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How does flooding affect trees?

Tree Talk: Here’s a look at what we learned following the 1993 flood.

May was a record-breaking month for precipitation. Many rivers and streams throughout the Midwest are still out of their banks, and fields are flooded. It reminds me of the 1993 Mississippi River flood. How does prolonged flooding impact trees?

First, the time of year that flooding occurs is an important factor in how trees respond and their long-term health. Similar to the 1993 flood, this year’s flooding began in April and has continued into early summer. Flooding in spring and early summer can have a much greater effect because this is when trees are coming out of dormancy, leaf-out is occurring and metabolic activity is kicking into high gear. Oxygen is in high demand during this time.

Also, the floodwaters are relatively warmer compared to winter and very early spring. Warmer water cannot carry as much oxygen as colder water. If tree roots are situated in water-logged or saturated soils, root respiration is greatly diminished. If soils stay saturated with little oxygen available, roots will begin to enter into an anaerobic condition (oxygen-deprived) and rot and decay.

All the time this is going on, the aboveground parts of the tree are trying to conduct photosynthesis, undergoing transpiration and attempting to grow. With the root system damaged or compromised, it is difficult for trees to conduct these vital functions. Remember, the root system is critical to tree health. If the roots are not able to function properly, the entire tree suffers.

If flooding continues long enough, trees may begin to decline and eventually die. We saw this in 1993 with flood-intolerant species such as lindens, sugar maples, upland oak species, many of our common fruit trees and conifers. Bottomland species such as hackberry, silver maple, willow, cottonwood, ash and elms fared much better and eventually recovered.

Duration is key

The second factor is the duration of the flood. Most tree species can tolerate short events of a few days or even a week or so. We see this quite often in spring. But when flooding and water-logged soils drag on for weeks and months, we pass the point of no return.

A third consideration is the speed of the current of the water, which can also damage trees by scouring the soil from around the trunk and bombarding the trunk with debris and harmful chemicals that may be present in the water. Once the waters have receded, siltation can be detrimental by adding “fill” around the tree and smothering roots.

Unfortunately, there is not much we can do about excessive precipitation and flooding. If you have individual landscape trees, diverting excess water away from them and providing mulch around them to increase infiltration may help. Planting flood-tolerant trees in flood-prone areas may lessen the impact. We hope for a drier summer, but we will probably see lasting effects of this year’s flooding for months and even years to come.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at fmiller@jjc.edu.

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