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

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 hemitomon-dominated marshes occupied 51% of the study area in 1968 and only 14% in 1992. This vegetation type was replaced withEleocharis baldwinii-dominated 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 evaluated three potential driving factors in this change-grazing, water-level increase, and water quality-but could not determine the cause of change definitively.