CHAPTER 5 



THE ROLE OF DISTURBANCES IN MODIFYING 



SALT MARSH 

 STRUCTURE AND FUNCTION 



The dynamics of species distributions and 

 growth rates have been documented in detail 

 for the salt marsh of Tijuana Estuary. A 

 general understanding of what controls 

 invasion and extinction of species at the 

 ecosystem scale has developed, as well as an 

 ability to predict expansions and declines at 

 the population scale. Catastrophic events, 

 though destructive in many ways, are 

 scientifically valuable. Their effects have 

 been documented through an extensive 

 monitoring program that began as a survey of 

 cordgrass habitats in 1979 and has continued 

 to date. This chapter provides extensive 

 evidence of the estuary's temporal variability 

 and indicates why no single year of data, or 

 general attributes such as productivity or 

 biomass, can fully characterize the salt marsh 

 ecosystem. 



5 . 1 MONITORING PROGRAM 



The lower salt marsh has been sampled 

 consistently at approximately 100 stations 

 along eight transects (Figure 5.1). The exact 

 number of stations varied slightly with our 

 ability to relocate station markers and our 

 need to track expanding cordgrass clones. The 

 transects were set up to characterize the 

 cordgrass community; thus they extended 

 from upper distributional limits toward 

 channels. In all cases, transects were named 

 for the nearest Army Corps of Engineers 

 benchmark (e.g., TJE-31). 



In 1984, the monitoring program was 

 expanded to include a larger proportion of the 



marsh plain. An additional 115 stations were 

 set up to extend from the upper distributional 

 limit of cordgrass inland. A topographic 

 survey then showed that the more inland areas 

 were still within the range of elevations 

 included in the lower marsh transects (Table 

 5.1). This emphasizes that the distribution of 

 cordgrass is not entirely predictable from 

 elevation. Other factors, including 



competition by succulent species, limit the 

 occurrence of cordgrass, and the monitoring 

 program has helped to elucidate these and 

 other distributional patterns. 



Measurements taken at the monitoring 

 stations included periodic measures of 

 elevation relative to Army Corps of Engineers 

 benchmarks. Each April, soil salinity was 

 measured in samples near all stations. In the 

 lower marsh, interstitial soil water was 

 collected by expressing soil water (from to 

 10 cm depth) through filter paper onto a 

 salinity refractometer. In the mid-elevation 

 marsh, the drier soil samples (0-10 cm 

 depth) were collected and taken to the 

 laboratory, where uniform soil pastes were 

 made and measured with a conductivity meter 

 (Richards 1954). In September, soil 



salinity sampling was repeated and vegetation 

 measured. Cordgrass was assessed by 

 measuring heights of stems within 0.25-m2 

 circular quadrats (or 25% of that area if 

 densities were too high for complete 

 measurements). Flowering stems were noted; 

 live and dead stems were counted. The percent 

 cover of each of the other species present was 

 estimated within cover classes ( <1%, 1%- 

 5%, 6%-25%, 26%-50%, 51%-75%, 

 76%- 100%). 



99 



