^■ 





S*2J**^^^Ii^' 





^.V^. 



Figure 36. A deltaic plain brackish marsh. Note the "hunmocky" appearance wh 

 typical of Spartina patens stands. The birds with black-tipped wings are 

 pelicans, the smaller ones ducks, mostly teal (Photograph by Robert Abernathy) . 



'«^ 



ich is 

 white 



The roles of chance and competition 

 in marsh plant distribution have not been 

 extensively studied in the delta marshes. 

 We usually assume that seed sources are 

 abundant so that a supply of propagules 

 does not limit invasion by a species and 

 the presence of one species does not 

 prevent another adapted species from 

 invading. In fact, competition is probably 

 a very strong distribution factor. With 

 the exception of a few true obligate 

 halophytes (represented on the gulf coast 

 by Batis mari tima and several species of 

 Sal icornia ), the salt-tolerant species will 

 all grow well in fresh or nearly fresh 

 substrates. Since these species are not 

 found in salt-free areas, presumably they 

 are confined to saline areas because they 

 cannot compete well with fresh marsh 

 species in a fresh environment. Another 

 example of competition is the observation 

 that the thick layer of dead vegetation 

 covering a stand of the perennial grass ^. 

 patens excludes S_. olneyi and annual 



grasses. It is common to burn S_. patens 

 stands to encourage these other species 

 which are nore desirable as food for ducks 

 and muskrats (Hoffpauir 1968). 



In early literature on delta marsh 

 plants it was assumed that the vegetation 

 modified the landscape so that the envi- 

 ronment was changed, allowing other spe- 

 cies to invade. For example, Penfound and 

 Hathaway (1938) outlined a successional 

 sequence fran saline through fresh marshes 

 to upland forests. The sequence was based 

 on the idea that marsh plants, by produc- 

 ing peat, could elevate the sites they 

 grew on until upland species could invade 

 and survive there. This idea of autogenic 

 succession arose before we understood the 

 rapidity of subsidence on the gulf coast. 

 It is clear now, I think, that most vege- 

 tation changes in the delta marshes occur 

 because of allogenic processes. In a 

 sense, the most the biota can do is resist 

 and slow down the inevitable change from 



40 



