Numerous studies have been conducted to help foresters and natural resource managers understand the impact of flooding on trees (see bibliography). The state-of-the-art, however; has not developed sufficiently to warrant a precise statement on the adaptability of a species to a specific flooding situation. Conclusions from different studies are often contradictory, caused in part by the physiological responses of the tree as it interacts with environmental conditions. Since these environmental conditions are not well understood, as well as the difficulty in categorizing tree species over their entire range, flood tolerance predictions must be carefully evaluated in general terms. A brief review of soil, tree, and flood characteristics indicates the complexity of these interactions.
Soils
The following soil-related points are important in understanding flooding effects on trees.
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Soil Aeration
Flooding results in poor soil aeration because the supply of oxygen to flooded soil is severely limited. Oxygen deficiency is likely the most important environmental factor that triggers growth inhibition and injury in flooded plants.
pH
Flooding of soil increases the pH of acid soils and decreases the pH of alkaline
soils.
Organic Matter
The rate of decomposition of organic matter in flooded soil tends to be only half
that in an unflooded soil. The major end products of decomposition of organic
matter in flooded soils are carbon dioxide, methane, and humic materials. In
addition, high concentrations of ethanol and hydrogen sulfide are produced in
waterlogged soils which can be damaging to root systems.
Sedimentation
Deposits of silt or sand as shallow as three inches may seal over and smother
tree roots by limiting the supply of oxygen. Species vary in tolerance to
sedimentation, but all seedlings are susceptible to root injury. Eastern cottonwood,
baldcypress, tupelo, and black willow seedlings can withstand moderate siltation.
Scouring
Strong currents, waves, or suspended particulates may cause soil around the
base of the tree to be washed away, exposing tree roots. Exposed roots can lead
to not only tree stress but can make the tree more vulnerable to windthrow.
Various characteristics of a tree affect its flood tolerance with the most prominent presented below.
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Height
Tree injury increases in proportion to the percent of crown covered by water. Species that can survive standing in several feet of water for months may die in less than one month when their foliage is completely covered. Few species can tolerate more than one month of complete submersion during the growing season.
Crown Class
Trees in the dominant crown class survive flooding much better than trees in
lower crown classes.
Age
Adult trees tolerate flooding better than overmature trees or seedlings of the
same species. Therefore, some species rated as flood tolerant may be quite
sensitive in the seedling stage. Seedlings often die because they are pushed over,
buried in mud, or uprooted.
Vigor
Tree vigor at time of flooding influences tolerance. Vigorously growing, healthy
trees withstand flooding better than less vigorous trees. Tree vigor may be
irrelevant, however; if the tree is totally submersed in water.
Roots
Long-term flooding leads to death and decay of large portions of a tree's root
system (see section on Management Implications for windthrow problems).
During flooding, some species can maintain normal roots in an active or dormant
condition; others rely upon new secondary and adventitious roots that may form
from the root collar or on the trunk near the water surface. Species unable to
either maintain normal roots or grow new ones can quickly die.
Species Variations
Flood tolerance variations within a species are not well understood. Flood tolerance may be an inherited trait and this may explain some of the discrepancies in
reports on survival. (Research methodologies also may vary from one study to
another; contributing to contradictory conclusions). However, it is generally
accepted that some species have greater tolerance for flooding than others (see
Tables 1, 2, and 3).
Determining flood tolerance is complicated by the diverse characteristics of floods.
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Season
Flooding during the growing season usually is more harmful to woody plants than flooding during the dormant season. Specifically, trees are most susceptible to flooding in late spring just after the first flush of growth. The timing of a spring flood influences species differentiation. For example, since silver maple flushes earlier than green ash, an early flood might be more damaging to silver maple while a later flood more injurious to green ash.
Duration
The longer trees are exposed to flooding, the greater the potential for injury.
Most trees can withstand only 1-4 months with water being continuously over
the soil surface. Short periods of flooding during the growing season can be
tolerated by most trees. However, if flooding is recurrent and keeps the soil
saturated or prevents recovery from previous flooding, injuries will accumulate
and serious damage may occur.
Water Level
The depth of water influences flood tolerance. The mortality rate is less for trees
in saturated soils than for trees with water covering the soil. After water covers
the soil, the depth may have little significance until the lower foliage is covered;
research results, however; differ on this point. Tolerance to complete submersion
is much lower than tolerance to shallower depths of water.
Temperature and Oxygen
Cold water is less injurious than warm water due to cold water's capacity to hold
more dissolved oxygen. Rapidly flowing water (with higher oxygen content) is
less harmful than stagnant water.
Mechanical injuries
An often overlooked aspect of flood damage is mechanical injury caused by cur-
rent, wave action, and floating debris. Young tree plantings may be especially
damaged by current and wave action. Floating debris can injure both small and
large trees.
