Change in species composition with 

 elevation has been attributed to 

 differences in inundation tolerance, 

 differences in salinity tolerance, and 

 competitive interactions of species (Purer 

 1942). Little experimental work has been 

 done to test these ideas which were put 

 forth nearly 40 years ago. But, we do 

 have a better data base for the physical 

 features which change with elevation and 

 some indication of the importance of 

 competition between certain marsh species. 



Inundation is obviously more frequent 

 and of longer continuous duration at the 

 lower elevations. Macdonald's (1969) 

 measurements of submergence at Mission Bay 

 show that the lower marsh boundary 

 coincides with MLHW (about 3.5 ft MLLW) 

 and with a sharp change in hours of 

 continuous submergence noted in January 

 1965 (Figure 5). Above this boundary, 

 maximum continuous submersion decreased 

 gradually to zero at 2.1 m (7 ft) MLLW 

 (mean lower low water). 



Soil organic matter content is often 

 lower in the upper marsh (Table 5) and 

 soils are sandier there as well (Figure 

 10). Together, these factors reduce the 

 water-retaining capacity of upper marsh 

 soils and cause the higher bulk densities 

 (dry wt/volume) of upper marsh soils noted 

 at Tijuana Estuary (Zedler 1977). 



These three environmental features, 

 marsh elevation, organic content and 

 percent sand, are unlikely to change 

 greatly from year to year and hence cannot 

 explain the variations in species 

 distribution which occur from year to 

 year. 



Soil salinity does change with time, 

 and this may well be the most important 

 physical variable which influences marsh 

 vegetation. Macdonald (1977a) suggested a 

 simple model of soil salinity differences 

 with elevation for California marshes, 

 namely that "soil salinities increase 

 landward to a maximum around MHW (i.e. the 

 low marsh-high marsh ecotone) and then 

 gradually decline" (Macdonald 1977a, p. 

 271). But since salinity extremes are 



Table 5. Soil organic matter content (%) 

 determined by loss on ignition. Data are 

 for the Tijuana Estuary, at stations where 

 vascular plant productivity was measured 

 by Winfield (1980) , 



high (8-11) 

 Monanthochloe 

 littoralis & 

 Salicornia 

 subterminalis 



0-10 19 5.2 6 

 10-20 18 1.5 2 



more likely to influence plant 

 distribution than are average conditions, 

 a more detailed description of soil 

 salinities is necessary. Data from a 

 transect at Tijuana Estuary (Figure 11) 

 illustrate the usual hypersalinity of 

 southern California wetlands and reveal 

 several patterns: (1) soil salinity is 

 relatively constant at low elevations 

 where cordgrass dominates; (2) soils in 

 the middle and upper marsh elevations 

 become leached of salts following rainfall 

 events; (3) on the average, upper marsh 

 soils are less saline than lower marsh 

 soils; (4) the variability of soil 

 salinity, and hence its unpredictability, 

 increases with elevation. These last two 

 points are made clearer by plotting 

 averages and standard errors of soil 

 salinity against elevation (Figure 12). 

 These results are consistent with 

 Macdonald's (1977a) model of increasing 

 salinity towards MHW, and decreasing 

 salinity thereafter, but inclusion of the 

 variability term (standard error) shows 

 the necessity of frequent salinity 

 measurement in order to characterize the 

 marsh soil environment. If this transect 



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