Janna Beckerman, Associate Professor, Department of Botany & Plant Pathology, Purdue University
Excessive rainfall across Indiana has resulted in flooding in many parts of the state. Although the impact of flooding on humans is almost immediate, how flooding affects trees is less obvious. There are several factors to take into account when considering the impact of flood stress on a particular tree. These include: 1) species tolerance to flooding, 2) overall tree health, 3) length of flood and 4) sediment accumulation around tree roots.
Some species of trees are better able to adapt to flooded conditions. Trees that have evolved for life in a river bottom have mechanisms to cope with the periodic flooding that may occur and are better able to handle flooding. However, urban areas are rarely forested by trees that are adapted to flooding. There are some notable urban exceptions: boxelder, sweet gum, red and silver maple, elm, hackberry, green ash, and willow are all considered relatively tolerant to flooding stress (they are often not considered desirable urban trees, though!). It should be noted that the spruce, pines, and oaks (with the exception of eastern pin oak and swamp white oak) are all relatively intolerant of flooded conditions. Even the most flood tolerant of trees will die if the flooding in prolonged.
When examining flood-damaged trees, keep in mind that urban life is stressful to many species of trees, making them particularly susceptible to flood-related damage. Flooding compounds these stresses, leaving trees highly predisposed to additional mortality due to insect and disease, particularly crown and root rots caused by Phytophthora species. Symptoms of flood damage include leaf yellowing (chlorois) followed by leaf browning and ultimately, leaf loss, defoliation, and death of the tree.
There is little that can be done to reduce the length of flood damage, but removing sediment accumulation around the roots, when possible, can minimize root compaction, and improve the plant survival. A light application of nitrogen fertilizer, (0.5 - 2.0 pounds of nitrogen per 1,000 sq ft) can be applied to encourage growth, but do apply any additional fertilizer with nitrogen again until the following spring, after full leaf expansion.
Those trees that weren’t killed in the initial flood event are considered predisposed, and can die quickly due to the combination of physical injury and rapid invasion from insects or diseases. Conversely, well-maintained and healthy trees can and do recover quickly. Most trees fall within these two extremes. How well a tree copes with flooding and the secondary agents of plant disease depends upon how vigorous the tree was prior to flooding, and how long the flooding occurred.
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Flooding can injury trees in a variety of ways, including drowning tree roots, changing the soil structure, and reducing the trees ability to anchor itself, resulting in blow down. This short duration flood would not be expected to cause any lasting effects to these trees.
The elm in the foreground will tolerate the flooding damage much better than the spruce in the background.