Physiography of the Lower Mississippi Valley Joint Venture:
The Lower Mississippi Valley Joint Venture (LMVJV) is a private, state, and federal partnership that consists of both the Mississippi Alluvial Valley (MAV) and West Gulf Coastal Plain/Ouachitas (WGCPO) Bird Conservation Regions (BCRs).
Mississippi Alluvial Valley
The Mississippi Alluvial Valley (MAV), also referred to as the Lower Mississippi Valley, Mississippi Alluvial Plain, or just “the Delta” historically was the largest forested wetland ecosystem in North America. This 22 million acre floodplain extends from the confluence of the Mississippi and Ohio Rivers at Cairo, Illinois, to the northern Gulf of Mexico, with a drainage basin encompassing 41% of the conterminous United States. The topographically complex floodplain features a mosaic of ridges, swales, meander belts and backswamps that supports a diverse and ecologically rich forested wetland ecosystem – one of the most productive in North America.
Small (<1 foot) changes in elevation in the MAV are associated with large shifts in hydrology, which in turn, strongly affect plant and animal community composition and structure. The forests and wetlands that formed on these alluvial soils and dominated the MAV prior to European colonization were sinks for sediments and nutrients, provided temporary storage of floodwaters, stored significant amounts of carbon in tree biomass and soils, and provided extensive wildlife habitat. However, the recent history of the MAV is one of agricultural expansion and change.
The rich alluvial soils of the forested floodplain proved to be a “gold mine” for the agrarian European settlers. Early clearing for agriculture focused on the higher landforms associated with braided stream terraces and the natural levees that were partially protected from the potentially devastating and relatively frequent flooding. Expansive federally sponsored flood control and drainage projects opened up new opportunities for agricultural development such that by the 1950s only 9 million acres of forested wetlands remained – confined primarily to the more poorly drained portions of the floodplain. However, continued flood control and drainage projects and high commodity prices over the next 30-35 years led to more than 4 million acres of the remaining forested wetlands being cleared, even though these lands were typically on poorly drained sites subject to regular flooding. By the early 1990's less than 25% of the MAV was forested, and most of this forest occurred on the unprotected side of the mainline Mississippi River levees or within the public land estate (e.g., federally-managed National Wildlife Refuges and state-managed Wildlife Management Areas).
Today, the MAV continues to support significant migratory bird habitats and populations and is home to many federally-listed fish, plant, invertebrate, and mammal species. Nearly 40% of North America’s waterfowl and 60% of all U.S. bird species migrate or winter in the MAV. The MAV was identified as a priority geography for waterfowl in the original North American Waterfowl Management Plan (1986), and the Lower Mississippi Valley Joint Venture partnership continues to improve waterfowl habitat conditions, as well as habitat for songbirds, shorebirds, and wading birds in this heavily degraded landscape.
West Gulf Coastal Plain/Ouachitas
The West Gulf Coastal Plain/Ouachitas (WGCPO) physiographic area occupies about 52 million acres in southeastern Arkansas, southwestern Oklahoma, western Louisiana, and eastern Texas. The area reaches from the Arkansas River Valley to the north in Arkansas and Oklahoma to the coastal prairies and marshes to the south in Louisiana and Texas, and from the Mississippi Alluvial Valley to the east in Arkansas and Louisiana to the Oaks and Prairies to the west in Oklahoma and Texas. The WGCPO lies within the Humid Southeast Region of the U. S. and comprises two subregions: all of the West Gulf Coastal Plain and the Ouachita Mountain portion of the Ozarks/Ouachitas.
The region is dominated by pine forests on the uplands, shortleaf (Pinus echinata) to the north and longleaf (P. echinata) and loblolly (P. taeda) to the south, and is dissected by numerous river systems characterized by forested wetlands, largely bottomland hardwood forests. Longleaf pine-bluestem savannahs formerly dominated the uplands in southeastern Texas and southwestern Louisiana, however these forests are much less common in today's landscape, comprising less than 3% of the land area of the WGCPO. Shortleaf pine mixed with oaks and hickories historically was the predominant forest type outside of the longleaf range. Today much of the shortleaf pines have been replaced by loblolly pines except in the Ouachitas and the drier areas to the west. Upland hardwood communities are intermixed with shortleaf pine in the Ouachitas and in drier (xeric) and sandier sites throughout the WGCPO. Shortleaf and longleaf pine communities and upland hardwoods typically are associated with sandy soils and are largely maintained by fire. Loblolly pines were formerly confined to flatwoods in the south and along moist (mesic) slopes in other areas, but now have largely replaced shortleaf and longleaf as plantations in most areas.
Outside of pine forests, the most extensive plant community type in the WGCPO is the mixed pine - hardwood that is often a successional type on lands previously occupied by other types. In more mesic areas and on slopes, hardwood species requiring greater amounts of moisture are dominant. Bottomland hardwood forests of various oak species, black gum, sweetgum, elms, and ash are found in stream and river bottoms. Swamps of cypress and/or tupelo are found in frequently to permanently flooded sites. Other wetlands dominated by herbaceous emergent and floating plants are occasionally found in some permanently flooded areas.
The Federally Endangered red-cockaded woodpecker is among the highest priority species in the WGCPO and occurs in open, park-like pine savannahs. Other high priority species that nest in this habitat type include Bachman's sparrow, northern bobwhite, and the brown-headed nuthatch. Pine savannahs are a conservation priority because of the numerous bird species supported in these habitats. These savannahs continue to be impacted by urban/suburban development, conversion to pasture, conversion to pine plantations, lack of thinning, and the lack of prescribed burning and/or suppression of naturally-caused fires.
Bottomland hardwood forests, cypress/tupelo swamps, and riparian habitats are distributed widely in association with the numerous rivers and tributaries within the WGCPO, and support priority species including Acadian flycater, Louisiana waterthrush, Swainson's, yellow-throated, and prothonotary warblers, and red-shouldered hawk. Bottomland forests also support substantial populations of several waterfowl species including the wood duck and mallard. The primary threats to these forests of high conservation priority include reservoir construction, stream modifications, destructive timber harvesting practices, and conversion to pine plantations, pastures, and other land uses.