By Tom Moorman, Ph.D.
Ten thousand years ago, planet earth was undergoing yet another swing in a continuous series of climate changes. This time, a period of warming was under way that spelled the end of the most recent Ice Age. So what? you say. What has this to do with bottomland forested wetlands?
Well, if you are a waterfowler, and have an interest in the natural history of waterfowl and their habitats, then you have benefited in more ways than one from this period of climate change. Not only did the retreating glaciers shape the northern Great Plains, leaving behind countless prairie potholes of the Duck Factory, but as all that ice melted, much of the runoff rushed southward, forming a vast river that gave birth to what Native Americans called "Mississippi," which, legend has it, translates to Father of Waters.
That's an appropriate moniker, as the glacial runoff gave birth to an enormous floodplain that we know today as the Mississippi Alluvial Valley (MAV). Over several thousand years, as the period of warming continued and the glaciers retreated, the volume of runoff declined, and an intricate secondary network of rivers and swamps developed on the face of this roughly 24-million-acre floodplain. Ultimately, the soils, climate, flooding regime, and other factors combined to shape one of the most unique forested wetland systems in the world, called by some the Amazon of North America, and known regionally as the Delta.
This by-product of the last Ice Age became the most significant wintering area for mallards in North America, with upwards of 40 percent of the mid-continent population settling in the region's network of sloughs, cypress-tupelo brakes, and, in some years, flooded oak flats. The key attraction for mallards was that at least one or more of the river systems in the MAV typically flooded annually, providing predictable habitat in the unpredictable world of wetlands.
Such annual winter floods created, in all but extreme drought years, hundreds of thousands of acres of flooded forested wetlands. These areas typically provided abundant, high-energy food, particularly acorns from several species of oak trees uniquely adapted to this system, including pin, water, willow, and Nuttall oaks. Acorns are loaded with carbohydrates, and because ducks don't have to concern themselves with the Atkins Diet, or any of the other diet plans on the market these days, acorns are a food that ducks can readily convert to fat. For ducks, fat is the substance upon which their very survival depends.
Research on wintering mallards and other species suggests that heavier (and hence likely fatter) waterfowl have higher overwinter survival rates. Research over the past two decades suggests that when winter habitat conditions are drier than average, survival rates are lower for mallards and other waterfowl, possibly because they are leaner, forced to become more mobile in search of food, and delay completion of important annual cycle events such as molting or forming pair bonds. So, from the ducks' perspective, their winter survival is dependent upon finding adequate habitat with adequate food that allows them to maintain a fat reserve to get them through nature's winter weather tantrums of ice and snow.
A "healthy" mallard typically maintains fat reserves that can provide energy for up to seven days. If they are able to eat enough to meet daily energy requirements and not dip into their fat reserves, mallards have a built-in emergency energy supply should snow and ice temporarily render feeding areas unavailable. In this case, the birds may elect to burn fat reserves for energy and wait out the inclement weather by reducing activity and related energy expenditure.
Forested wetlands such as the bottomland hardwood wetlands of the MAV are ideal habitat in this instance, in that they provide many places where the birds can loaf and feed out of the wind, and conserve critical fat reserves. Green-timber hunters know that timber hunting is best on clear mornings accompanied by cold north winds. Under those conditions, mallards predictably seek out forested wetland habitat to gain a thermal advantage, or put more simply, they seek out a windbreak just as any of us would do given a choice between freezing in a penetrating, cold winter wind, or slipping behind a row of trees to avoid the wind and create a slightly warmer environment.
Such winter weather conditions develop in the Deep South after passage of cold fronts, which, during active winter weather patterns, can occur every three to four days. The lower temperatures and higher wind speeds that occur post-front are a cue for mallards to seek out habitats where they can minimize energy use related to weather exposure.