Chemicals
Floods may carry various chemicals that have been picked up as runoff from
agricultural fields and other areas or from sewage released when treatment
facilities become unable to handle large volumes of water. The impact depends
upon the type and dosage of chemicals.
Tolerance Categories
Table 1 and Table 2 present a summary of the research pertinent to flood-tolerant trees and shrubs for three geographical divisions (districts) of the U.S. Army Corps of Engineers: Lower Mississippi Valley, Missouri River; and North Central (see Figure 1). These three divisions include a majority of the forestland flooded during 1993.
Table 1 combines research results from the Lower Mississippi Valley and the Missouri River divisions. Since classification is relative, flood tolerances are best viewed as overlapping from one tolerance category to the next. Where research results differed between Lower Mississippi Valley and Missouri River studies, species are classified into two tolerance categories. The tolerance categories in Table 1 should be interpreted as follows:
| Very Tolerant | Able to survive deep, prolonged flooding for more than one year. |
| Tolerant | Able to survive deep flooding for one growing season, with
significant mortality occurring if flooding is repeated the following year. |
| Somewhat Tolerant | Able to survive flooding or saturated soils for 30 consecutive days during the growing season. |
| Intolerant | Unable to survive more than a few days of flooding during the growing season without significant mortality. |
Table 2, which presents the results of an Illinois study, is the most comprehensive in the North Central Division. It is important to note that the flood tolerance categories in Table 2 differ from Table 1 in both name and definition. Table 2 tolerance categories should be interpreted as follows:
| Tolerant | Most individuals survived more than 150 days of flooding during the growing season. |
| Somewhat Tolerant | Some individuals killed by less than 90 days of flooding and some individuals survived greater than 150 days of flooding. |
| Slightly Tolerant | Most individuals survived more than 50 days but less than 100 days of flooding. |
| Intolerant | Severe effects with less than 50 days of flooding. |
Table 3 provides flood tolerance ratings for cultivated woody plants in New York subjected to a growing season flood (June 1972). The species listed in Table 3 are commonly available in the landscape trade and are frequently used in park landscapes and urban settings in the Midwest and the Great Plains. Because Table 3 is based on a short duration flood (10 days), information on the intolerant species (those killed or damaged) will be of the most use to practitioners.
It should be noted that Tables 1-3 classify tree and shrub species tolerance relative to continuous, rather than intermittent, flooding. Some species, for example, might tolerate one year of continuous inundation but only 3-4 months of intermittent flooding. Also, some sites affected by the 1993 floods in the Midwest and the Great Plains had soil saturation up to 90 days prior to flooding. Consequently, the factors of soil saturation prior to flooding and continuous versus intermittent flooding must be considered when predicting the relative flood tolerance of species.
With the exception of the tolerance ratings for hackberry, green ash, and shingle oak, Table 1 is more conservative in its tolerance ratings than Table 2. Table 1 includes more species than Table 2 and is based on a summary of studies from a broader geographical area. Consequently, Table 1 is recommended as the field guide" for foresters and other resource managers who are evaluating flood- damaged trees in the Midwest and the Great Plains.
References
Barry, P. J., Anderson, R. L., and K. M. Swain. Undated. [How to Evaluate and Manage Storm-Damaged Forest Areas.] Southeastern Area, U.S. Forest Service. 15 p.
Bell, D. T., and E L. Johnson. 1974. [Flood-Caused Tree Mortality Around Illinois Reservoirs.] Trans. Ill. State Acad. Sci. Vol 67 (1): 28-37.
Broadfoot, W M., and H. L. Williston. 1973. [Flooding Effects on Southern Forests.] Journal of Forestry. Vol 71 (9): 584-587.
Hook, D.D. 1984. [Waterlogging Tolerance of Lowland Tree Species of the South.] Southern Journal of Applied Forestry. Vol 8 (3): 136-149.
Kozlowski, T. T., Kramer; P. J., and S. G. Pallardy. 1991. [Soil Aeration, Compac- tion, and Flooding.] in The Physiological Ecology of Woody Plants, New York: Harcourt Brace Jovanovich. 303-337.
Loucks, W. L. 1987. [Flood-Tolerant Trees.] Journal of Forestry. March: 36-40.
Missouri Department of Conservation. 1993 (Summer). [The Effect of Flooding on Trees.] Missouri Forest Management Notes. Vol 5 (3): 1-2.
White, R. M. 1973. [Plant Tolerance for Standing Water: An Assessment.] Cornell Plantations. Vol 28: 50-52.
Whitlow, T. H., and R. W. Harris. 1979. [Flood Tolerance in Plants: A State-of-the- Art Review.] National Technical Information Service, U.S. Dept. of Commerce. August: 1-161.
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