Dangers of Rising Flood Waters

6/16/2017 (Permalink)

HIGH WATER

Rising floodwaters can take down forests from the roots.

After the Great Midwest Flood of 1993, which caused nearly $20 billion in damage and covered 400,000 square miles, nearly all the trees in four stands of Sanganois Wildlife Management Area near Beardstown, Ill., were submerged for more than six months and died.D

Whether it’s a river overflow or a coastal storm surge due to a hurricane, floodwaters can mean trouble for trees and forest ecosystems. First, floodwater currents, loaded with debris and silt, can erode the soils around shrubs and trees, exposing roots. In strong currents, debris may even strip bark from trees, uproot plants or strip shrubbery.

In addition to the initial physical impact, because many of a tree’s fine, oxygen-absorbing roots grow in the upper six inches of soil, these roots can die off as the soil becomes waterlogged. Without a robust root system, trees don’t absorb enough water and become weakened. As a result, foliage wilts or dies, and these weakened trees are a target for insects and disease.

The area around St. Louis, Missouri, in August 1993 after the Great Midwest Flood. Credit: NASA

Coastal flooding — often the result of a storm surge or hurricane — can cause significant damage by flooding freshwater areas with salt or brackish water. This saltwater intrusion can spell trouble for plants that rely on freshwater. Increased salts in the soil make it more difficult for tree roots to take up water because of changed osmotic pressure.

“The result is what’s called physiological drought. Even though there is water, the tree can’t take it up, and it’s the same as if there was no water,” explains U.S. Forest Service research ecologist John Stanturf of the Forestry Sciences Laboratory in Athens, Ga. “Under severe conditions, an already-stressed tree, such as one that has lost its leaves by shearing in hurricane winds, can’t recover and refoliate and may die outright.”

A few years after the Great Midwest Flood, land managers noticed that new shrubs and seedlings began to thrive in the affected areas, though the types of trees differed from the older, established stands that were killed by the flood. Some tree species — such as silver maple, green ash, sweet gum and black willow — are thought to be more tolerant to flooded soils than other trees like black locust, black walnut, tulip poplar, sugar maple and American beech. Regardless of species, though, it may take mature, surviving trees several years to recover from a summer of flooding according to the South Dakota Department of Agriculture.