Flooding in bottomland hardwood sites is an important natural occurrence and provides much of the disturbance needed for regeneration of these ecosystems. What has been unusual about the floods of 1993 is the combination of length of inundation, growing season occurrence, and flooding of urban and non-floodplain forests.References
Tree recovery following flooding is relative. A logger, urban forester, or wildlife manager (as examples) may view a tree's recovery from vastly different perspectives. For example, the logger who seeks a long, defect-free straight trunk will be discouraged if a few inches of terminal dieback results in vigorous lateral branch growth and subsequent poor form. An urban forester, however, may be concerned about the potential hazard of dead branches. The loss of a mast crop may be considered a serious problem to a wildlife manager, but the logger may see it as a minor one.
Regardless of the perspective used to define tree recovery, trees need to build up food reserves for future stress conditions such as another flood or a drought. However, environmental conditions (prolonged flooding or rapid drying of soil) often limit recovery.
Factors Affecting Management
Natural resource managers should be aware of the following management implications when evaluating the consequences of the Floods of '93.
- There will be a significant increase in disturbed sites as a result of siltation and scouring. Normally this would provide an excellent seedbed for many of the pioneer species that occur on bottomland sites. The problem is timing; seeds will not be available again until early next growing season and a percentage of these sites may be lost to herbaceous competition. In addition, herbaceous plants will replace the woody understory in many forest stands.
- There may be a bumper seed crop in 1994 as a result of the stress induced by the prolonged 1993 inundation.
- The loss of litter and the duff layer may make the soil more susceptible to temperature variances. This may affect seed germination (particularly of non-floodplain species) as well as delaying spring leaf-out.
- There will be mortality across the continuum from mature trees to new regeneration due to the prolonged inundation. Death will probably come slowly to some species over the next 1-4 growing seasons. Consequently, when low vigor trees are harvested, one should not depend on sprout origin material making up much of the stocking in the next stand.
- Opportunities may increase for planting in bottomland sites as a result of severe scouring or siltation of agricultural fields. The only long-term practical solution for some of these sites may be restoration to bottomland hardwood species by planting. (Refer to Tables 1 and 2 in the Flood Tolerance of Trees section of this resource packet when selecting species for planting.)
- Diameter growth of most flood-intolerant species usually is reduced by prolonged flooding of soil during the growing season. Sometimes the reduced growth occurs so long after flooding that other causes are sought. In contrast, flood tolerant trees may make a spurt of diameter growth during the first year of flooding. For example, a Mississippi study found that the diameter growth of green ash, which is often categorized as relatively tolerant of flooding, was 80 percent greater than normal when water remained on the ground from spring through August.
- Sometimes it is difficult to make valid estimates of wood production in flooded trees by measuring changes in stem diameter, especially in young trees. For example, short-term flooding of some woody ornamentals increased stem diameter largely because of an increase in bark thickness rather than increased wood production. Forest trees may respond to flooding in a similar manner.
- Reduced height growth of many flooded conifers and hardwood trees has been shown in experiments with potted plants. Additional studies have found that height growth of some flood-tolerant species may be increased by flooding if the water is flowing.
- The shallow root systems of trees growing in poorly aerated (flooded) soils often make them prone to windthrow. Windthrow is a management concern in both urban and forest settings.
- Landowners who are investors" in timberland can deduct a flood-related casualty loss if the value of the loss is 10 percent or more (minus any income from salvage) of their adjusted gross income. Flood victims who plan to reforest their land for timber production are eligible for a 10 percent tax-credit and can amoritize 95 percent of the cost over a 7-year period.
Little information is published on how much time is available to salvage dead or declining trees before stain-causing fungi begin to significantly degrade sawlog material. Since much of the flood-induced mortality will probably occur over the next two growing seasons, there may be a one to two-year period to accomplish salvage.
Stain-causing fungi enter first through points of injury such as damaged bark and broken limbs, so if there is no apparent mechanical injury, there may be a little more time before stain develops. Light colored hardwoods (ash and birch, for example) stain faster than dark colored species such as oak and walnut. Cottonwood and silver maple will probably deteriorate the fastest.
Foresters must be prepared to develop "salvage plans" as a part of an overall management plan for flooded forestland. Local sawmills, however, may be unprepared to process the dead and dying timber especially if the supply exceeds mill capacity. Species composition may also pose a problem in the marketplace.
Timber harvesting frequently contributes to increased soil wetness and reduced soil aeration by compacting soils and damming subsurface water flow. High soil moisture also results from the large reduction in transpiration which follows harvesting. These factors can impact residual trees following a large-scale salvage cut.
Broadfoot, W. M., and H. L. Williston. 1973. [Flooding Effects on Southern Forests.] Journal of Forestry. Vol 71 (9): 584-587.
Kozlowski, T. T., Kramer, P. J., and S. G. Pallardy. 1991. [Soil Aeration, Compaction, 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. Vol 85 (3): 36-40.
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