Floods and Trees
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