This study was conducted at three locations in a bottomland hardwood forest with a distinct elevation and hydrological gradient: ridge (high, dry), transition, and swamp (low, wet). At each location, concentrations of soil greenhouse gases (N 2 O, CH 4 , and CO 2 ), their fluxes to the atmosphere, and soil redox potential (Eh) were measured bimonthly, while the water table was monitored every day. Results show that soil Eh was significantly (Po0.001) correlated with water table: a negative correlation at the ridge and transition locations, but a positive correlation at the permanently flooded swamp location. Both soil gas profile analysis and surface gas flux measurements indicated that the ridge and transition locations could be a sink of atmospheric CH 4 , especially in warm seasons, but generally functioned as a minor source of CH 4 in cool seasons. The swamp location was a major source of CH 4 , and the emission rate was higher in the warm seasons (mean 28 and median 23 mg m À2 h À1 ) than in the cool seasons (both mean and median 13 mg m À2 h À1 ). Average CO 2 emission rate was 251, 380 and 52 mg m À2 h À1 for the ridge, transition and swamp location, respectively. At each location, higher CO 2 emission rates were also found in the warm seasons. The lowest CO 2 emission rate was found at the swamp location, where soil C content was the highest, due to less microbial biomass, less CO 2 production in such an anaerobic environment, and greater difficulty of CO 2 diffusion to the atmosphere. Cumulative global warming potential emission from these three greenhouse gases was in an order of swamp4transition4ridge location. The ratio CO 2 /CH 4 production in soil is a critical factor for evaluating the overall benefit of soil C sequestration, which can be greatly offset by CH 4 production and emission.

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.• On former agricultural floodplains, some floristic differences between afforested sites and mature bottomland forests are a natural result of differing successional age. However, species-composition attributes of understory vegetation offer useful indicators of restoration progress. • Relative growth form composition (herbaceous vs. woody) reflects the role of tree-planting practices in establishing a forest overstory. Functional-group composition (based on wetland-fidelity rankings) can be an important indicator of hydrologic conditions and practices. • Active afforestation can restore forest habitat structure, but restoring local hydrology is key to enhancing functional ecosystem services such as nutrient retention and carbon sequestration.

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