Environmental Description:  This willow oak forest occurs in bottomland flatwoods environments of major rivers in the south-central United States. This type most often occurs on first bottoms and terrace flats with poorly drained, heavy silty clay soils that are flooded with shallow water each winter and spring. The relatively high clay content (sometimes forming a fragipan) in these soils produces a perched water table that contributes to prolonged wet conditions and equally severe summer dry periods when soils become hard and develop excessive cracks. This restricted soil permeability is characteristic of flatwoods, which contain species typical of drier upland sites on slight rises and shallow ephemeral seasonal ponds in depressions. Some examples (e.g., in Kentucky) are not in bottomlands but are in distinctly upland positions, but otherwise fit the description (M. Evans pers. comm.).

Soils include dominantly level or nearly level soils that formed in water-laid clayey or loamy sediments on the floodplains of the Mississippi and Ohio rivers. Although flatwoods are best developed on soils with massive, brittle fragipans, they are also well represented on relatively impermeable clay soils. In the Midwest, fragipans (hardpan) are usually developed from glacial till of Illinoisan age (White and Madany 1978). This glacial material forms a loamy, brittle subsurface horizon that is low in porosity and organic matter, low or moderate in clay, but high in silt or very fine sand. A fragipan appears cemented and restricts root growth and water flow. Soils high in clay also exhibit low porosity and are slowly to very slowly permeable. Clays also display an affinity for water, which makes it unavailable to plants when soils begin to dry. Clay soils have a high shrink-swell capacity and crack excessively as they dry. During the rainy season, water slowly permeates these soils and ponds in depressions. The soil dries rapidly in the summer or during drought, and herbaceous vegetation wilts in response to this stress. The penetration of tree roots in the subsoil and burrowing by animals is limited by the hardpan (Nelson 1985, White and Madany 1978). Order: Entisol, Mollisol; Subgroup: Typic Fluvaquents, Vertic Haplaquent, Fluvaquentic Halplaquolls; Family: fine-silty, montmorillonitic-mixed, non-acidic, mesic, udic; Series (IL): Belknap, Bonnie, Cape. Bedrock is deeply buried Paleozoic sandstone or stratified Cenozoic marine deposits.

Ponding in shallow depressions encourages the growth of hydrophytic vegetation, while droughty conditions encourage the growth of vegetation typical of drier uplands on slight ridges. Shallow flooding is common, occurring seasonally (winter and spring) for short durations. Tree species in this type may not tolerate deep flooding (Robertson et al. 1984).