The Great Flood of 1993 will be remembered as one of America's most devastating natural disasters. Record high flows of flood water breached over 1,100 levies and flooded communities up and down the Mississippi River and its tributaries. In the Mississippi River Basin, flooding covered 10 million acres, destroying or damaging more than 40,000 buildings and killing at least 47 people. Much of the attention of the flood was focused on the loss of buildings, roads, and crops. A factor that is hard to calculate into flood loss is the mortality of community trees.
There is considerable information available on the survival of trees that are subjected to flooding during the dormant season or for relatively short periods during the spring. These studies indicate the response to flooding depends upon the season, depth and duration of flooding, and the age and health of the tree species affected. Late season prolonged flooding, like that which occurred in 1993, has not been thoroughly examined.
Unlike native stands of timber along rivers that have an evolutionary history of surviving floods, community trees are typically transplanted from nurseries and may or may not be native to the area where they are growing. The flood provided a unique opportunity to research the effects of long-term, late-season flooding on trees in urban and community settings.
Strong water currents washed away soil around the base of trees, exposing the roots. (photo by Fredric D. Miller, Affiliate, INHS Center for Economic Entomology.)
A research project to examine the immediate and long-term effects of the flood on urban trees was initiated in fall of 1993 as a collaborative effort among scientists at the Illinois Natural History Survey, the University of Illinois, Western Illinois University, and the University of Northern Iowa with cooperation from the U.S. Army Corps of Engineers and the city of Davenport, Iowa, and other flood-affected communities. The goals of the project represent the various interests of the scientists involved. Our foremost objective is to determine the "urban" tree species most susceptible to prolonged late-season flooding. Secondarily, we are interested in determining the cause of mortality in years following the flood.
Unlike crop plants, mortality related to the flood will continue for several years in long- lived species like trees. Flooding saturates the soil and rapidly depletes oxygen available to roots in the soil. When flooding occurs, the soil has less oxygen for root respiration, and the conditions become anaerobic. Anaerobic respiration produces compounds that are toxic to roots. Water and nutrient-absorbing roots die, creating a void in the nutrient cycle of the trees, that results in less photosynthesis. Eventually, if conditions don't change, the tree may die. Root mortality caused by the flooded soils will continue after the water has receded, making the trees susceptible to various secondary pests.
Trees may succumb to diseases or insect attack in years following the flood due to the stress of flooding. Determining what diseases and insects are attacking these stressed trees and which trees are less likely to be attacked is another significant aspect of the project.
Flooded and nonflooded sites in close proximity were selected for our studies. Trees in the flooded sites were submerged in water 5 to 6 feet deep for periods up to six weeks during the growing season in 1993. Preliminary "urban" tree inventories were taken of the study sites in 1994. Tree species and flood-related tree mortality were recorded along with diameter (growth) measurements and tree condition ratings of the living trees. Inventories on the trees will continue through 1998.
Many trees were killed by the flood in 1993. The overall influence of the flood on tree survival will be seen in years to come. (photo by Fredric D. Miller, Affiliate, INHS Center for Economic Entomology.)
Early observations show heavy first-year mortality of sugar maple, crabapple, hackberry, ornamental pear, linden, eastern white pine, junipers, and other evergreens. Limited mortality was observed on pin oaks and walnuts. Rot and decay fungi were identified on dead trees, but were not implicated in causing mortality. Similarly, bark beetles and other insects that feed in dead wood were identified on many of the dead trees, but were not implicated in the trees' demise.
Initial inventories were provided to the affected communities to assist them in replanting and replacing plant materials damaged in the flood. The information gained from the studies will assist landscapers and communities in using flood-tolerant plantings in flood-prone areas. It will also give us a better understanding of insect, disease, and tree interactions associated with flooding stress on tree species not adapted to that environment.
John E. Lloyd, Center for Economic Entomology and Patrick J. Weicherding, Department of Forestry, University of Illinois
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