| | Figure 1. A map of the flotant in the Barataria and Terrebonne basins, 1996. |
| | | Figure 2. The below-ground material of maidencane forms an extensive root and rhizome distribution that creates a fibrous and buoyant organic root mat. (Photo by Jenneke M. Visser) |
| | | Figure 3. Maidencane growth within exclosures at the Terrebonne Basin thin-mat floating marsh restoration demonstration project field site. (Photo by Charles E. Sasser) |
| | | Figure 4. Testing of artificial floating marsh system designs on Aug. 22, 2005, at the LSU AgCenter Aquaculture Research Station in Baton Rouge. (Photo by Jenneke M. Visser) |
| | | Figure 5. Deployment of vegetated artificial floating marsh system structures at the Mandalay marsh site in April 2006. (Photo by Daniel Sasser) |
| | | Figure 6. Condition of vegetation in artificial floating marsh system at the Mandalay marsh sites on June 1, 2006. In this photo the structures are plainly visible. (Photo by Brian Milan) |
| | | Figure 7. Condition of vegetation in artificial floating marsh system at the Mandalay marsh sites on Sept. 11, 2006. In this photo, the robust growth of maidencane hides the structures. (Photo by Jenneke M. Visser) |
| | | Figure 8. Measurement of root mat depth at the Mandalay marsh field site, October 2006. (Photo by Charles E. Sasser) |
|
Charles E. Sasser, Michael D. Materne, Jenneke M. Visser, Guerry O. Holm and Elaine Evers
Coastal wetland loss has been widespread in Louisiana over the past half century and is related to a variety of causes. These include hydrologic changes from increased flooding duration and frequency, saltwater intrusion, plant-eating animals – particularly nutria – and water quality.
Louisiana coastal wetlands include large areas of salt marsh nearest the Gulf of Mexico shoreline with decreasing salt water influence in the brackish, intermediate and freshwater marshes farther inland from the coast. In the freshwater areas of the coast, major losses have occurred in the floating marshes that have historically covered extensive areas, particularly in the Mississippi River Deltaic Plain (MRDP) region, and continue to form a significant portion of the Louisiana coastal marshes. Landscape data from 1990 in the Barataria and Terrebonne basins of the MRDP indicate that buoyant marshes covered about 70 percent of the freshwater and low salinity marsh zones, an area of more than 350,000 acres.
The classic example of floating marsh (flotant) in Louisiana is a marsh dominated by maidencane. It has a 1- to 2-foot thick, buoyant, organic mat of densely intertwined roots and rhizomes in a mostly organic matrix that floats continuously, rising and falling with water level changes. This ability to float vertically as water level increases effectively neutralizes flooding as a stress, while providing a continuously wet environment for vegetation growth.
Through the middle of the last century, thick-mat flotant marsh was expansive and nearly unbroken covering a large part of the freshwater regions of the coast. Since then, however, the maidencane floating marsh has changed in many areas to open water or a low productivity, highly disturbed form of floating marsh called the thinmat flotant, dominated by spikerush. In contrast to the highly productive maidencane marsh, spikerush marshes grow on thin (less than 10 inches) floating mats, which are relatively fragile. In most cases, the thin-mat flotant marshes will not support the weight of a person. By contrast, a healthy thick-mat flotant is easily strong enough to support the weight of large animals.
Maidencane Decreases
Both the thick-mat maidencane and thin-mat spikerush marshes are supported by highly organic substrates with very low mineral densities and high organic matter content. Aboveground vegetation productivity is considerably higher in the maidencane marsh compared to the spikerush marsh. Between 1968 and 1992, maidencanedominated marshes in the Barataria and Terrebonne basins decreased from 67 percent to 19 percent of the fresh and low-salinity marshes. At the same time, spikerush marshes increased from 3 percent in 1968 to 53 percent in 1992. The vegetation habitat map (Figure 1) illustrates the widespread distribution of the thin-mat floating marsh in the MRDP.
We are increasingly optimistic that restoration of deteriorated freshwater floating marshes is possible by initiating new growth in shallow, open water areas and improved biomass production and soil stabilization in the remnant thin-mat marshes. Floating marshes appear to be unique in their origin, the way they function and their responses to changing environmental conditions. For successful coastal restoration, the implications of these unique characteristics must be considered in the design of restoration projects.
It is important to recognize the plant-substrate unit on which plants grow as the foundation of a floating marsh. Because the floating substrate is formed by the plants that grow in it, it is not a separate unit as, for example, soil in an agricultural field. In the Louisiana coastal ecosystem, the native freshwater plant, maidencane, plays a key role in the formation and maintenance of coastal freshwater peat soil mats and is the primary plant species used in restoration efforts. The belowground material of maidencane forms an extensive root and rhizome distribution that creates a fibrous and buoyant organic root mat (Figure 2). This extensive network of fibrous roots and rhizomes is crucial for building the floating marsh mats characteristic of most of our highly organic freshwater marshes. Other marsh plants that commonly occur in this highly buoyant floating mat do not on their own produce the extensive belowground material necessary for thick-mat development.
Two Enhancement Projects
The Coastal Wetlands Planning, Protection and Restoration Act of 1990 provides federal funds for planning and implementing projects that create, protect, restore and enhance coastal wetlands of the United States, including Louisiana. Two demonstration projects were designed and implemented under this act to develop methods for restoration of degraded freshwater floating marsh.
