Vegetation communities in floodplain systems have developed over hundreds of years as a function of soil type, topography, and hydrology. The type of vegetation growing on a particular floodplain will be dominated by trees or shrubs adapted to the environmental conditions of that floodplain. Hydroperiod is the most important local environmental condition determining composition, and the species found respond to elevation differences relative to the river's flooding regime. Typical floodplain forests begin at the natural levee where coarse-grained deposits result in quickly draining soils and continue as surface elevations decrease away from the river and become more poorly drained.
Structural characteristics of floodplain forests vary depending upon location (Table 1). Stem density and basal area are generally greater in the southeastern United States and the humid tropics than in arid areas, but in arid areas basal area can still exceed 50m2ha~\ Basal areas in floodplain forests tend to be as high as or higher than that of upland forests. Almost without exception, the number of tree species increases as flooding decreases. The greatest number of tree species occurs in wet, tropical floodplains such as the Amazon. The understory of floodplain forests is generally lower in density and species numbers, probably due to reduced light levels and the extended flooding conditions.
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