During 1993 the Upper Mississippi River System experienced floods of exceptional magnitude and duration, especially at its more downstream reaches. The flood had widespread effects on the vegetation. Submerged species such as Potamogeton pectinatus significantly decreased in abundance, especially at sites with more severe flooding. However, many species were able to regenerate in 1994 from seeds or storage organs. Emergent species such as Scirpusfluviatilis were similarly affected, but in the upstream reaches were able to regrow in the autumn following the flood and at many sites showed exceptionally high productivity in the following year, probably due to nutrientrich sediment deposition by the flood. Many tree species were very severely impacted, although Acer saccharinum and Populus deltoides have shown some seedling regeneration on newly deposited sediment beneath stands of mature trees, which would have out-shaded the seedlings if they had not been killed by the flood.
Water elevation in the Upper Mississippi River (UMR) is highly regulated by an extensive system of locks and dams. Completion of this system in the 1930s created productive, biologically diverse backwater habitats. The status of plant communities in these backwater areas may now be threatened by several factors, including sediment accumulation, recreational use, and navigation traffic. Aerial photography, taken in 1975 and from 1991 to 1995, was used to describe vegetation changes occurring in four UMR backwater areas of Navigation Pool 8. The objectives were to determine (1) if changes occurring in these areas are consistent with hydrarch succession, (2) if the diversity of their plant communities has declined since 1975, and (3) how a large flood event that occurred in 1993 affected the composition and diversity of plant communities in these areas. Three general cover classes were recognized, representing an aquatic to terrestrial gradient. Coverages of specific vegetation types were estimated and evaluated using two indices of community diversity (vegetation richness and the Shannon diversity index). Though some vegetation changes were consistent with expected successional patterns (e.g. increased terrestrialization), other changes were not (e.g. loss of marsh vegetation). Diversity indices and coverages of most aquatic macrophytes declined from 1975 to 1991/1992 but then increased following the 1993 flood. The results suggest that disturbance-diversity concepts, including the flood pulse model, are applicable to the vegetation dynamics of these systems. Published in
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