VEGETATION TYPES: 



^■1- FRESH MARSHES 



INTERMEDIATE 

 MARSHES 



BRACKISH MARSHES 

 SALINE MARSHES 

  NL>N-MARSH AREAS 



Figure 35. Vegetation zones in the Mississippi River Delta marshes (Chabreck and 

 Linscombe 1978). 



Chabreck': data are for the coastal 

 marshes of the whole state. There is some 

 difference in the species found in the 

 western chenier plain compared to the 

 delta, but these are minor. More impor- 

 tant is that the species list is a com- 

 posite from many different, sites. No one 

 site would be expected to contain all the 

 species, especially in the intermediate 

 and fresh marshes. Each major zone is 

 actually a complex mosaic of many sub- 

 associations. The primary zones are, as 

 the names indicate, determined by the 

 salinity tolerance of the plants. Within 

 each zone detailed mosaics result from 

 much more complex factors including soil 

 nutrients and elevation (hence flooding 

 frequency and duration). 



For example, a 90-km^ site in the 

 intermediate marsh in the Barataria basin 

 was mapped from aerial imagery, and 

 intensive ground surveys were conducted. 

 Six plant associations were identified 

 using statistical clustering techniques 

 (Figure 38), and even more complex visual 

 patterns are seen in the aerial imagery. 



The observed patterns seem to result from 

 the interaction of brackish water entering 

 the marsh from the east and south, and 

 fresh upland runoff from the west, com- 

 bined with slight elevation differences 

 (Sasser et al . 1982). 



Vegetation studies in the Atchafalaya 

 basin fresh marshes show the importance of 

 elevation and exposure to direct river 

 flow versus stagnating backwater flooding 

 in controlling the species distribution 

 (Johnson et al., LSU Center for Wetland 

 Resources; unpublished). Greenhouse 



studies on salt marsh species from the 

 delta clearly show differences in the 

 ability of different species to tolerate 

 flooding (Parrondo et al . 1978). In these 

 studies, although S^. al terniflora and S. 

 cynosuroides appeared to be equally weTl 

 adapted to salt, the latter was far less 

 tolerant of flooding (Figure 39). The 

 greenhouse studies quantify qualitative 

 observations that S^. cynosuroides is found 

 in slightly elevated locations in the 

 marsh. 



39 



