Bringing Bottomland Forests Back to the Mississippi Alluvial Valley

Understory plants as an indicator of restoration progress

A Mississippi Alluvial Valley WRP site, with planted bottomland hardwood trees adjacent to a moist-soil area. Photo courtesy of U.S. Geological Survey, National Wetlands Research Center.
A Mississippi Alluvial Valley WRP site, with planted bottomland hardwood trees adjacent to a moist-soil area. Photo courtesy of U.S. Geological Survey, National Wetlands Research Center.

The vast Mississippi Alluvial Valley (MAV) that stretches along the Mississippi River from southern Illinois to the Gulf of Mexico once supported 24 million acres of bottomland and wetland forest — rich stands of oak, gum, ash, hickory, baldcypress, and water tupelo.

The hydrology of the original floodplain was drastically altered by flood-control levees built to support intensive agriculture across the region, which reduced forest extent by nearly 80 percent and resulted in the loss of valuable wildlife habitat and other ecosystem services.

Over 20 years ago, U.S. Department of Agriculture (USDA) programs such as the Wetlands Reserve Program (WRP) started offering incentives to private landowners in the MAV to restore marginal cropland to bottomland hardwood forest, primarily by planting trees in protective easements. In the beginning, the success of these afforestation efforts was evaluated in terms of forest structure — tree density, growth, and species composition. Because the WRP also establishes managed moist-soil areas as a means of hydrology restoration on the easements, the focus has broadened to include recovery of ecological functions, prompting a need for additional ways to measure the success of afforestation.

As part of the USDA Natural Resource Conservation Service’s Conservation Effects Assessment Project, U.S. Forest Service and Geological Survey scientists recently published findings from a study that compared afforested WRP sites and mature bottomland forest sites to determine if understory vegetation could serve as an indicator for MAV floodplain forest restoration success.

“The plants that grow in the understory of bottomland hardwood forests are sensitive to water and soil site conditions, so the variation in understory plant composition could be a useful indicator of ecological success on different sites,” said Diane De Steven, research ecologist for the Forest Service Southern Research Station Center for Bottomland Hardwoods Research, and lead author of the article.

Trees can be planted to achieve desired forest structure, but the herbs, shrubs, and vines that make up the understory must come from seed banks or dispersal of seeds from nearby forests, which are now scarce in the MAV. The researchers wanted to know if those passive mechanisms would establish an understory flora similar to that of natural bottomland hardwood forests. They surveyed 37 WRP restoration sites and compared them to 26 mature bottomland hardwood sites. Both groups of sites were distributed across three states in the MAV and were similar in terms of landscape position, soils, and elevation.

“The WRP seeks to recover multiple ecological services that include more natural hydrologic function, so we used both species composition and functional vegetation attributes to assess the developing understories of afforested sites,” said De Steven. “Our findings indicated that the understory vegetation reflected several factors that influence restoration progress.”

The analyses found that the understories of WRP and bottomland forest sites differed substantially in species and growth-form composition, largely due to greater prevalence of herb species in young  afforested sites compared to mature forests. All bottomland hardwood sites and most WRP were populated with a majority of hydrophytic  species (plants that are associated with wet conditions), qualifying the understory vegetation as meeting “wetland” criteria.

The study also found considerable variation among WRP sites. Distance to nearby forest and overstory tree density were important determinants of understory development, as woody species were more prevalent relative to herbs when trees were denser and sites were closer to forest. Site wetness conditions were also important, as WRP areas near enhanced-hydrology features (either managed moist-soil areas or natural wet swales) had relatively more wetland species, which has implications for restoration practice. Where WRP sites lack the majority of hydrophytic species, wetness conditions may be inadequate for restoration.

“Given that recovering the historic flooding regime of the MAV as a whole is impractical, local hydrology restoration is critical for improving habitats and functional conditions,” said De Steven. “As indicators of hydrologic status, the relative prevalence of hydrophytic and wetland species can suggest whether a restored site has the potential to support the functional processes associated with ecological services such as reducing nutrient run-off to streams and sequestering carbon in the soil.”

Access the full text of the article.

For more information, email Diane De Steven at ddsteven@fs.fed.us.

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