The other unique feature that forested wetlands provide for wintering mallards is pair isolation habitat. Mallards and a few other species of ducks form pair bonds in fall and winter, so that, by January, most females have chosen a mate. As with most species of ducks, there tends to be a surplus of males in the population. Hence, unpaired males continue to seek mates aggressively, so much so that they may interrupt the daily activity pattern of females, perhaps limiting the time females can spend feeding. This, in turn, may cause females to be leaner, and we have already seen what the implications are for skinny ducks in winter. So, there is strong pressure for females to pair early in winter as it allows them to feed more and maintain larger fat reserves because the male defends the female from unpaired males.
To further reduce interference from unpaired males, pairs often seek out pair-isolation habitat. Wetlands with lots of cover offering small, especially remote areas where pairs can be isolated from unpaired males allow the pairs to maximize time spent feeding or resting to build or conserve fat reserves for periods of inclement weather or future migration activities.
Thus, mallards have evolved to use forested wetlands in winter for food, for refuge from post-frontal winds and cold temperatures, and as places to seek refuge from unpaired birds that may interfere with daily activities. All of these factors affect the delicate energy balance the birds have related to the maintenance of fat reserves, which has implications for overwinter survival rates.
Imagine the thoughts of Native Americans, or the first explorers or settlers of European descent, as they penetrated the vast forests, cane breaks, and wetlands of the MAV. One conjures up images of giant oak and cypress trees, some supporting vines as thick as a man's leg, their canopy creating a heavily shaded forest interior. Panthers, black bears, and red wolves wandered the forest, while alligators and beavers were abundant in wetlands. The prehistoric-sounding call of the now likely extinct ivory-billed woodpecker resounded from deep within the heart of the forest. It must have been a magical place. However, the mindset of the time was to tame these lands, and make such "waste areas" productive and useful to humans. It didn't take long for the settlers to discover the source of the MAV's bounty—its deep, rich alluvial soils.
The growing human population in the East needed food and fiber, and in the early to mid-1800s clearing of the higher elevation lands in the MAV commenced. Higher elevation is a relative term, and in the MAV, changes of a few inches or a foot often determined the flooding regime and associated vegetation of any given site. These soils were ideal, and still are, for the production of cotton. However, the Mississippi River and its tributaries periodically attempted to reclaim these lands via extensive floods. During the late 1800s, levee systems were built, and thereafter frequently raised and expanded, to control or preclude flooding of homes and farmlands. As man gained more and more control of the floods, more and more forest was cleared from lower elevation sites, sites that produce bountiful crops of rice and soybeans today. Much of this flood control and clearing was subsidized by government agencies, the work done to improve the human condition in the region.
However, as is almost always the case, such changes had costs. Extensive levees and drainage systems resulted in the clearing of an estimated 80 percent of the MAV's bottomland forest. While winter flooding still occurs, the area affected by natural flooding has been substantially reduced. Further, the timing of the floods has shifted toward late winter through spring, occasionally into early summer. Finally, the duration of flooding has changed, with some higher elevation sites that formerly were important waterfowl habitat now flooding infrequently for short durations, while some low elevation sites that offer less valuable waterfowl habitat flood later and longer into the growing season. Growing season flooding is detrimental to the survival of existing forests, and is particularly detrimental to seedling survival. The future of some remaining forested wetlands, and the future of some key waterfowl and wildlife habitat, remains at risk.
Fortunately, from a duck's perspective, all is not lost. Ducks Unlimited and its many conservation partners have worked diligently over the last decade to assure that the MAV continues to provide adequate feeding habitat, so that birds can maintain their fat reserves and maximize overwinter survival and the number of birds that return north to breed each spring. Most of this work has been in cooperation with rice farmers and other private landowners. Indeed, approximately 275,000 acres of land are enrolled in DU and partner private lands conservation programs, of which about 110,000 acres are flooded in most years. Harvested rice fields flooded in fall and winter provide waste rice and weed seeds as food for wintering waterfowl. Mallards and other puddle ducks, in particular, make extensive use of this food resource in the MAV.
