Ecological characteristics of floodplain forest reference sites in the Upper Mississippi River System

Historical and present day disturbances have contributed to long-term changes in the extent, composition, and structure of Upper Mississippi River System (UMRS) floodplain forests. Loss of forest habitat, low tree species diversity, a lack of successful regeneration in many areas, and continued declines in forest health over time are serious threats to this ecosystem. Floodplain forest restoration has therefore gained importance as a management goal throughout the UMRS. A key component of the restoration process is assessing the ecological characteristics of remnant, high quality reference sites to identify appropriate targets for forest restoration efforts. In this study, fourteen reference sites were located and established along a ∼1140 km longitudinal stretch of the Upper Mississippi River, lower Illinois River, lower Big Muddy River, and lower Cache River watershed. Biological and abiotic data were sampled from 1000 m2 permanent plots located at each study site. Overstory floodplain forest communities were generally dominated by silver maple (Acer saccharinum) and a variable combination of green ash (Fraxinus pennsylvanica), American elm (Ulmus americana), cottonwood (Populus deltoides), pin oak (Quercus palustris), and hackberry (Celtis occidentalis). Common understory trees capable of overstory recruitment included silver maple, green ash, American elm, and hackberry. Overstory species richness and diversity were generally high, but species richness and diversity in the understory layer were much lower, a trend that was most pronounced in the northernmost sites. In addition, the scarcity of mast producing and/or shade intolerant species in the understory was evident throughout the study, even at sites where they were present in the overstory. Herbaceous communities were notably variable between study sites. Despite the latitudinal gradient, ordination of the floodplain forest communities revealed substantial compositional overlap between sites. However, patterns in the ordination were significantly correlated with local variability in canopy cover, elevation, soil texture, and several hydrologic parameters. Species richness and diversity generally increased from north to south. Results suggest that regional differences and local characteristics such as microtopographic features should be considered when selecting target restoration conditions and evaluating the suitability of individual species for specific restoration projects.