Floods and Trees

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Published on: Aug. 2, 1995

Last revision: Oct. 20, 2010

The dry weather during the summer of 1994 was a sharp contrast to the situation the previous year, when flood waters covered much of the Missouri and Mississippi river flood plains for up to two months. Losses to homes, businesses, crops and human life were severe. These losses included significant death of bottomland trees.

Flooding generally creates favorable conditions for the growth of bottomland forests. Tree species native to bottomland forests rely on flooding to provide nutrients and a proper seed bed for the start of young seedlings.

Normally, flooding in the Missouri and Mississippi River systems occurs during the winter and early spring, when trees are best able to withstand flood waters. The flood of 1993 was notable for its duration and occurrence during the growing season.

The most drastic effects of the 1993 flood were seen in forests that had been protected from periodic flooding over the years. Because of the less frequent flooding, species susceptible to floods were able to grow and mature. These trees tended to be older, and few were able to withstand the flood.

Trees tolerate flooding for varying lengths of time, some up to four months, depending on species, age and size. Species native to upland habitats, such as pines, white and red oak, sugar maple and flowering dogwood cannot tolerate water covering the soil for long. These and other flood-susceptible species, which frequently have been planted as ornamentals in urban areas protected by levees, did not fare well once levees were breached during the flood.

Seedlings and saplings of most species that were completely inundated suffered the same fate. Similarly, overmature and stressed trees did not do well. Healthy, native bottomland species, such as silver maple, green ash, sycamore and cottonwood, seemed to fare best. Some species, such as baldcypress, black willow and water tupelo, can even go beyond the limits sustained during the 1993 flood.

Growing season floods are generally more damaging to trees than dormant season floods. When actively growing, tree roots use oxygen at higher rates than when dormant. Flooding restricts the amount of oxygen in the soil, especially in slowly moving or stagnant water.

Silt deposited when flood waters recede further restricts oxygen supplies, especially on newly planted trees or young seedlings. Tree roots also must contend with toxic compounds carried by the flood waters or produced as a byproduct of anaerobic (without oxygen) decomposition of dead plant materials.

Even trees that survived the flood are not home free. At the least, many trees were under severe stress for several months. Evidence of stress could be seen in the yellowing leaves, crown dieback and peeling bark.

Some roots died or suffered mechanical injury from sediment and objects carried by floodwaters. Sprouting often occurs along the stems of wounded trees. The early coloration seen on flooded trees during late summer was a stress symptom, too.

Stress sets the stage for invasion of trees by insects and disease organisms. These "secondary attackers" prey on weakened trees, presumably because the defense systems of the trees are impaired. Stem borers are an important group of insects to be concerned with after a flood or other severe stress event. Borers affect the water and food conducting systems in trees and weaken stems, leading to later breakage.

Diseases most likely to occur are root rots and cankers. This is because root systems are stressed, and stems and branches have wounds providing easy entrance for diseases.

What will happen in future years as a result of the 1993 flood is still unknown. We have limited experience with floods of this duration and timing. Experts seem to agree that we haven't seen the last effects of this flood, however.

To help us learn more about what happens following a flood of this magnitude, the Conservation Department has begun several studies to monitor the aftermath. One is a study of trees in urban areas. The objectives of this study are to determine how different species respond to floods and what secondary attackers are common. Another study is concerned with the effects of the floods in natural bottomland settings. Both involve several states and are cooperatively funded by the U. S. Forest Service.

We can be certain more trees will die over the next five growing seasons, as secondary pest organisms victimize stressed trees. But is this good or bad?

The mortality is viewed as a problem by loggers and tree farmers interested in using a crop. Dead trees need to be salvaged quickly to avoid defects that will soon occur because of rotting. Surviving trees and saplings may be prone to poor form because of mechanical damage and sprouting.

Dead and dying trees in urban areas can present hazards and cost dollars to replace. And there is the fear that the loss of mast species, such as pin oak, will reduce waterfowl, turkey and deer numbers.

On the other hand, flooding is a natural occurrence. Floods and similar disturbances act as "reset buttons" for tree succession and growth. Carpets of silver maple, cottonwood and green ash were visible the summer following the flood, thanks to the favorable conditions created for these pioneer species. Silt deposited by the flood provides a perfect seed bed.

Most competing understory plants were killed, and light now penetrates to the forest floor through the dead overstory trees. Seeds cast in the fall and spring by these colonizing plants sprouted with ease and took hold. This is the way forests work.

One good result from the floods is that the acreage of bottomland forest will increase in Missouri. Bottomland hardwood is the only forest type showing a decline over the last 15 years, according to forest survey statistics. The decline is attributed to increasing development in flood plains, including farming, industrial and residential uses.

Levees allowed these uses with protection from flood waters. However, the levees really only provided a false sense of security. Floods of this magnitude have happened before and they will happen again. Realizing this, some flood plain land will be allowed to revert to bottomland forests, thus increasing their overall total acreage.

We will continue to monitor the effects of floods on trees to increase our management skills. The point to remember is that our native bottomland habitats and the species that live there are the ones that tolerate flooding best and recover from flooding the quickest. When trying to determine the best uses for your riverside land, keep in mind the resiliency of bottomland forests.

Bottomland Forests and the Flood

by Phil Covington

Walking in the woods on a windy day can be dangerous at the Ted Shanks Conservation Area. Most of the trees in the floodplain here were killed by the 1993 floods and are now breaking off at what was the high water line of the flood.

This 7,636-acre wetland is located in the northeast corner of Pike County, near Louisiana. It lies at the confluence of the Salt and Mississippi rivers. Seventeen miles of exterior levee keep both rivers at bay, while 23 miles of interior levees separate the wetland into 19 manageable pools. The 2,920 acre bottomland forest here is the largest found along the Mississippi River between Rock Island, Ill. and Reelfoot Lake, Tenn.

On July 1, 1993, Mississippi flood waters overtopped our levee, and by July 4th the entire wetland area was under 15 to 17 feet of water. Our primary concern was for the bottomland forest. Mature pin oaks were obviously stressed. Leaves were small and light green, branch tips were dying back and dead limbs appeared in the crowns.

These trees shunted their remaining energy into producing a tremendous mast crop and then died. This might have been our greatest loss, because mature pin oaks made up over 80 percent of the trees in the forest. Mast crops are important as a food source for the migrating waterfowl and other wildlife this area was created for.

The prolonged 1993 floods, occurring just at the peak of the growing season, took a terrible toll: 90 percent of the bottomland trees at the Ted Shanks Conservation Area were dead. The same was true for nearby islands. In fact, a large island heron rookery went unused because nest-bearing sycamore trees had died.

Dead hickory, oak, hackberry, hawthorn, dogwood, holly, persimmon, sassafras and wild plum were among the trees that stood starkly against the spring sky in 1994. The few mast producing trees that survived included pecan, green ash, silver maple and button bush.

In the spring of 1994 we found young pin oak seedlings sprouting from the acorn crop of the previous fall. They were nestled close to the earth beneath a carpet of spanish needle, smartweed and ragweed. The process of renewal had begun.

Change is what wetlands do best. Long term static water levels lead to stagnation and a reduction in species diversity. Dynamic changes, like those at the Ted Shanks Conservation Area, ensure that wildlife will continue to find food and shelter in years to come.

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