Jenneke M. Visser

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.

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.

Landscape conservation in a forested wetland watershed.

M ore than one-half of the 40 million ha of wetlands in the coterminus United States is forested (Frayer et al. 1983). Most of these wetlands (57%; Abernethy and Turner 1987) are in the southeastern United States. They are characterized as permanently, semipermanently, or intermittently flooded and are dominated by cypress (Taxodium spp.), tupelo (Nyssa spp.), and oak (Quercus spp.). The broad Mississippi River alluvial floodplain, which extends from the Gulf of Mexico to southern Illinois, historically supported the largest United States expanses of forested wetlands, but since the 1950s these areas have been rapidly converted to the production of cotton, corn, and soybeans (OTA 1984). Brinson et al. (1981) estimated the loss of riparian forest at more than 70% since presettlement days. Abernethy and Turner (1987) calcu-

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 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.

A Computer Model to Forecast Wetland Vegetation Changes Resulting from Restoration and Protection in Coastal

ABSTRACT Visser, J.M.; Duke-Sylvester, S.M.; Carter, J., and Broussard, W.P., III, 2013. A computer model to forecast wetland vegetation changes resulting from restoration and protection in coastal Louisiana. The coastal wetlands of Louisiana are a unique ecosystem that supports a diversity of wildlife as well as a diverse community of commercial interests of both local and national importance. The state of Louisiana has established a 5-year cycle of scientific investigation to provide up-to-date information to guide future legislation and regulation aimed at preserving this critical ecosystem. Here we report on a model that projects changes in plant community distribution and composition in response to environmental conditions. This model is linked to a suite of other models and requires input from those that simulate the hydrology and morphology of coastal Louisiana. Collectively, these models are used to assess how alternative management plans may affect the wetland ecosystem through explicit spatial modeling of the physical and biological processes affected by proposed modifications to the ecosystem. We have also taken the opportunity to advance the state-of-the-art in wetland plant community modeling by using a model that is more species-based in its description of plant communities instead of one based on aggregated community types such as brackish marsh and saline marsh. The resulting model provides an increased level of ecological detail about how wetland communities are expected to respond. In addition, the output from this model provides critical inputs for estimating the effects of management on higher trophic level species though a more complete description of the shifts in habitat.

The Effect of Multiple Stressors on Salt Marsh End-of-Season Biomass

It is becoming more apparent that commonly used statistical methods (e.g. analysis of variance and regression) are not the best methods for estimating limiting relationships or stressor effects. A major challenge of estimating the effects associated with a measured subset of limiting factors is to account for the effects of unmeasured factors in an ecologically realistic matter. We used quantile regression to elucidate multiple stressor effects on end-of-season biomass data from two salt marsh sites in coastal Louisiana collected for 18 yr. Stressor effects evaluated based on available data were flooding, salinity air temperature, cloud cover, precipitation deficit, grazing by muskrat, and surface water nitrogen and phosphorus. Precipitation deficit combined with surface water nitrogen provided the best two-parameter model to explain variation in the peak biomass with different slopes and intercepts for the two study sites. Precipitation deficit, cloud cover, and temperature were significantly correlated with each other. Surface water nitrogen was significantly correlated with surface water phosphorus and muskrat density. The site with the larger duration of flooding showed reduced peak biomass, when cloud cover and surface water nitrogen were optimal. Variation in the relatively low salinity occurring in our study area did not explain any of the variation inSpartina alterniflora biomass.

Changes in tree species composition, structure and growth in a bald cypress-water tupelo swamp forest, 1980–1990

Abstract Changes in forest vegetation during a 10 year period in a second growth bald cypress-water tupelo swamp were analyzed. The vegetation composition of this swamp forest is characteristic for deep-water alluvial river swamps in the southeastern United States. Bald cypress (Taxodium distichum) dominates the lowest elevation sites, while water tupelo (Nyssa aquatica) increases in importance with increasing elevation. Species composition and diameter at breast height (DBH) of the trees in the swamp forest were measured in permanent plots established in 1980 and growth rates were estimated from the change in DBH between 1984 and 1990. Repeated measures analyses indicated that forest canopy opening had no significant effect on density, basal area, or relative importance of the three dominant tree species. In the 10 years of the study, the densities of bald cypress and water tupelo have stayed relatively constant, while red maple (Acer rubrum) and ash (Fraxinus spp.) densities are declining rapidly. For all four tree species, mortality exceeded recruitment between 1984 and 1990. Since this 65–100 year old second growth forest has a closed canopy, low recruitment was expected. However, saplings of all species, except ash, were observed in all plots. The most important change in species composition in the study area was the large decrease in the number of red maple trees. Water tupelo growth rates were the highest at 9.8 ± 3.4 cm2 year−1, followed by bald cypress at 7.0 ± 1.5 cm2 year−1, and red maple at 1.4 ± 0.2 cm2 year−1. Bald cypress growth rate was significantly correlated with site elevation. Variation in growth rates of the three dominant tree species was not explained by total tree density or total basal area.