( Forestiera acuminata ) are also found in 

 the baldcypress ( Taxodium distichum ) and 

 tupelogum ( Nyssa aqua tic a ) community. 

 Herbs and vines are common along bayou 

 banks and in any open spaces where light 

 penetrates the canopy. Poison ivy ( Rhus 

 radicans ) , evening trumpet flower ( Gel- 

 semium sempervirens ) , green briar 

 (Smilax spp.,), peppervine ( Ampelopsis 

 spp.,), and Virginia creeper ( Partheno- 

 cissus quinquef olia ) are the most pre- 

 valent vines in this area (Day et al. 

 1981). A complete list of species found 

 in both Louisiana bottomland hardwood 

 and deep swamp communities is found in 

 Conner et al. (1975) . 



BRACKISH MARSH (4) 



The brackish marsh is the second 

 largest habitat in areal extent in the 

 MDPR (as shown in Figure 21), covering 

 404,000 ha (997,880 acres) or 11.7% of 

 the entire area (Table 5). While the 

 average salinity values for MDPR salt 

 marshes is 16 to 19 ppt, the brackish 

 marsh is found in less saline waters, 

 with average values ranging from 7 to 9 

 ppt (Rainey 1979) . Since brackish marsh 

 is defined here to include those marshes 

 classified as intermediate by Chabreck 

 (1972), the average salinity range for 

 this habitat is from 2 to 9 ppt. The 

 overall salinity range in brackish marsh 

 varies much more widely, however, from 

 fresh conditions to almost full ocean 

 salinity. 



The dominant brackish marsh plant 

 is saltmeadow cordgrass, ( Spartina 

 patens ) , which is also found in salt 

 marsh. Saltmeadow cordgrass grows best 

 in waters with an average salinity of 

 8.6 ppt (Chabreck 1972) and it occupies 

 about 54% of the brackish marsh commu- 

 nity, as measured by percent cover. The 

 brackish marsh includes some plants with 

 lower salt tolerances than plants found 

 in the salt marsh. Being in a slightly 

 lower salinity, hence less stressful 

 setting, the brackish marsh is charac- 

 terized by a higher diversity of plant 

 life. Whereas the salt marsh habitat 

 supports only 17 plant species, the 

 brackish and intermediate marshes con- 

 tain 63 (Chabreck 1972). Saltmarsh 



cordgrass ( Spartina alternif lora ) also 

 occurs in brackish marsh, but it covers 

 only about 4% of the area. Other macro- 

 phytes that make up major portions of 

 the plant community are saltgrass ( Dis- 

 tichlis spicata ) , black rush ( Juncus 

 roemerianus ) , Scirpus spp., and several 

 three square rushes. 



Biomass and productivity studies of 

 brackish marsh macrophytes in the MDPR 

 have been carried out by de la Cruz 

 (1974), Payonk (1975), Hopkinson et al . 

 (1978b), White et al. (1978), and Cramer 

 et al. (1981). Average brackish marsh 

 has an estimated aboveground productiv- 

 ity of 2,800 g dry wt/m /yr compared 

 with a value of 2,000 for the salt marsh 

 (Costanza et al. 1983). This value for 

 brackish marsh is higher than that for 

 any other wetland habitat, implying that 

 the brackish marsh occupies an optimum 

 set of conditions. Stress levels are 

 high enough to reduce excessive competi- 

 tion, while source levels, or natural 

 work services, are high enough to ensure 

 excellent growing conditions. "Stress" 

 and "source" follow the usage developed 

 by Odum et al. (1974). 



When belowground production is in- 

 cluded, brackish marsh is estimated to 

 produce 11,300 g dry wt/m 2 /yr (Costanza 

 et al. 1983). This estimate is high 

 compared with values usually cited for 

 marshes, and is not very precise because 

 of the difficulty in measuring below- 

 ground production. 



Two other poorly understood param- 

 eters are community respiration and 

 detrital export. As a result, total 

 export to other habitats cannot be pre- 

 cisely quantified. By using an estimate 

 of total net primary production (11,300 

 g dry wt/m 2 /yr) and two of the available 

 estimates of respiration, the brackish 

 marsh can be calculated to be either ex- 

 porting 7,300 g dry wt/m 2 /yr, or 

 importing 3,000 g dry wt/m 2 /yr. Our 

 understanding of brackish marsh dynamics 

 is dependent upon more reliable data for 

 production, respiration, and export. 



Other biological studies conducted 

 in the brackish marsh have included the 

 use of this habitat by birds (Mabie 



45 