DU and its partners have been so successful at working with MAV landowners to provide habitat that some waterfowl hunters in the Deep South recently have wrongly associated this habitat with shortstopping, or an effort to scatter birds to make them less vulnerable to hunters. Nothing could be further from the truth. The fact is, this habitat likely represents a fraction of what historically existed in the MAV, and it serves only to provide the birds with options to seek and find habitat in the Mississippi Valley during winter as they have since the end of the last Ice Age.
Further, this habitat has been developed under a carefully conceived effort under the North American Waterfowl Management Plan's Lower Mississippi Valley Joint Venture. "Effective waterfowl conservation can only be achieved by working in cooperation with private landowners, because they own more than 70 percent of the remaining wetlands. We're very pleased to be one of many partners leading restoration and enhancement efforts under the auspices of the Lower Mississippi Valley Joint Venture. Our work has been carefully planned, and we annually evaluate our progress relative to planned objectives so that we know we are meeting the needs of the birds that historically wintered in the MAV," says Dr. Curtis Hopkins, director of conservation at DU's Southern Regional Office.
Some important bottomland hardwood forested wetland areas remain in the MAV. Some of these are in the public domain, such as White River National Wildlife Refuge in Arkansas, or Delta National Forest in Mississippi. Others remain in private ownership. Ducks Unlimited works to protect privately held tracts by accepting conservation easements from landowners. In exchange for giving up certain development rights and agreeing to maintain the land as forested wetland habitat, these landowners receive tax relief from the federal government while at the same time retaining land ownership, including use of the land for hunting. Such easements provide protection to important forested wetlands in perpetuity.
Another important aspect of DU's work in the MAV is restoration of forested wetlands. DU has worked in partnership with the U.S. Department of Agriculture's Natural Resources Conservation Service's (NRCS) Wetland Reserve Program (WRP), a federally funded program that is part of the Farm Bill. WRP pays landowners to retire marginal, flood-prone farmland. Landowners typically provide a perpetual easement to the NRCS in exchange for a one-time easement payment and payment of restoration costs. WRP enhances the land's value as waterfowl habitat, and, by extension, provides the landowner with improved duck hunting opportunities.
Under WRP, restoration activities include reforestation with a suite of bottomland hardwood tree species, particularly heavy-seeded species such as cypress and Nuttall, water, willow, and overcup oak. The species mix for each site is determined by careful consideration of the site's elevation, the degree to which it floods, and other factors that influence which species are suited to the area. DU's role in WRP is related to reforestation and restoration of hydrology. DU's biologists work with the NRCS to complete tree-planting activities, and DU's engineers design levees and water-control structures to restore wetlands on the sites. "Ducks Unlimited's expertise in wetland restoration has been instrumental in helping the NRCS make WRP a successful program and a highly sought Farm Bill program in the MAV" says Don Gohmert, NRCS state conservationist in Louisiana.
WRP is offered nationwide, but its greatest impact has been in the MAV, where DU has partnered with the NRCS and private landowners to reforest approximately 160,000 acres over the last 10 years. In addition, more than 34,500 acres of emergent wetlands have been restored through re-establishment of hydrology. Taken together, WRP in the states of Louisiana, Arkansas, and Mississippi alone constitute the single largest bottomland forested wetland restoration effort anywhere in the world. "The Wetland Reserve Program is a vitally important wetland restoration program that enables restoration of forested wetlands of value to waterfowl, other wildlife, and people at a scale that is truly meaningful. The forested wetlands restored under WRP will provide not only excellent forested wetlands for mallards and wood ducks, but will also provide a host of other functions important to people, including recreation, flood control, enhancements to water quality, and other ecological functions," says Ken Babcock, director of DU's Southern Regional Office.
The Mississippi Alluvial Valley, a product of the last Ice Age, the premier mallard wintering area in North America, and one of Ducks Unlimited's highest conservation priority areas on the continent. As the old saying goes, it's important to see the forest for more than the trees.