Potential Natural Vegetation of the Mississippi Alluvial Valley

Over the past three decades, extensive field studies have been conducted to characterize the composition and distribution of plant communities in the Mississippi Alluvial Valley. These studies have been carried out for various purposes under the auspices of federal and state research programs or in conjunction with Corps of Engineers project planning efforts. In the process, a wetland site classification approach has evolved based on hydrology, soils, and geomorphic setting. The research data and classification system have been recently used to create a set of Potential Natural Vegetation (PNV) maps covering more than 26,000 square miles within the region. The purpose of PNV maps is to help guide restoration planning and prioritization. Because the hydrology of the landscape has been permanently changed by major flood control projects, the PNV maps do not represent the distribution of the original, pre-settlement vegetation. Rather, they identify the natural communities that are appropriate to the modern altered hydrologic conditions. The PNV maps are available for use in a Geographic Information System, where a range of complex restoration scenarios (such as the development of wildlife travel corridors or refuge areas) can be explored efficiently, and alternatives can be compared in terms of costs and ecological effectiveness.
PNV maps have been assembled for each of 7 sub-basins in the Mississippi Alluvial Valley, providing coverage of all of the region between the northern border of the Arkansas Delta and the Red River in Louisiana.  They were developed using existing soils and geomorphology maps, hydrologic maps that generally reflect flood frequency, and a variety of other spatial data that helped define the distribution of the PNV communities in the modern landscape.  The sources of these data sets varied among the basins, and the mapping criteria were adjusted to compensate as necessary in order to maintain consistency in the site classification, community classification, and levels of detail across all basins. While these maps are suitable for planning and landscape analysis purposes as presented here, they should be considered critically when used in designing restoration projects and users should be prepared to adjust community boundaries as dictated by conditions observed in the field.
Each of the basin maps is accompanied by a table indicating the typical site features and species composition that should be restored in each community type.  The dominant species designated in the map legend are generalized over the entire area and may include species that are less abundant in some basins than in others.  Therefore, persons using these maps for restoration should consult the appropriate descriptions associated with each basin in planning site preparation and species selection.  It is particularly critical to recognize that the communities identified on these maps must have appropriate topography and drainage conditions restored prior to planting, especially where decades of farming or land-leveling have significantly altered the natural drainage patterns.  Many lowland hardwood forests are maintained as wetlands due to shallow ponding of water rather than flooding, and while flooding cannot be restored in most instances, the pattern of micro-relief and vernal pools that characterized those sites in the past can be approximated, and must be if restoration of the mapped communities is to be successful.
Note that these maps are not appropriate for use to identify specific flood frequency zones – the hydrologic data used are an amalgam of flood modeling, observed past flood conditions, and the inferred influence of topography and infrastructure, and they reflect the relative “wetness” of a site but not necessarily the occurrence of flood events.  Similarly, these maps do not reflect the distribution of jurisdictional wetlands, as defined under the provisions of the Clean Water Act. Only the appropriate Corps of Engineers District Regulatory Office can make that determination.  These maps are intended to provide restoration guidance and serve as planning tools only.