The first project demonstrated the enhancement and recovery of the remnant thin-mat floating marsh in deteriorated freshwater marshes by transplanting maidencane and protecting from nutria grazing. After four growing seasons, maidencane coverage from transplants into the thin-mat marsh protected from nutria expanded from an initial 4 percent at planting to greater than 80 percent after four years. The maidencane plantings doubled the biomass above- and below-ground, relative to controls, and improved soil strength, which is an indicator of marsh health. The overall conclusion from the project is that thin-mat floating marsh can be enhanced and restored by transplanting maidencane into the mat and protecting from nutria grazing (Figure 3).
The second floating marsh restoration demonstration project is under way at the LSU AgCenter and is aimed at restoring open water areas within degraded freshwater floating marsh. Implementation of this project began in 2004 and consists of two phases. The first phase is the development of artificial floating marsh systems. This part of the project included the development of a floating system that provides the structure to keep the substrate in place and adequate buoyancy for maidencane plants to become established and grow a self-sustaining mat. Thus, structures using a variety of materials were evaluated during testing in ponds at the LSU AgCenter’s Aquaculture Research Station in Baton Rouge (Figure 4). At the same time researchers at the University of New Orleans are assisting through a subcontract provided by the LSU AgCenter to look at the plant response to nutrients, flooding and substrate in order to develop methods to maximize the establishment and growth of maidencane in the floating structures.
Over the first 18 months of the project, 27 designs were constructed and tested in research ponds at the Aquaculture Station. The designs used various structural materials – pine and cedar wood, bamboo, PVC, Styrofoam – or combinations of these materials. Mat materials tested included burlap, jute netting, coconut, straw-coconut, birch and hydroponic growth on a poultry wire base. Substrate material used included hardwood mulch, peat and a peat-bagasse mixture. Plants were established from maidencane harvested from Louisiana donor marshes and grown in LSU AgCenter greenhouses at Baton Rouge. Experiments included testing whole plants, rhizomes, stem material and plant pieces as a starter material to determine the types of plant material most suitable for plant establishment.
Field Testing Designs
The second phase of the project is field testing the best designs in a natural marsh setting. Two successful designs based on findings related to structural integrity, buoyancy and growth response from the research pond investigations were brought forward for field deployment at the Mandalay National Wildlife Refuge in Terrebonne Parish. The refuge is located within a wetland area that once supported a large expanse of thick-mat floating maidencane marsh that has now degraded to open water and remnant freshwater marsh. The two successful designs were a 4-foot by 10-foot terrace design using capped PVC pipe for buoyancy and a 4-foot by 4-foot bamboo square design using bamboo material for buoyancy – both with wire containment baskets for plant material. Two hundred terrace structures and 100 bamboo square structures were deployed at Mandalay Refuge in the spring of 2006 using either maidencane whole plants in peat pots or stem material as the plant material source (Figure 5). Because nutria grazing in the open marsh system is a major problem, most structures were fenced.
After a full growing season in the field, all of the structures at the Mandalay Refuge have remained intact, with no loss of structural integrity. Fortunately, the 2006 hurricane season was mild, with no tropical storms or hurricanes causing problems to the project area or the Louisiana coast. The vegetation planted in the structures has performed well, with all maidencane treatments having average cover values exceeding 60 percent. Total cover of all emergent plants averaged 90 percent across all treatments. Maidencane was dominant (Figures 6 and 7).
Establishment with potted plants resulted in more rapid cover increases than establishment from stems. By the end of the first growing season, however, differences in cover between establishment techniques were small, especially in the sites planted first.
The below-ground growth of roots and rhizomes was equally impressive (Figure 8). Maidencane rhizomes started spreading outside of the structures as the growing season progressed. The expectation is that this lateral spread will continue and eventually join the floating maidencane islands into a continuous marsh, achieving the desired restoration goal. In addition to the lateral spread of maidencane, other species including water hyacinth and frogbit rafted against the structures, providing some additional opportunity for marsh formation.
Preliminary Findings
Our preliminary findings after the first year of deployment are that the floating island structure designs using PVC and bamboo materials for buoyancy were successful. All structures remained buoyant and structurally intact in the first growing season. Coconut, birch, coconut-straw mats and chicken-wire baskets were successfully used to establish maidencane plant material. Maidencane cover increased whether established from marsh plugs, whole plants (rhizome and stem material), belowground material, stems or peat pots.
Based on the preliminary results of these demonstration projects and previous research, restoration and enhancement of much of Louisiana’s coastal freshwater marshes appear to be possible by controlling nutria grazing and transplanting exceptionally productive mat-forming plant species such as maidencane directly into the degraded areas. This includes both transplanting directly into existing remnant thin-mat flotant marshes, as well as growing back marsh using maidencane planted artificial floating islands in adjacent shallow open water areas where conditions are favorable.
Pilot-scale efforts are under way at the LSU AgCenter to develop a more cost-effective method of reintroducing maidencane into degraded marsh areas. Because maidencane is not a prolific producer of seeds, the reintroduction of this desirable species on remnant marsh mats in degraded areas by aerial distribution of seeds is not possible at this time. Preliminary work at the LSU AgCenter to aerially distribute plant material from small root and stem pieces, however, may provide a valuable option for helping restore coastal floating marshes.
Charles Sasser, Professor, School of the Coast & Environment and School of Plant, Environmental & Soil Sciences; Michael D. Materne, Instructor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Jenneke M. Visser, Associate Professor, Guerry O. Holm, Postdoctoral Researcher, and Elaine Evers, Research Associate, School of the Coast & Environment, LSU, Baton Rouge, La.
(This article was published in the spring 2007 issue of Louisiana Agriculture.) |