative, which is consistent 

 with the measured increases in density in 

 the nearby Tijuana Estuary marsh. As 

 further evidence that the flooding was 

 responsible for the expansion of cordgrass, 

 patches were again censused in 1981, a 

 year of little rainfall and high soil 

 salinities throughout the growing season. 

 The original patches of cordgrass had 

 their lowest expansion rates, indicating 

 significantly poorer growing conditions. 

 Several new patches were located, but the 

 rate of establishment was clearly below 

 that of 1980. To date, the evidence 

 strongly suggests that flooding is a 

 stimulant for major expansion of 

 cordgrass . 



Prolonged Excessive Freshwater Input 



Small pockets of freshwater marsh 

 have often been observed to develop next 

 to storm drains or other areas of 

 continuous freshwater input to salt marsh 

 habitats, but it is rare to witness 

 large-scale replacement of salt marsh by 

 freshwater marsh. However, exactly that 

 occurred in southern California (Zedler 

 1981b, c). Vegetation at the mouth of the 

 San Diego River shifted from dominance by 

 pickleweed ( Salicornia virginica ) to 

 dominance by cattails (Typha dominguensis ) 

 when the natural floods of 1980 were 

 followed by the deliberate release of 

 water from El Capitan Reservoir. A 

 two-month period of inundation early in 

 the growing season killed large areas of 

 pickleweed-dominated salt marsh vegetation 

 at the seaward end of the river. Leaching 

 reduced soil salinities to ppt in many 



parts of the marshland seeds of freshwater 

 marsh species, brought downstream by the 

 floodwaters, germinated rapidly. Cattails 

 developed a nearly continuous, tall, 

 robust canopy within two to three months 

 after flooding. The rapid shift in 

 species composition demonstrates that 

 pickleweed is sensitive to inundation and 

 that mudflat habitats are readily 

 colonized by opportunistic species (Figure 

 29). 



Dominance by freshwater species was 

 short lived, however, because the 

 freshwater input ceased and tides resumed 

 their role as the major water source. By 

 July 1981 nearly all the cattails had died 

 and the marsh was a nearly uniform canopy 

 of standing dead vegetation. At this 

 time, a few patches of salt marsh 

 vegetation were visible ; some of these 

 were patches of cordgrass which had 

 survived inundation and subsequent 

 competition with cattails. (Many had been 

 planted as part of the author's marsh 

 restoration project.) Other patches were 

 Jaumea carnosa or saltwort ( Bat is 

 maritima) which either survived the floods 

 or germinated shortly after. All three 

 species began expanding vegetatively 

 following the demise of the cattails, and 

 may produce a dense canopy before 

 pickleweed is able to re-invade by seed 

 and dominate the area. 



Chance events, such as the survival 

 or establishment of small patches of 

 halophytes, may have profound influence on 

 the species composition of a marsh. 

 Historical features such as local patches 

 of mortality and establishment may explain 

 some of the differences in species 

 composition among southern California's 

 coastal salt marshes. Even if identical 

 habitats are available for salt marsh 

 growth, one need not expect the ultimate 

 species lists to be identical, let alone 

 their respective abundances. 



Other Disturbances 



Denudation of marsh vegetation by 

 vehicle abuse is one of the most visible 



37 



