Additional changes in elevation no doubt 

 occurred with drying and shrinkage of the 

 clay-dominated soils. Resurvey of the marsh 

 elevations in August 1984 (Table 5.2) 

 documented elevations similar to pre-1980 

 levels for all but transect TJE-2, which is 

 closest to the area of maximum channel 

 sedimentation. There, an increase of 28 cm 

 appears to have been caused by sand and mud 

 deposited during the same winter storm that 

 moved much of the coastal dune into the 

 estuarine channels. These before/after data 

 document local sedimentation rates during that 

 extreme event. 



Caution in interpretating these elevation 

 data is needed, since all are keyed to Army 

 Corps of Engineers benchmarks that have not 

 been professionally resurveyed. Whether or 

 not the swelling and shrinkage of the marsh 

 surface affects the positioning of these metal- 

 pipe benchmarks is unknown, as is the effect 

 of local shifts due to subsidence (e.g., 

 following groundwater pumping) or tectonic 

 activity. 



Table 5.2. Changes in elevation in the lower 

 marsh before flooding (1979), after flooding 

 (1980), and after sea storms (1984). Data 

 are mean elevation (cm NGVD). 



Transect 



1979 1980 1984 



5 . 3 EFFECTS OF MAJOR DISTURBANCES ON 

 SALT MARSH COMPOSITION 



For southern California, rare events are 

 very important to overall wetland structure 

 and functioning (Zedler and Onuf 1984). This 

 section focuses on the role of extreme 

 conditions in controlling the most basic 

 structural feature of a plant community: its 

 species composition. The presence of a species 

 is determined first by its ability to establish 

 and second by its ability to persist. It is 

 hypothesized that germination and 

 establishment are limited to the "low salinity 

 gap" that follows winter rainfall, and that 

 expansion and persistence are limited by the 

 environmental stresses of hypersaline 

 drought and/or excessive inundation. 



At Tijuana Estuary, the salt marsh 

 monitoring program begun in 1979 was 

 critical for documenting increased growth, 

 vegetative expansion, and seedling 

 establishment of cordgrass (Zedler 1983b) 

 following the 1980 flood. Associated field 

 research projects that were suggested by the 

 monitoring program have in turn identified 

 causes of observed vegetation changes. 



5.3.1 Dynamics of the Cordgrass Marsh 



Cordgrass and its associated succulents 

 underwent tremendous variation in growth 

 and distribution between 1979 and 1988. 

 The changes were neither cyclic nor 

 predictable from past understandings of salt 

 marsh ecology. The vegetation dynamics 

 appear to be related to both salinity and 

 elevation changes that occurred between 1979 

 and 1985. 



From 1979 to 1983, the distribution of 

 cordgrass was relatively constant (Figure 

 5.3). Debris deposited by the 1980 flood 

 eliminated some patches of cordgrass through 

 smothering, but there was some progression 

 landward, such that the net decrease in 

 occurrence was small. Then, during the 1984 

 drought, there was substantial mortality of 

 cordgrass, which was severe at the lowest 

 elevations. Each tidal creek was lined with a 

 band of dead cordgrass. The following year, 



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