Forested bottomland conservation areas in the midwestern and southern United States are subject to an increasingly diverse range of demands for recreational use and other ecosystem services, many dependent upon the maintenance of specific plant communities. Historical land use and other disturbances have shaped present vegetation composition, but these impacts are poorly understood. This study examined historical land use records, dendrochronological evidence, and pre- and post-tornado vegetation, with and without salvage logging, to assess forest composition changes over approximately 125 years at Mermet Lake Conservation Area in southern Illinois. This site has land use history, vegetation cover, and a management mandate common to many large river bottomland forests in the Midwest and southern USA.
The vegetation of the area prior to Euro-American settlement was primarily a forest dominated by Taxodium distichum (L.) Rich. and Nyssa aquatica L. A period of drainage and conversion to agriculture began ca. 1900 and was followed by public ownership as a conservation area since 1950. Management during this latter period was characterized by partial hydrologic restoration and complete fire suppression. The post-agriculture forest was dominated by oaks (Quercus spp.) with a transition to mixed mesophytic and bottomland hardwood forests. Following a tornado, composition and diversity within the developing stand varied along a wind intensity gradient but tended toward increasing dominance of mixed mesophytic species at the expense of Quercus. Subsequent partial salvage logging further increased vegetation complexity in response to mineral soil exposure and creation of microtopographic variation. Grading and seeding of skid trails following salvage operations produced compositionally distinct vegetation communities. Increasing prevalence of the invasive exotic Microstegium vimineum (Trin.) A. Camus., especially on salvaged plots, is expected to continue to impact vegetation communities at Mermet Lake.
Our results suggest that historical alterations in site hydrologic conditions, from pre-drainage to drainage to partial hydrological restoration, in combination with associated land use changes, produced drastic changes in forest community composition over the last century. Managers of this and similar bottomland forest areas need to consider disturbance regime changes and appropriate silvicultural strategies needed to create or maintain the historical range of vegetation types associated with sometimes disparate conservation objectives