DIVERSITY OF FLOODPLAIN FORESTS OF LA  GRANGE  REACH  (RM  80-158),
ILLINOIS RIVER, FOLLOWING SIGNIFICANT DISTURBANCE

Michelle M. Cripps and Thad R. Cook.
Illinois Natural History Survey, Long Term Resource Monitoring Program, Havana Field 
Station, 704 North Schrader Avenue, Havana, IL 62644.

The floodplain forests of the Illinois River have been subject to developmental 
disturbance since modern European settlement.  Logging, changes in hydrology, and 
conversion to agriculture have had significant effects on the composition of floodplain 
forests.  The 1993 flood, which lasted almost the entire growing season, gives us the 
unique opportunity to study the effects of another significant disturbance to this very 
important ecosystem.  Random quadrat sampling at 46 sites throughout the La 
Grange Reach of the Illinois River (RM 80-158) revealed mature tree mortality ranged 
between 0% and 100% at each site with an average of 24.7% mortality per sampling 
site.  Trees in smaller diameter classes had a higher mortality than larger diameter 
classes.  The average density of trees per hectare dropped from 203 to 146 due to 
the flood.  Simpson's Diversity Index showed only a slight decrease in species 
diversity.  Importance Values indicated Silver Maple (Acer saccharinum) to be the 
most important mature tree species and to have increased in dominance after the 
1993 flood.  Willow species, the second most abundant species, decreased in 
dominance.  Pecan (Carya illinoensis), the only mast species encountered, was found 
in very low numbers.  Of the 15 species found during sampling, 6 can be considered 
rare species.

Keywords:  floodplain forest, Illinois River, La Grange Reach, species diversity, 1993 
flood
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