Evaluation and description of a floodplain system : the Middle Fork Forked Deer River, West Tennessee

This study was conducted on the Middle Fork Forked Deer River in West Tennessee from 2000-2003 to evaluate the severe degradation of floodplain and aquatic habitats resulting from upland erosion and coarse sedimentation. Land use practices have resulted in the erosion of coarse Coastal Plain sands from the uplands into the tributaries and river system, eventually resulting in sand deposition on the floodplain, increased overbank flooding, a rise in the groundwater table, and ponding of upstream timber (Diehl 1999). In 2000, the West Tennessee River Basin Authority expressed interest in restoring specific segments of the river. Thus, information is needed regarding the extent of bottomland hardwood forests, and the major factors affecting forest structure and composition. The objectives of our study were: 1) to quantify the loss and current extent of bottomland hardwood forests along the 9.7 km study reach, 2) to evaluate the major environmental factors structuring the wetland plant communities, and 3) to identify and evaluate potential restoration options. Black and white 25.4 cm X 25.4 cm aerial photographs from 1941 through 1960 were georeferenced and analyzed to identify potential losses of bottomland hardwood forest on the study site. In addition, a minimum of 5 study plots of 400 m2 each were established in each of six predefined geomorphic features on the Middle Fork Forked Deer River. Vegetation communities, environmental characteristics, and mean tree ages were measured on the study plots to identify important species, environmental changes in response to river degradation, and to estimate the temporal extent of disturbances on the study site. Simple linear regression of change in forest area over time revealed no loss of total forest area in the 60-year period studied (P = 0.23). However, systemic responses to channelization have resulted in significant changes in tree species composition. Historically common species like sweetgum (Liquidambar styraciflua) and oak (Quercus spp.) were not important, with an Importance Value (IV) of less than 1, and occurring in less than 20 % of forested plots, while red maple (Acer rubrum ), a disturbance-tolerant species, was the most important species on the site (IV = 78.1) and occurred in 87% of forested plots. Cluster analysis and indicator species analysis conducted on overstory species revealed 6 distinct vegetation associations identified by river birch (Betula nigra), bald cypress (Taxodium distichum), water tupelo (Nyssa aquatica), black willow (Salix nigra), and red maple. Subsequent non-metric multidimensional scaling (NMMS) ordination techniques indicated that the forest communities were not closely correlated with relative elevation-a result atypical of most bottomland hardwood studies. Correlations with NMMS axes indicate that species composition is likely defined by individual site hydrology, but results are confounded by the ubiquitous presence of red maple across elevation gradients and geomorphic features. The types of forest composition changes on the study site varied spatially and temporally with geomorphic response to channelization. Dendrochronological analysis of cores from the pervasive red maple suggests that widespread disturbance occurred across the study site approximately 35-45 years ago, which coincides with the halt of maintenance of channelized rivers in west Tennessee. Because of the system-wide nature of the disturbance that has occurred on the study site, restoration of the study reach will require a long-term approach across broad spatial scales. Suggestions for restoration include developing collaborative plans with landowners in the river basin, identifying and arresting sediment sources, restoring hydrology through meander reconstruction and levee dissolution, and reforesting with hardwood species based on landowner desires