Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.
Major Land Resource Area (MLRA): 134X–Southern Mississippi Valley Loess
The Southern Mississippi Valley Loess (MLRA 134) extends some 500 miles from the southern tip of Illinois to southern Louisiana. This MLRA occurs in Mississippi (39 percent), Tennessee (23 percent), Louisiana (15 percent), Arkansas (11 percent), Kentucky (9 percent), Missouri (2 percent), and Illinois (1 percent). It makes up about 26,520 square miles. Landscapes consist of highly dissected uplands, level to undulating plains, and broad terraces that are covered with a mantle of loess. The soils, mainly Alfisols, formed in the loess mantle. Stream systems of the MLRA typically originate as low-gradient drainageways in the upper reaches that broaden rapidly downstream to wide, level floodplains with highly meandering channels. Alluvial soils are predominantly silty where loess thickness of the uplands are deepest but grade to loamy textures in watersheds covered by thin loess. Underlying the loess mantle are Tertiary deposits of unconsolidated sand, silt, clay, gravel, and lignite. Crowley’s Ridge, Macon Ridge, and Lafayette Loess Plains are discontinuous, erosional remnants that run north to south in southeastern Missouri - eastern Arkansas, northeastern Louisiana, and south-central Louisiana, respectively. Elevations range from around 100 feet on terraces in southern Louisiana to over 600 feet on uplands in western Kentucky. The steep, dissected uplands are mainly in hardwood forests while less sloping areas are used for crop, pasture, and forage production (USDA, 2006).
This site is of large extent in MLRA 134 with a maximum distribution and occurrence in the Loess Plains (EPA Level IV Ecoregion: 74b) from western Kentucky to the Southern Rolling Plains (EPA Level IV Ecoregion: 74c) in southwestern Mississippi. The site extends into the adjoining Southern Coastal Plain, MLRA 133A to the east.
All or portions of the geographic range of this site falls within a number of ecological/land classifications including:
-NRCS Major Land Resource Area (MLRA) 134 – Southern Mississippi Valley Loess (USDA-NRCS, 2006)
-Environmental Protection Agency’s Level IV Ecoregion: Loess Plains, 74b (Griffith et al., 1998; Woods et al., 2002; Chapman et al., 2004)
-231H - Coastal Plains-Loess section of the USDA Forest Service Ecological Subregion (McNab et al., 2005)
-LANDFIRE Biophysical Setting 4514730 Gulf and Atlantic Coastal Plain Floodplain Systems (LANDFIRE, 2008)
-East Gulf Coastal Plain Small Stream and River Floodplain Forest CES203.559 (NatureServe, 2012)
-East Gulf Coastal Plain Large River Floodplain Forest CES203.489 (NatureServe, 2012)
-Low-gradient Riverine Wetlands (Wilder and Roberts, 2002)
-Western Mesophytic Forest Region - Mississippi Embayment Section (Braun, 1950)
Ecological site concept
The Northern Moderately Wet Alluvial Flat is characterized by very deep, somewhat poorly drained soils that formed in silty and loamy alluvium. Soil reactions range from very strongly acid to moderately acid. This site occurs on level to nearly level alluvial flats and within localized, shallow depressions that are often pocked throughout the better drained areas of narrow to broad floodplains. It may also be viewed as a transitional zone between landforms and positions with better drainage, such as natural levees, and the wet, low-lying flats and backswamps. On narrower floodplains, this site is positioned at slightly lower elevations landward of the stream bank. Slopes are commonly less than 1 percent but range from 0 to 3 percent. Flooding ranges from rare to frequent during winter and spring, and duration is brief to very long depending on stream and drainage basin size and flood magnitude. Outside of overbank flooding, a seasonal high water table generally occurs between 1 and 2 feet during wet seasons. Vegetation associated with this site exhibits signs of increasing soil wetness compared to the better drained sites. Species requiring drier conditions begin dropping out and are replaced by plants more tolerant of oxygen deficits. Overstory components generally include swamp chestnut oak, water oak, willow oak, overcup oak, Nuttall oak, pin oak, American sycamore, sweetgum, red maple, hickory, green ash, American elm, and ironwood. Loblolly pine is an additional component farther south. Understory density is typically lower than adjoining better drained flats, levees, and rises. Where ground cover is present, a mixture of facultative wetland and wetland obligate species frequently occur.
Northern Alluvial Flat - PROVISIONAL
This site occupies the drier landforms and positions within the floodplain environment (e.g., natural levees, rises, well-drained to moderately well drained flats).
Northern Wet Alluvial Flat - PROVISIONAL
Other than backswamps, this site is representative of the wettest portion of the floodplain environment.
Southern Rolling Plains Loess Drainways - PROVISIONAL
This site is the southern counterpart to the Northern Moderately Wet Alluvial Flat provisional ecological site.
Baton Rouge Terrace Southern Loess Drainageway - PROVISIONAL
This site may represent a southern counterpart to the Northern Moderately Wet Alluvial Flat.
West Central Somewhat Poorly Drained Flats - PROVISIONAL
This site is a southwestern counterpart, occurring within the Macon Ridge physiographic subsection.
Western Moderately Wet Alluvial Flat - PROVISIONAL
This site is the western counterpart, which occurs mainly along streams and alluvial fans associated with Crowley's Ridge.
Table 1. Dominant plant species
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