Lower Mississippi River Flatwoods

This system encompasses forests, prairies and woodlands on Pleistocene terraces in the Mississippi Alluvial Plain of Arkansas, Missouri and Louisiana. It occurs primarily west of Crowley's Ridge on Pleistocene glacial outwash deposits in Arkansas and Missouri, and on Macon Ridge in Louisiana and adjacent Arkansas. The sites are above modern floodplains, but have poor internal drainage and are flat with poor runoff, leading to very wet conditions in winter and spring. They also often have a claypan that restricts both internal drainage and, later in the year, water availability. Therefore, they are very wet in the winter/spring and very dry in the summer, a moisture regime termed hydroxeric. Because of this moisture regime, the communities are variable, ranging from willow oak flats to post oak flats to prairies. In the 1940s, the Arkansas Game and Fish Commission produced a wildlife habitat map of Arkansas in which these sites were classified as "terrace hardwood forests." These communities have a large variety of upland and lowland tree species, ranging from post oak to overcup oak in a small area. Such species diversity may be explained by regeneration of species with dramatically different moisture tolerances on the same site in dry and wet years on these hydroxeric sites. Because the sites are above current floodplains and susceptible to being drained, they have been cleared at an even greater rate than nearby floodplain forests.

The communities of this system are variable, ranging from willow oak flats to post oak flats to prairies. In examples on Macon Ridge (Louisiana), overstory dominants include Carya tomentosa (= Carya alba), Nyssa sylvatica, Quercus alba, Quercus pagoda, Quercus nigra, Quercus michauxii, and Liquidambar styraciflua. In addition, Quercus shumardii and Quercus falcata are fairly frequent but not usually abundant. Common midstory trees include Cornus florida, Ostrya virginiana, Aralia spinosa, Ulmus alata, Sassafras albidum, and Acer rubrum. Important shrubs/small trees are Vaccinium arboreum, Vaccinium virgatum, Viburnum rufidulum, Crataegus marshallii, Aesculus pavia, Frangula caroliniana, Asimina triloba, Hypericum hypericoides, and Euonymus americanus. Although infrequent, Hamamelis virginiana can be locally abundant. Important woody vines include Toxicodendron radicans, Parthenocissus quinquefolia, Vitis rotundifolia, Vitis aestivalis, and Smilax smallii. Toxicodendron radicans and Parthenocissus quinquefolia are usually thick on the ground, as well as being represented by high climbing individuals. Common and characteristic herbaceous plants include Chasmanthium sessiliflorum, Dichanthelium boscii, Podophyllum peltatum, Carex cherokeensis, Elephantopus carolinianus, Elephantopus tomentosus, Scleria oligantha, Aristolochia serpentaria, Botrychium virginianum, Passiflora lutea, Dioscorea villosa, Clitoria mariana, Sanicula canadensis, Geum canadense, Galium circaezans, Agrimonia rostellata, Spigelia marilandica, Clematis virginiana, Phryma leptostachya, Ruellia caroliniensis, and Smallanthus uvedalius (LNHP 2004).

The sites where this system is found are above modern floodplains, but have poor internal drainage and are flat with poor runoff, leading to very wet conditions in winter and spring. They also often have a claypan that restricts both internal drainage and, later in the year, water availability. Therefore, they are very wet in the winter/spring and very dry in the summer, a moisture regime termed hydroxeric. In Louisiana, distinct mesic and wet community variants are recognized (LNHP 2004, 2009).

Threats include clearing, grazing pressure, invasion by woody plants, conversion to exotic cool-season grasses and lack of fire (Nelson 2005). Loss of habitat and fragmentation of the remaining flatwoods habitat have been pronounced. The small size of remaining flatwoods areas contributes to the lack of fire and invasion by invasive exotic plants from seed sources in surrounding ruderal habitat areas.

This system is found in the Mississippi Alluvial Plain from the Missouri "bootheel" south to Louisiana. It occurs primarily west of Crowley's Ridge on Pleistocene glacial outwash deposits in Arkansas and Missouri. In southeastern Arkansas and northeastern Louisiana it is found on Macon Ridge (Ecoregion 73j (EPA 2004, LNHP 2009)). It is not reported from Kentucky, Tennessee, or Mississippi.