Forested wetlands differ from true swamps in that they lack continuously standing water, although repeated flooding is common.  Differences in the length of inundation give rise to a variety of community types within this classification.  For example, large timbered areas bordering swamps or rivers with frequent flooding (PF01C) often have a poorly developed, very open understory.  Silver maple, sycamore, and cottonwood are common, and the forest floor is littered with rotting logs and woody debris deposited by flood water. In contrast, drier areas (PF01A), where flooding is not as prolonged, have a greater diversity of plant species, with oaks, elms, and hickory common in the canopy.  The forest floor in these areas is often covered by a variety of annual and/or perennial plants. The soils in this community consist of silty and clayey alluvial materials.  They are subject to annual flooding with brief to long duration.  Reaction of the materials ranges from strongly to slightly acid.  The major soils include Bonnie silt loam, Birds silt loam, Piopolis silt loam, Petrolia silty clay loam, Karnak silty clay, and Sharon silt loam. *Used throughout the community descriptions to indicate the most commonly occurring code in Illinois