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Louisiana

Mississippi River Alluvial Plain


Mississippi River Alluvial Plain

The largest ecoregion in Louisiana is the Mississippi River Alluvial Plain, which covers some 12,350 square miles in the state. This ecoregion occupies parts of seven states from southern Louisiana to southern Illinois. Within Louisiana, this region encompasses all lands in the historic Mississippi River flooplain.

Alluvial plains are lowland areas adjacent to major rivers, which receive periodic flooding and significant deposits of silt and clay material suspended in the flood waters. Bottomland hardwood forests and cypress swamps, also referred to as forested wetlands, are the dominant natural plant communities in this region. A key factor in the development and maintenance of these communities is their ability to survive extended periods of flooding.

Historic Condition

Historically, species-rich bottomland hardwood forest created an essentially unbroken expanse of some 24 million acres of forest along both sides of the Mississippi River. Minor variations in elevation within the floodplain had dramatic effects on local ecology. Elevational differences are reflected in frequency and duration of flooding, which in turn influenced the soil type present. In general, if you examine a cross-section of the landscape from the river to the adjacent uplands, there are five ecological zones recognized in southern alluvial plains. As you move from Zone 1, which includes permanently flooded areas such as rivers and lakes, to Zone V, which only rarely is exposed to flooding, plant community composition changes greatly. For example, Zone II, the first zone not subjected to permanent flooding, is dominated by bald cypress and water tupelo. In contrast, Zone IV, which is only flooded for a month or so each year, supports a tremendous variety of tree spices including hackberry, American elm, green ash, sweet gum, nuttall oak and water oak.

These floodplains contained a series of natural levees, meander scrolls, sloughs, and oxbow lakes in which different zones can occur practically anywhere in the landscape. Within each zone, natural disturbance from wind storms, disease, rare fires, etc., created a mosaic of age and structure classes, each preferred by a different assemblage of plant and animal species. Because of this structural and compositional diversity and the fact that they are wetlands, mature bottomland hardwood forests ranked among the most productive ecosystems in North America.

Current Condition

The forested wetlands of the Mississippi River alluvial plain is an imperiled ecosystem. Over the past two centuries the extent of bottomland hardwood forests in the region has decreased from 24 million acres to only 4.9 million acres. Of equal importance to the actual absolute loss of habitat is the change from an essentially unbroken forest in pre-settlement times to a landscape of some 40,000 distinct patches scattered throughout the floodplain. This high degree of fragmentation has had dramatic effects on many species including Louisiana black bear and migratory songbirds.

Remaining forests are also affected by modification of natural flooding and drying cycles. Construction of levees along the Mississippi and other major rivers in this ecoregion, along with periodic dredging and straightening of the channels blocked the critical flow of water, sediment, and nutrients to these wetlands. Recent data indicate that the loss of large, contiguous blocks of forest along the Mississippi River and river channel modification have been accompanied by the loss of species such as Bachman's Warbler, Ivory-billed Woodpecker, Florida panther and red wolf.

Biodiversity Significance

The aquatic and terrestrial habitats of the Mississippi Valley are extremely important habitat for countless species of plants and animals, many of which are found in greatest abundance within this ecoregion. The future of such well-known animals as Louisiana Black Bear depends upon successful conservation of the forested wetland ecosystem. Other species that are not widely recognized, such as freshwater mussels also depend upon protection and restoration of high-quality natural habitats. Recent data from surveys sponsored by TNC indicate that some of the rivers in the Louisiana portion of the ecoregion may support more species and greater overall densities of fish and mussels than any other region in the Southeast.

The Louisiana black bear, Interior Least Tern, and pallid sturgeon are now listed as threatened or endangered and over 70 species of Neotropical migrant songbirds (which are declining significantly as a group) are found in this ecoregion. Some of the species of Neotropical migrants that are of most concern to bird conservationists are found in this region, including Swainson's Warbler, Prothonotary Warbler, American Swallow-tailed Kite and Wood Thrush.

Conservation Efforts

Successful conservation work by The Nature Conservancy has always included a sincere effort to find common ground to ensure that mutual goals are achieved. The enormity of the task of restoring the Mississippi Valley ecosystem emphasizes the importance of partnerships with a wide spectrum of state and federal agencies as well as with private landowners.

The Conservancy recently completed the ecoregional conservation plan for the Mississippi Valley, which will form the blueprint for our conservation work in this ecoregion in the decade ahead. The plan focused on the conservation needs of black bear, migratory song birds, migratory waterfowl, shorebirds and freshwater communities (e.g., fish and mussels). This plan will further our conservation efforts in the valley, which have run the gamut from acquisition of lands to create national wildlife refuges (e.g., Cat Island), to the acquisition of preserves that will serve as platforms for our conservation and restoration work in the region (e.g., Cypress Island Preserve), to development of the Black Bear Conservation Committee. The Black Bear Conservation Committee has won numerous national awards for their nonconfrontational, inclusive style of resolving a potentially contentious problem--the listing of the Louisiana Black bear as federally threatened.

In total, The Nature Conservancy has been instrumental in the permanent protection of nearly 45,000 acres in this ecoregion, most of which is now available to the general public for hunting, fishing and other forms of outdoor recreation. Our efforts have led to the establishment of four new national wildlife refuges, two Conservancy preserves (Frederick's Swamp and Cypress Island), and the acquisition of Bluebonnet Swamp Nature Center in Baton Rouge.

Opportunities for Public Visitation

There are numerous opportunities for the public to visit state wildlife management areas and federal refuges within this ecoregion. Clearly, the Conservancy's Cypress Island Preserve is one of the premier destination for those interested in experiencing the swamps and hardwood forests of the Mississippi Valley. Developed hiking trails, numerous alligators, and one of the nations largest waterbird nesting colonies make this a "don't miss" destination.

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The Nature Conservancy and Disney have collaborated to offset carbon emissions, improve wildlife habitat, and enhance freshwater quality by reforesting marginally productive agricultural lands in the Lower Mississippi River Valley.

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