EROSION RATHER THAN PLANT DIEBACK AS THE MECHANISM OF MARSH LOSS IN AN ESTUARINE MARSH

Coastal marsh loss in Louisiana is attributed to plane dieback caused by processes that stress vegetation, and a common landscape pattern is broken marsh that expands at the expense of surrounding unbroken mars h. We tested the hypothesis that vegetation is more stressed in broken marsh than in adjacent unbroken marsh, as indicated by vegetation abo veground biomass, species diversity and soil Eh, on transects that ext ended from broken marsh to unbroken marsh at Marsh island, Louisiana. Soil Eh, vegetation above-ground biomass and species diversity did not differ between broken marsh and unbroken marsh, and above-ground biom ass was similar to that reported from other marshes. Thus, we rejected the hypothesis that marsh loss is related to vegetation stress. Two f actors were related to vegetation vigour: soil drainage and soil bulk density. Surprisingly, significant soil drainage occurred in broken ma rsh but not in unbroken marsh. Aboveground biomass of the dominant pla nt, Spartina patens (Aiton) Muhl., was lowest where soil bulk density was less than 0.08 g cm(-3), which illustrated the importance of miner al matter accumulation in submerging coastal marshes. The mechanism of marsh loss appeared to be erosion below the living root zone, as indi cated by the vertical and often undercut marsh-water interface, and by the separation of sod clasts. This is different from more rapid marsh loss associated with plant stress which we observed in other Louisian a marshes only 135 km away, indicating that marsh loss mechanisms can vary spatially even within a relatively small region.