Response of floodplain forest communities to modified hydrology in the Upper Mississippi River Basin

Background/Question/Methods

The Great Flood of 1993 was a catastrophe that inundated Midwestern urban and agricultural lands for weeks, and resulted in combined damages estimated at $15 billion. In addition, a large expanse of natural floodplain forest was seriously affected. Modifications in river hydrology, precipitated by levees and navigation dams, may help determine forest recovery patterns after the 1993 flood. The presence of these structures reduces the size of floodplains, and their capacity to influence the movement of water downstream. Thus, floodplains experience deeper and more frequent flooding than they did historically. Modern hydrologic conditions favoring different community types will greatly affect restoration efforts by floodplain land managers. This study was intended to (1) quantify differing patterns in forest community recovery in response to varying pool hydrology after the 1993 flood, and (2) describe the differential responses of species in relation to the various hydrological differences. Forest survey, water level (1993-2011), and elevation data were processed and integrated using a GIS. River level and elevation data were used to calculate inundation at survey sites.

Results/Conclusions

Initial analyses revealed that absolute basal area became greater with increasing mean maximum inundation depth (p < 0.02), but H’ diversity and evenness were negatively correlated (both p < 0.02).  Species richness (s) was not related mean maximum inundation depth (p = 0.87).  Ordination of relative basal area data separated sites along three axes. The first axis revealed a compositional gradient ranging from silver maple (Acer saccharinum) dominance to cottonwood (Populus deltoides) and willow (Salix spp.) dominance.  The second axis separated mixed green ash (Fraxinus pennsylvanica) and sycamore (Platanus occidentalis) stands from cottonwood stands. The third axis delineated Kentucky coffeetree (Gymnocladus dioicus) and elm (Ulmus spp.) stands from willow stands. These results suggest Upper Mississippi River Basin floodplain forest communities differ in species dominance patterns as a result of differing hydrologic regimes and soil disturbances which favor cottonwood and willow.