Charles E. Sasser

Marsh vegetation types of the Mississippi River Deltaic Plain

Marshes of the Mississippi River Deltaic Plain represent 17% of the coastal marshes in the continental United States. However, only a few detailed descriptions of the diverse plant communities that occur in this large expanse of wetlands exist and none are based on detailed vegetation analysis. The objective of this study was to quantitatively analyze the vegetation data collected in the wetlands of the Barataria and Terrebonne estuary to determine naturally occurring vegetation associations. Two-way indicator species analyses (TWINSPAN) revealed nine vegetation types: polyhaline mangrove, polyhaline oystergrass, mesohaline mix, mesohaline wiregrass, oligohaline wire grass, oligohaline mix, fresh bulltongue, fresh maidencane, and fresh cutgrass. These nine types form a logical expansion on the four salinity zone described for the region by previous studies and form a basis to compare the vegetation types of the Mississippi River Delta region with other regions of the Atlantic and Gulf coasts.

Vegetation dynamics in the emerging Atchafalaya Delta, Louisiana, USA.

1. The wetlands of the Atchafalaya Delta, Louisiana are characterized by a warm climate, highly fertile sediments, and an absence of moisture limitations. Consequently, vegetation succession (on islands that emerged in 1973) was expected to occur relatively rapidly. 2. From 1980 until 1986, annual surveys of vegetation cover and substrate elevation were conducted on 110 permanent plots along six transect lines crossing four islands. In addition, to evaluate the effect of herbivory (predominantly by Myocastor coypus, nutria or coypu), four 50-m × 40-m exclosures and four control plots were constructed on two of the islands

Hurricane-induced sediment deposition in a gulf coast marsh

Abstract Rates of sediment accumulation in four coastal marsh communities in the Mississippi River deltaic plain, Louisiana, were studied using feldspar clay marker horizons. The results indicate that normal riverine flooding contributes relatively little (

Spatial and temporal changes in Louisiana's Barataria Basin marshes, 1945–1980

A computerized geographic information system with site-specific change-detection capabilities was developed to document amounts, rates, locations, and sequences of loss of coastal marsh to open water in Barataria Basin, Louisiana, USA. Land-water interpretations based on 1945, 1956, 1969, and 1980 aerial photographs were used as input, and a modified version of the Earth Resources Laboratory Applications Software developed by the National Aeronautics and Space Administration was used as a spatial data base management system. Analysis of these data sets indicates that rates of marsh loss have increased from 0.36% per year in the 1945–56 period, to 1.03% per year in 1956–69, and to 1.96% per year in 1969–80. The patterns of marsh loss indicate that the combination of processes causing degradation of the marsh surface does not affect all areas uniformly. Marsh loss rates have been highest where freshwater marshes have been subject to saltwater intrusion. The increase in the wetland loss rates corresponds to accelerated rates of subsidence and canal dredging and to a cumulative increase in the area of canals and spoil deposits.

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.

The impact of a severe drought on the vegetation of a subtropical estuary

In order to document the effect of the recent drought and the resulting marine intrusion event on plant-community shifts in a Louisiana estuary, we analyzed two vegetation data sets collected in Barataria estuary in 1997 and 2000 and compared community shifts to surface salinity changes at four points along the estuarine gradient within the study area. We used the major vegetation types identified in our previous research of larger data sets and tested the use of a simple vegetation classification technique. This vegetation classification technique is based primarily on the dominant and co-dominant species, and secondarily on the number of taxa observed. To distinguish vegetation types with similar dominant species but different associated species, the vegetation classification technique used a salinity score derived from the species composition. Surface water salinity increases were reflected by a change in species composition in the mesohaline to fresh marshes. The largest species composition shift observed was the shift from oligohaline wiregrass (species rich vegetation type dominated bySpartina patens) to mesohaline wiregrass (vegetation type dominated byS. patens with few other species). Shifts in vegetation composition may have been enhanced by the presence of the major dominant species at a low abundance in other vegetation types. The vegetation classification technique used could classify over 95% of the stations. This vegetation classification technique provides a simple method to classify Louisianas coastal vegetation based on plant species composition.

A PROPOSED COAST-WIDE REFERENCE MONITORING SYSTEM FOR EVALUATING WETLAND RESTORATION TRAJECTORIES IN LOUISIANA

Wetland restoration efforts conducted in Louisiana under the Coastal Wetlands Planning, Protection and Restoration Act require monitoring the effectiveness of individual projects as well as monitoring the cumulative effects of all projects in restoring, creating, enhancing, and protecting the coastal landscape. The effectiveness of the traditional paired-reference monitoring approach in Louisiana has been limited because of difficulty in finding comparable reference sites. A multiple reference approach is proposed that uses aspects of hydrogeomorphic functional assessments and probabilistic sampling. This approach includes a suite of sites that encompass the range of ecological condition for each stratum, with projects placed on a continuum of conditions found for that stratum. Trajectories in reference sites through time are then compared with project trajectories through time. Plant community zonation complicated selection of indicators, strata, and sample size. The approach proposed could serve as a model for evaluating wetland ecosystems.

Marsh mat flotation in the Louisiana delta plain

Vertical mat movement in relation to surface-water fluctuations was measured for 1 year at three marshes differing in dominant emergent vegetation and location in the Mississippi River delta plain of coastal Louisiana, U.S.A.. The freshwater marsh, dominated by Panicum hemitomon, floated directly with ambient water levels, provided they were high enough to float the mat. Water levels varied by c. 70 cm and mat movement by 55 cm. An intermediate-salinity marsh closer to the Gulf of Mexico and dominated by Sagittaria falcata moved 35 cm vertically during the study period, and water levels moved 70 cm. A brackish marsh, dominated by Spartina patens, moved only 3 cm in response to c. 40 cm of vertical water movement (...)

Marsh Vegetation Types of the Chenier Plain, Louisiana, USA

The Chenier Plain of Louisiana contains 3.085 km2 of coastal marshes and stretches from the Texas border to Vermilion Bay at approximately 91°30′W. The objective of this study was to describe the vegetation types of the Chenier Plain in 1997, compare the vegetation types of the Chenier Plain with those described previously for the Mississippi River Deltaic Plain, and compare the distribution and composition to previous descriptions of vegetation types in the region. Two-way Indictor Species Analysis (TWINSPAN) revealed seven major vegetation types that occurred in the region in 1997: fresh bulltongue, fresh maidencane, oligohaline bulwlwhip, oligohaline paspalum, oligohaline wiregrass, mesohaline wiregrass, and mesohaline mixture. These vegetation types are a logical expansion of the habitats previously described for the region. Five of the seven vegetation types were also identified by similar analyses and descriptions for the Mississippi River Deltaic Plain. Vegetation in the fresh marsh substantially changed since it was first described by O’Neil in the 1940s. The largest change was the disappearance of the sawgrass habitat, although this change occurred before 1968. We show a continued trend in increase of oligohaline marsh at the expense of mesohaline wiregrass marsh, although it is not clear if this change is genuine or arises from the difference in classification methods among years. The mesohaline mixture, labeled saline marsh in previous studies, has remained relatively stable over time.

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.