Long-term vegetation change in Louisiana tidal marshes, 1968–1992

Abstract

The Louisiana coastal marshes form some of the most extensive wetlands within the continental United States. The problem of land loss in these coastal marshes is well-documented, but very little is known about possible changes in vegetation composition that might be associated with this loss. We analyzed vegetation data collected from 1968 to 1992 in the tidal wetlands of Terrebonne parish and described five vegetation types that occur in this region. Our data did not show the predicted change to more salt-tolerant vegetation. This is probably due to the influence of the Atchafalaya River in the study area. However, we documented a large change in the dominant vegetation of the fresh marsh. Panicum hemitornon-dominated marshes occupied 51% of the study area in 1968 and only 14% in 1992. This vegetation type was replaced with Eleocharis baldwinii-domJnated marshes (3% in 1968 to 41% in 1992). This change occurred adjacent to an area of significant conversion to open water. Based on limited available data from the literature, we ev',duated three potential driving factors in this change-grazing, water-level increase, and water quality-but could not determine the cause of change definitively.

Growth and allocation by a keystone wetland plant, Panicum hemitomon, and implications for managing and rehabilitating coastal freshwater marshes, Louisiana, USA

“…The extent of floating marsh in coastal Louisiana declined significantly over the period 1960-1990(Sasser et al 1986Evers et al 1992;Visser et al 1999) largely as a result of physical disturbance, saltwater intrusion and grazing by the introduced herbivore Myocastor coypus M. (Sasser et al 1986;Visser et al 1999). In areas where degradation has occurred, TMFM has converted predominantly into either open water or thin-mat floating marsh, a less structurally sound form of floating marsh characterized by a different plant community assemblage lacking P. hemitomon.…”

“…The ecology of TMFM is well studied (e.g., DeLaune et al 1986;Sasser et al 1986Sasser et al , 1991Sasser et al , 1995Swarzenski et al 1991;Visser et al 1999;Holm et al 2000) and interest in its rehabilitation and sustainable management is well supported (CWPPRA 1993;Sasser et al 1993Sasser et al , 2005. However, P. hemitomon growth response to key environmental drivers such as hydrologic regime, and to a lesser degree nutrient regime, have largely remained unstudied or the results inconclusive.…”

Sawgrass (Cladium jamaicense) responses as early indicators of low-level phosphorus enrichment in the Florida Everglades

“…The spatial distribution of key species and changes in community composition are perhaps the most widely used vegetation indicators of anthropogenic impacts in wetlands and the basis for adaptive management (Galatowitsch and van der Valk 1996;Weieher et al 1996;Posey et al 1997;Weinstein et al 1997;Stein and Ambrose 1998;Detenbeck et al 1999;Visser et al 1999;Mushet et al 2002;Hinkle and Mitsch 2005;Teal and Weishar 2005). Research on the effects of eutrophication on Everglades vegetation also has focused on community change, in particular the conversion of sawgrass and slough communities to monotypic cattail stands (e.g., Davis 1994;Craft et al 1995;Rutchey and Vilchek 1999).…”

Hydrological Dynamics I: Surface Waters, Flood and Sediment Dynamics

“…Most of the marshes in the Louisiana delta plain no longer receive water from the Mississippi River, as it is confi ned within large levees (e.g. Visser et al 1999;Reed and Wilson 2004). When combined with high subsidence rates (potentially exacerbated by oil and gas extraction (Ko and Day 2004)) and sea level rise, these reduced freshwater infl ows can be expected to result in seawater intrusion into the freshwater marshes.…”

“…In addition, large decreases in the amount of suspended sediment carried by the Mississippi River have resulted from the trapping of sediment behind dams on the Missouri and Arkansas rivers and many stream bank protection works (Mossa 1996). As discussed above, Visser et al (1999) report that most of the marshes in the Louisiana delta plain are now no longer nourished by the Mississippi River and its sediment. An accretionary defi cit, resulting from the lack of sediment, subsidence and sea level rise is leading to a shift from freshwater to saltwater marshes and a loss of land (Baumann et al 1984;Kuhn et al 1999).…”