break-up of the substrate. The rapid rate of subsidence in Louisiana's coastal zone (Baumann et 

 al. 1984), in combination with the predicted sea level rise of 50 to 200 cm during the next 100 

 years (Titus 1986), will lead to increased salinity and flooding stresses. 



A salinity range of 21-28 ppt produced a significant reduction (compared to controls at 11-15 

 ppt) in aboveground biomass of S. patens. Since S. patens possesses salt glands (Anderson 1974), 

 this species has the capability of extruding excessive salt, thus aiding in control of internal 

 electrolyte concentrations. Although toxic ion effects cannot be totally excluded, the significant 

 reduction in aboveground biomass of S. patens in response to simulated saltwater intrusion was 

 most likely caused by a water deficit, which resulted in substantial tissue death upon sudden 

 exposure to more saline conditions. Two months after transplantation to the higher salinity marsh, 

 approximately 50% of the original tissue was dead. However, new green shoots had also appeared 

 in these plots. Although these results indicate that S. patens may recover to some extent from 

 sudden influxes of saltwater that kill the original biomass, the potential for biomass production 

 would be nevertheless significantly reduced. 



Flooding with saltwater may cause stresses in addition to those resulting from increased 

 electrolyte concentrations or physiological drought. If the influx of saltwater is accompanied by an 

 increase in depth or duration of flooding, then the plants may also experience root oxygen 

 deficiencies, decreased nutrient uptake, and/or a buildup of toxic compounds such as hydrogen 

 sulfide in the highly reducing soil environment (Hook 1984; Kozlowski 1984). Many inland brackish 

 and salt marshes in Louisiana are characterized by low productivity, decreased elevations due to 

 subsidence, increased waterlogging, and high soil sulfide concentrations. Factors associated with 

 increased soil waterlogging have been implicated in decline in growth and dieback of S. alterniflora 

 (Linthurst and Seneca 1980; Mendelssohn and Seneca 1980; Howes et al. 1981; Mendelssohn et 

 al. 1981; King et al. 1982; DeLaune et al. 1983; Mendelssohn and McKee 1988). The results of 

 this study demonstrate that dieback of S. /?a/ms-dominated brackish marshes may in some cases also 

 be caused by similar mechanisms. The results showed that the aboveground growth of S. patens 

 can be adversely affected when the soil becomes highly reduced and sulfide accumulates (2.5 mM). 

 The strong negative effect of greater soil waterlogging on plant growth was evident in the saltwater 

 intrusion-subsidence experiment (Figure 2), as well as the dieback site experiment (Figure 3). The 

 significant reduction in aboveground biomass and stem density of S. patens caused by an in situ 

 decrease in elevation alone further emphasizes the effect of elevation on plant growth (Figure 2). 

 The inhibitory effects of waterlogging were ameliorated, however, when the elevation of the plots 

 was increased 10 cm. This result was evident in both experiments. The positive effect of increased 

 elevation suggests that restoration of such a dieback area with S. patens would require sediment 

 additions to raise the elevation of the inland marsh. 



In contrast to S. patens, standing biomass of S. alterniflora in sods transplanted to the same 

 dieback site was significantly higher that of the controls (Figure 3). These results indicate that S. 

 alterniflora can tolerate sulfide levels of about 2.5 mM for a growing season and that 

 transplantation of this species to a dieback brackish marsh would possibly succeed in restoring the 

 area without the need for sediment additions. Although S. alterniflora was more tolerant of 

 waterlogged brackish sites- than S. patens, S. alterniflora standing biomass can be significantly 

 reduced in salt marsh dieback sites where sulfide concentrations increase to extremely high levels 

 (4-5 mM) (McKee and Mendelssohn, unpubl. data). Also, there is evidence that belowground 

 biomass of S. alterniflora is significantly reduced by moderate levels of sulfide (2.5 mM) (Koch and 

 Mendelssohn, in press). Thus, depending on the site, restoration with S. alterniflora may require 

 sediment additions to achieve a long-term recovery. 



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