F L O O D I N G and its effects on TREES

Flood Tolerance of Trees

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.

Trees

Various characteristics of a tree affect its flood tolerance with the most prominent presented below.

Floods

Determining flood tolerance is complicated by the diverse characteristics of floods.

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.

Fig. 1 U.S. Army Corps of Engineers Divisional Areas

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.


Return to the Table of Contents