disappearing much more often than was 

 obvious from its net changes. A tendency to 

 shift about, either through vegetative 

 expansion or recruitment from seed, indicates 

 high recoverability and resilience. Such 

 species are more likely to recruit when space 

 becomes available. 



The analysis of small-scale dynamics 

 underscores the need to sample large areas in 

 order to represent species distributional 

 changes. Sampling a small area that happens 

 to coincide with quadrats undergoing local 

 expansion or elimination may not represent 

 the range of the species. As a correlate, 

 sampling a large area and ignoring changes at 

 the quadrat scale can underestimate the 

 dynamics of species occurrences. 



5.3.4 A Conceptual Model of Compositional 

 Changes 



Extreme events have altered wetland 

 structure dramatically in southern California. 

 At the same time, they have helped to reveal 

 cause-effect relationships. Invasion and 

 expansion of species are controlled by the 

 annual "low-salinity gap" (Figure 5.6), 

 which varies both in duration and degree of 

 salinity reduction (Zedler and Beare 1986). 

 Population declines and local extinctions are 

 caused by drought, hypersalinity, and 

 prolonged inundation, to which species have 

 differential tolerance. 



The species characteristic of southern 

 California coastal wetlands do not conform to 

 the classical separation of halophytes and 

 glycophytes on the basis of tolerance to 0.5 

 ppt salinities (Waisel 1972). There is a 

 spectrum of tolerances, and establishment is 

 determined by the degree and duration of 

 freshwater influence: the low-salinity gap. 

 Salinity determines germination, while 

 duration of the required salinity and soil 

 moisture conditions determines seedling 

 establishment. This is consistent with the 

 regeneration niche concept of Grubb (1977), 

 who hypothesized that multiple character- 

 istics of both species and environment 

 influence establishment. Once established, 



LOW- 

 SALINITY 

 GAP 



none 



normal 



extreme 



prolonged 



MONTH OF YEAR 



JFMAMJJASOND 



JFMAMJJASOND 



JFMAMJJASOND 



JFMAMJJASOND 



Figure 5.6. Conceptual model of low-salinity 

 gaps in the salt marsh soil. Density of dots 

 indicates salinity, with the usual condition 

 being hypersaline (>40 ppt). The degree of 

 salinity reduction and its duration determines 

 seed germination and seedling establishment 

 (see text). 



salt marsh plant populations may persist or 

 go extinct, depending on environmental 

 conditions that may differ greatly from those 

 controlling establishment. 



The following is a summary of how low- 

 salinity gaps appear to control invasions and 

 how extreme stresses appear to control local 

 extinctions, based on conditions at both 

 Tijuana Estuary and San Diego River and 

 changes following wetter and drier conditions. 



• No low-salinity gap: With little or no 

 winter rainfall, only a few of the salt marsh 

 species (i.e., Salicornia species) are able to 

 germinate and establish seedlings. 



• Normal low-salinity gap: With typical 

 winter rainfall, brief salinity reduction 

 stimulates seed germination and allows 

 seedling establishment. Species of the 

 brackish marsh are not able to establish. 



• Extreme low-salinity gap: Extreme 

 flooding reduces salinities substantially, and 



108 



