There’s nothing quite like sitting in a duck blind on a chilly December morning waiting for first light. Wind moves the grasses around the blind as unseen mottled ducks call at the edge of the pond. Groups of green-winged teal buzz past on their way farther into the marsh, dark shadows against the greying sky. A cacophony of snow geese honk at each other, resounding from a rice field to the north. Then the sun rises, bringing with it group after group of birds searching for a spot to eat — and the possibility of shooting a bag limit before the morning is over.
The prospect of mornings like this is what brings duck hunters out in the marsh in the wee hours of the morning and a large part of what drives wetland management. For nearly a century, wetland management has centered on providing habitat and food for declining waterfowl populations. These management efforts, along with hunting regulations, have been successful in restoring waterfowl populations to historic numbers. Today, the Chenier Plain — a region of forested ridges called cheniers interspersed with freshwater marshes along the coast of southwestern Louisiana and southeastern Texas — winters millions of waterfowl each year.
Part of the reason the Chenier Plain of southwestern Louisiana and southeastern Texas is such good waterfowl habitat is that vast portions of coastal marsh have been blocked off with levees to control water levels. Within these impoundments, land managers can manipulate water levels to create optimal conditions for waterfowl. One such technique involves completely drying the marsh during the summer by draining or pumping water out of the impoundment. This encourages the growth of plants that produce high quality seeds and tubers that waterfowl love to eat. When waterfowl move in for the winter, managers re-flood the marsh, making it attractive to migrating birds in search of food. These seasonal drawdowns are excellent at creating highly productive marshes that provide abundant food for waterfowl. However, these same management efforts may be affecting the long-term health of the wetlands.
In the Chenier Plain, marshes maintain elevation through accretion of organic matter, adding height through root growth and accumulated plant litter. Flooded conditions slow down decomposition of the plant material, allowing it to be stored as soil carbon. However, during seasonal drawdowns, decomposition of plant material in the soil is accelerated. As decomposition increases, soil volume decreases and wetlands may lose elevation. Over decades of seasonal drawdowns, this practice may lead to dramatic elevation losses. As elevation declines, it becomes harder to manage water levels within impoundments, and marshes may eventually disappear entirely as plants become stressed and die off. When marshes lose elevation and water deepens, managers lose the ability to grow enough food for wintering waterfowl and their usefulness as wetland habitat diminishes. In coastal areas that support large wintering populations of waterfowl, elevation loss endangers crucial waterfowl habitat.
We are already beginning to see this phenomenon in marshes of the Chenier Plain that have been intensively managed for waterfowl over the past few decades. While we have an excellent understanding of how management practices influence food resources and waterfowl use of managed wetlands, little research has focused on understanding how these same management practices affect elevation. In the LSU AgCenter School of Renewable Natural Resources, researchers are working to change that.
At Rockefeller Wildlife Refuge in Grand Chenier, Louisiana, researchers from the School of Renewable Natural Resources are studying how management practices affected historic rates of accretion within coastal impoundments. Taking cores of soil from managed and unmanaged marshes, researchers use cesium-137 dating to determine the age of soil layers and infer whether management changes how much soil accretes over time. In a heavily managed, seasonally drawn down unit, historic accretion was significantly lower than in an unmanaged unit. However, this technique does not provide insight into exactly how management is changing accretion or elevation.
To better understand the specifics of how management changes accretion and elevation, researchers are also studying the processes that drive elevation change in impounded coastal marshes in Louisiana and Texas. Surface elevation tables are used to measure millimeter-level changes in elevation along with root growth, decomposition and other processes known to affect elevation. In a seasonally drawn marsh, researchers also assess how carbon gas flow from the soil changes as the marsh is dried out. As carbon gas flow is directly related to decomposition, this experiment may help explain why seasonally drawn down marshes accrete less over time.
While managing wetlands for waterfowl is crucial for producing enough food and habitat for the millions of waterfowl that visit the Chenier Plain each year, seasonal drawdowns are affecting elevation of coastal marshes. In a region that already experiences significant wetland loss and sea level rise, managing wetlands for elevation should be a primary concern. In a future without wetlands to manage, there can be no waterfowl productivity.
Ashley Booth is a doctoral student in the School of Renewable Natural Resources.
Acknowledgements: Booth is researching waterfowl habitat management along with Andy Nyman, professor, and Sammy King, associate professor, and another graduate student, Scott Graham, all in the School of Renewable Natural Resources.
(This article appears in the winter 2020 issue of Louisiana Agriculture.)
Ashley Booth and recent School of Renewable Natural Resources graduate Cameron Field (standing) evaluate millimeter-level changes in surface elevation using a surface-elevation table in the marsh at Rockefeller Wildlife Refuge. Photo by Karis Ritenour
Scott Graham walks neck-deep in water to a marsh field site at Rockefeller Wildlife Refuge. Photo by Ashley Booth
A seasonally drawn-down wetland impoundment at Rockefeller Wildlife Refuge alone hosts thousands of waterfowl each year but has a much lower accretion rate than nearby unmanaged areas. Photo by Ashley Booth