the number of halophytes that can establish 

 from seed increases. Cordgrass (Spartina 

 foliosa) and spike rush (Juncus acutus) are 

 able to expand their distributions through 

 seedling recruitment. However, because the 

 low-salinity gap is brief, brackish marsh 

 species are unable to establish. 



• Prolonged low-salinity gap: Artificially 

 impounding freshwater or increasing 

 streamflow via reservoir discharge allows a 

 variety of fresh and brackish marsh species to 

 germinate and become established. If they 

 grow to rhizome stage while salinities are 

 low, they may persist after hypersaline 

 conditions return. 



• Impoundment effects: If either fresh or 

 saline water is impounded, inundation 

 becomes stressful, especially if waters 

 completely submerge the mature plants or 

 seedlings. Species with little aerenchyma will 

 undergo heavy mortality. Pickleweed 

 (Salicornia virginica) dies out in areas that 

 have prolonged periods of inundation, 

 especially in summer. In the Netherlands, 

 experimental studies of several halophytes 

 indicated that warm temperatures reduce 

 inundation tolerance (Groenendijk 1984); 

 this is reasonable, since respiration rates 

 would increase and growth would decline. 



• Drought and hypersalinity effects: When 

 an estuary closes to tidal flushing, soils will 

 dry and salinities will rise simultaneously, 

 unless rainfall or artificial discharges 

 maintain water supplies. Most vegetatively- 

 reproducing perennials can survive long 

 periods of hypersalinity, perhaps by growing 

 longer roots and tapping water stored below 

 0.5 m depth (Griswold 1988). Even though 

 cordgrass had high mortality in 1984, its 

 population did not go extinct at Tijuana 

 Estuary. However, short-lived species that 

 have shallow roots and annuals that rely on 

 seedling recruitment will undergo heavy 

 mortality. 



Competition among species is also a factor 

 in local extinction. The region's rather 

 diverse plant communities may well result 

 from the highly variable environment, 

 because no one species has optimal 

 environmental conditions or an indefinite 



competitive advantage. For example, 

 pickleweed usually outcompetes cordgrass 

 (Zedler 1983b; Covin 1984, Griswold 

 1988), but the conditions that limit 

 cordgrass (hypersaline drought) differ from 

 those that limit pickleweed (inundation). 

 With prolonged flooding, pickleweed loses its 

 advantage, and complete exclusion cannnot be 

 achieved through competition. 



5 . 4 EFFECTS OF MAJOR DISTURBANCES ON 

 CORDGRASS GROWTH 



The monitoring of cordgrass at Tijuana 

 Estuary has tracked its responses to major 

 flooding (1980, 1983) and drought (1984), 

 as documented by the total stem length (TSL) 

 data (Figure 5.4, Zedler 1983b, Zedler et al. 

 1986, Zedler 1991a). In this section, the 

 various responses of the cordgrass are 

 explored by comparing effects on height, 

 density, and mortality. All summaries are for 

 stations where the species occurred (variable 

 n; see frequency data in Figure 5.3). 



There was a significant increase in TSL in 

 1980 and a rapid decline in 1981. This 

 growth response has been interpreted as an 

 effect of reduced soil salinity that resulted 

 from catastrophic flooding (Zedler 1983b). 

 At the time, it was thought to indicate a 

 maximum growth response for the site, 

 because floodwater volumes were enormously 

 high (the 1980 water year had 28 times the 

 average streamflow of previous years). On 

 the average, plants grew over 10 cm taller in 

 1980 than the previous year (Figure 5.7). 

 In addition, plants reached maximum heights 

 that were 20 cm greater than in 1979 

 (Figure 5.8). The tallest plants exceeded 100 

 cm. 



The second major growth response 

 occurred in 1983. In contrast to the 1980 

 response of increased height, the 1983 

 response was largely one of increased density 

 (Figure 5.9). Height also increased, as 

 maximum heights averaged 90 cm (Figure 

 5.8), but it did not show up as an increase in 

 mean height (Figure 5.7) because production 

 of new, short individuals dominated the 

 vegetation response. The high TSL in 



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