Flood damage to trees develops in three primary ways:
1) acute soil and tree changes because of saturated and inundated soil conditions; 2) flood water physically knocking over trees; and, 3) chronic problems associated with a changing environment and modified tree reactivity.

Most trees and woody shrubs are not adapted well to flooded conditions. There are a range of flood tolerance levels among different species and individuals. There are a number of growth form, anatomy, and physiology changes available in tolerant plants to try and minimize flooding damage and growth constraints.

Major Impacts of Flooding:
The major impacts of flooding on the tree and site resources are:

1. Poor Aeration — Flooding initiates an immediate constraint on oxygen movement from the atmosphere to the respiring surfaces of the tree.
As a site floods with water, water moves into and occupies the macro-pores that previously held gas. The tree root interface then does not have a gaseous component.
The diffusion rate of oxygen across a water interface is 7,000 times slower than across a gas interface.
Available oxygen is quickly (l-3 hours) used by the respiring roots and the soil microorganisms.
The only area where oxygen is available is in the top l/10 of an inch of soil that is in contact with oxygenated water. Lack of oxygen leads to production and accumulation of carbon-dioxide, methane, hydrogen, and nitrogen gas.
2. Soil Structure — Flooding alters soil structure by allowing soil aggregates to fall apart from reduced cohesion, dissolving of metallic and organic coatings, and dispersing of clay particles.
3. Anaerobic Ecology — Flooding causes anaerobic organisms to replace aerobic organisms in the soil. Anaerobic organisms are primarily bacteria.These
bacteria cause denitrification and reduction of manganese, iron, and sulfur.
4. Reduced Chemical Activity — Flooding reduces redox potential, increases pH of acid soils, and decreases the breakdown of organic matter. The breakdown or decomposition of organic matter in normal soils helps hold cations and anions, releases essential elements, and prevents leaching of elements. This decomposition process is a rich assemblage of organisms.
In flooded soils, the decomposition of organic matter is by anaerobic bacteria. These organisms are less diverse and much more inefficient at decomposition. Slick, slimy layers of partially decayed organic matter may be present as a result.
In normal soils, decomposition of organic matter yield carbon-dioxide and humus. Carbon- dioxide moves into the atmosphere and components of the humus are bound to clays and oxides of aluminum and iron. This binding process improves soil structure. The nitrogen in the soil that is released as ammonia is converted to nitrate. Sulfur compounds are oxidized tosulfates.
In flooded soils, decomposition of organic matter yield carbon-dioxide, humus, methane and a large number of other materials. The gasses produced included carbon-dioxide, methane, and hydrogen.
Other materials, some extremely volatile and some very toxic are produced. Examples of other materials produced by decomposition of organic material under anaerobic conditions include various hydrocarbons, alcohols, carbonyls, fatty acids, phenolic acids, sulfur compounds, acetaldehyde, and cyanogenic com- pounds. Many of these materials escape as gas bubbles, dissolve in the water, or float to the water surface.

Flooding effects trees at every stage of their development, from seed germination and flowering to sprouting and vegetative growth. At each life stage, flooding can cause injury, changes in anatomy and growth form, decline, and death.

Seed Problems:
Seed germination requires oxygen and water. As the seed takes in water, its respiration increases. Flooding fills the gas exchange pores on the seed and limits oxygen transport. Depending upon the species and duration of flooding, seed viability quickly declines.
For example, bald cypress, water tupelo and black tupelo seeds remain viable for extended period of flooding. These seeds do not germinate under the flood water but when the water recedes. Cottonwood, willow, and sycamore seeds can germinate under water to hasten establishment on new areas caused by flooding. Green ash and boxelder, both wetland species, have seeds that loose vitality quickly after flooding.

Seedling Problems:
Flood tolerance of seedlings vary greatly.
Coverage of foliage with water is a critical feature to initiating flood damage — the longer the leaves are under water the worse damage becomes. Seedlings can also be buried by sediment and pushed over or swept away by flood water. Major damage can occur along the young stem and root collar area that permanently damages the tree.

General Tree Problems:
Soil anaerobic conditions initiate several symptoms of flood damage including: no growth or internode elongation; poor leaf expansion; limited leaf formation; leaf chlorosis; premature senescence; and, abscission (old leaves first). Leaf abscission can take from two weeks in upland species to eight weeks in wetland species after flooding. Wetland species can be damaged just as badly as upland species by grow- in season / warm weather flooding.
The usual symptom of a tree to flooding is a decline in growth. It is possible to have trees show in the first year after a flood an apparent increase in diameter growth. This is caused by the bark, phloem tissue, and xylem tissue producing more intercellular spaces and lower density cells. This low density material makes the tree look like it has made a large growth spurt. In some trees this growth pattern may. be a prelude to decline and death.
One growth response of trees to flooding is production of proportionally greater numbers and enlargement of thin-walled cells called parenchyma. These cell types are found in both the xylem and phloem. Resin duct number are also increased in species with these features. Flooding initiates production of low density cells and more intercellular spaces in the xylem, phloem and bark. The stem becomes lower in density to facilitate oxygen transport and removal of toxic materials.

Root Problems:
Tree root response to flooding is a reduction of root initiation and growth. Within seven days there is noticeable root growth loss. Many types of root limitations can be seen in normal soils with a shallow water table. Rooting depth is related to water table depth and the lower limits of adequate oxygenation.
A shallow water table generates a shallow root system. Roots only growth where soil atmosphere has 5% oxygen.
Flooding causes a loss of extent, reach and health of the roots. Over time, decline, death and decay are the results. Additionally, pathogens such as Phytophthora fungi attack the tree roots. This fungus tolerates low soil oxygens levels and is stimulated by poor host vigor and root membrane leakage. Generally, under flooded conditions, the woody roots survive and non-woody roots die. Loss of root mass through attack and decay leave the tree prone to drought damage the following growing period and to wind-throw.

Physiology Effects:
Flooding effects on the physiology of the tree generically include energy production, root membrane health, and coping with toxic materials in the soil.
Within five hours, photosynthesis is shut- down. Photosynthesis is limited by loss of carbon fixation enzymes, loss of chlorophyll, reduction of leaf area, and leaf abscission. Photosynthesis is an expensive system to maintain and very sensitive to internal and external environmental changes. Once flooding is over, it takes an extended time for photosynthetic activities to return to normal.
Root problems include the poor uptake of macro-essential elements and additional resource loss through leakage from root membranes. Nitrogen is lost in the soil and throughout the tree under flooded conditions. Nitrate in the soil is lost by denitrification to atmospheric nitrogen and through production of nitrogen oxides. At the same time, the roots are not taking-up available nitrogen. Flooding also suppresses mycorrhizae fungi which are strongly aerobic. Poor phosphorus and potassium uptake is an associated problem.
Flooding initiates many growth regulator production changes and effects how they are destroyed once they have carried their message. Auxin, ethylene and abscisic acid concentrations increase in flooded trees while gibberellic acids and cytokinins decrease. The result is leaf epinasty, senescence and abscission; stem growth disruption; and, adventitious root production.

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