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Loess Plains

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About the Loess Plains

The Loess Plains are a region extending north-south from far western Kentucky, south through western Tennessee, and through much of Mississippi. Although unified by geology and soil type, this region has considerable differences in climate and land use over its length. Overall though, this region has been heavily altered and degraded in a variety of ways.

This region consists of gently rolling plains, with broad bottomlands and terraces along rivers and streams. Soils here are mostly formed on thick loess, fine-textured wind-blown sediments. Loess is thickest in the west; in the east, there are also some loose sands and gravels originating as coastal plain sediment, forming hills dissected by gullies. The bedrock, shale and sandstone, is deeply buried in most areas. Soils here are varied, but tend to be deep, fertile, and erosion-prone throughout much of the region. Although most of the soils here are well-drained, impermeable clay layers beneath the surface often impede drainage, which, combined with the low gradient of streams here, leads there to be abundant wetlands along the larger waterways, especially in the west.

The climate is humid and subtropical throughout, but with considerable variability over the length of the region. Winter lows in the south average as much as 10°F (5.5°C) warmer than in the north, and daytime highs in winter can average 13°F (7°C) warmer. Summer temperatures are more similar, but still average about 3-4°F (2°C) warmer in the south. The growing season varies from 185 days in the north to 230 in the south. Precipitation also follows a different seasonal pattern throughout the region: the north has wetter seasons peaking in late April and late November, and a drier season peaking in August, with late summers tending to be sunniest. In the south, however, the winter is consistently wetter, from December through March, and the remainder of the year has less rainfall, but summers are cloudier, and relative humidity is higher year-round, mitigating the effect of the drier season.

Originally this region mostly supported oak-hickory forest. There was also some expanses of bluestem prairie, and areas with mosaics of prairie and oak-hickory forest. Bottomlands also supported southern floodplain forest. The forest composition varied considerably over the length of this region, due to significant differences in climate. The oak hickory forest, on uplands, featured white oak (Quercus alba), mockernut hickory (Carya tomentosa), pignut hickory (Carya glabra), southern red oak (Quercus falcata), post oak (Quercus stellata), black oak (Quercus velutina), and northern red oak (Quercus rubra), and in the south, American beech (Fagus grandifolia) and tupelo (Nyssa sylvatica), and on a few sites, blackjack oak (Quercus marilandica). The south of this region also supported some oak-hickory-pine forest, with a similar mix of hardwoods along with significant amounts of shortleaf pine (Pinus echinata), and some loblolly pine (Pinus taeda).

In the north, bottomlands supported pin oak (Quercus palustris), American sweetgum (Liquidambar styraciflua), eastern cottonwood (Populus deltoides), tuliptree (Liriodendron tulipifera), white ash (Fraxinus americana), red maple (Acer rubrum), silver maple (Acer saccharinum), pecan (Carya illinoinensis), American sycamore (Platanus occidentalis), swamp chestnut oak (Quercus michauxii), swamp white oak (Quercus bicolor), bald cypress (Taxodium distichum), and tupelo. In the south of this region, the wettest sites with standing water supported, bald cypress and water tupelo (Nyssa aquatica), whereas sites with only periodic flooding supported overcup oak (Quercus lyrata), swamp chestnut oak, water oak (Quercus nigra), water hickory (Carya aquatica), red maple, and green ash (Fraxinus pennsylvanica).

Most of the forests here have been cleared, and this region is currently intensively farmed, with both cropland and pastureland. The region produces corn, soybeans, wheat, hay, livestock, tobacco, and poultry in the north, additionally grain sorghum especially in Tennessee, and in the south, soybeans, cotton, corn, hay, and beef cattle. In the south, there are also pine plantations. There is also gravel, sand, and clay mining. Streams in the north have been channelized and many have minimal to no riparian buffers; however in the south there has been less drainage, and more small lakes, ponds, and small wetlands remain. Throughout the region, the floodplains of larger rivers still have fairly extensive expanses of bottomland forest. Uplands in the east also have some oak forest. In the north, outside of bottomlands and dry uplands, the only remaining forest is in small, isolated and highly fragmented woodlots. The bluestem prairies in the north have been almost entirely cleared for agriculture. In the south, however, the land has been more severely degraded by soil loss and erosion, and much of the agriculture has been abandoned, leading to more wild forest cover.

This region is moderately populous, containing Memphis, TN, as well as the smaller city of Jackson, MS, and numerous smaller cities and towns. There is significant suburbanization around the larger cities.

There is some public protected land here, almost exclusively in the state of Tennessee. The largest area is Hatchie National Wildlife Refuge, which includes mostly bottomlands but some uplands as well; although this refuge only covers a limited length of the Hatchie river, almost the entire length of this river's floodplain in this region is forested. There are also numerous smaller wildlife refuges, including Obion River Wildlife Management Area, Big Cypress Tree State Park, Hop-In Wildlife Refuge, Three Rivers Wildlife Management Area, and Gooch Wildlife Management Area.

The western border of this region varies irregularly between different regions. Over much of its length it borders the Bluff Hills; where this region is absent, it directly borders the Northern Holocene Meander Belts, except in a small part of Mississippi where it instead borders the Northern Pleistocene Valley Trains. The far north of this region borders the Wabash-Ohio Bottomlands to the north, and the far south borders the Southern Rolling Plains to the south. In the north of this region, in Kentucky, it is bordered to the east by the Western Highland Rim. In most of Tennessee and northern Mississippi, it is bordered to the east by the Northern Hilly Gulf Coastal Plain; farther south it is bordered to the east by the Southern Hilly Gulf Coastal Plain, except in a small area where it borders the Jackson Prairie.

References

1. Woods, A.J., Omernik, J.M., Martin, W.H., Pond, G.J., Andrews, W.M., Call, S.M, Comstock, J.A., and Taylor, D.D. "Ecoregions of Kentucky (Poster)", U.S. Geological Survey, Reston, VA (2002) Web.

2. Griffith, G.E.; Omernik, J.M.; and Azevedo, S.H. "Ecoregions of Tennessee (Poster)", U.S. Geological Survey, Reston, VA (1998) Web.