Understory vegetation as an indicator for floodplain forest restoration in the Mississippi River Alluvial Valley, U.S.A.

In the Mississippi River Alluvial Valley (MAV), complete alteration of river-floodplain hydrology allowed for widespread
conversion of forested bottomlands to intensive agriculture, resulting in nearly 80% forest loss. Governmental programs have
attempted to restore forest habitat and functions within this altered landscape by the methods of tree planting (afforestation)
and local hydrologic enhancement on reclaimed croplands. Early assessments identified factors that influenced whether
planting plus tree colonization could establish an overstory community similar to natural bottomland forests. The extent
to which afforested sites develop typical understory vegetation has not been evaluated, yet understory composition may be
indicative of restored site conditions. As part of a broad study quantifying the ecosystem services gained from restoration
efforts, understory vegetation was compared between 37 afforested sites and 26 mature forest sites. Differences in vegetation
attributes for species growth forms, wetland indicator classes, and native status were tested with univariate analyses;
floristic composition data were analyzed by multivariate techniques. Understory vegetation of restoration sites was generally
hydrophytic, but species composition differed from that of mature bottomland forest because of young successional age and
differing responses of plant growth forms. Attribute and floristic variation among restoration sites was related to variation
in canopy development and local wetness conditions, which in turn reflected both intrinsic site features and outcomes of
restoration practices. Thus, understory vegetation is a useful indicator of functional progress in floodplain forest restoration.