PARENT SESSION
Poster Session #46: Riparian Ecology.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Ongoing mortality and forest change in response to the Mississippi River Flood of 1993.

Deutsch, Charles^1,2, Manar, Katy^*,1,2, Jamison, Kenneth^1,2, Schulz, Kurt¹, Rocks, Jim^1,3, Jones, Cynthia^1,3, ¹ Southern Illinois University Edwardsville, Edwardsville, IL² US Army Corps of Engineers, Riverlands Demonstration Project, St. Louis, MO³ URS Corporation, Portland, OR

ABSTRACT- The Great Flood of 1993 had direct effects on lowland forest health and composition. Early studies conducted in Illinois sites near St. Louis, MO revealed about a 20% reduction in stand basal area and the destruction of the sapling and seedling strata. Erratic and unnatural flood patterns caused by water management activities delayed recovery by causing nearly complete mortality of tree seedlings until 1998. We measured subsequent forest changes over the period 1998-2001 in permanent plots. Stand basal area continued to decline over the period, from an estimated 51.2 m²/ha before the flood, to 37.8 m²/ha in 1998, to 28.6 m²/ha in January 2002. Tree density followed a similar trend. The relative proportions of the dominant tree species, Acer saccharinum and Fraxinus pennsylvanica, increased from 52 and 16% relative basal area in 1993, to 64 and 18% in 1998, and 73 and 21% in 2001. Density showed a similar trend. Over 1998-2001 tree mortality was high among the remaining large Populus deltoides and currently abundant Fraxinus. Tree species combined diversity (Shannon-Wiener H') and equitability showed continuing declines. The density of the saplings (stems 2.5-10.0 cm dbh) increased, but sapling densities remained low to the point of insignificance (53 stems/ha). Tree seedling success was markedly better in the last few years, corresponding to a reduction in growing season floods. The seedling understory (> 1 m tall but <2.5 cm dbh) is now dominated by Acer and Fraxinus (540 stems/ha). Current forest dynamics continue to reflect the bottleneck in canopy structure and species richness caused by the 1993 Flood. In light of the historical record, lowland stands near St. Louis may be more depauperate than at any other time in the last two centuries.

KEY WORDS: floodplain forest, Mississippi River, Flood of 1993, Acer saccharinum