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Defoliation and Its Effect on Tree Vitality

A Technical Report from The Bartlett Tree Research Laboratories

The plant leaf functions primarily in the manufacture of sugars and carbohydrates. These substances are the basic food or energy sources for all metabolic processes in the plant including growth, root development, flower and seed production, disease resistance, etc. Leaves also provide many indirect benefits such as emitting oxygen, scr eening out particulates and other air pollutants, intercepting precipitation to minimize erosion and shading the ground to modify surface temperatures.

Shade trees in an urban environment are subject to premature defoliation from a variety of factors including insects, diseases and environmental and cultural adversities.

Effects of defoliation on trees can range from a slight reduction in vigor to total death. Defoliation harms plants by eliminating their food production capability. The refoliation process, which frequently occurs immediately following defoliation, also requires energy for budbreak and leaf expansion, which causes further depletion of stored food reserves. The inability of the tree to manufacture food (energy) together with the depletion of stored food weakens the tree and results in reduced growth, stunted, pale-green new leaves and possibly twig and branch dieback. Mortality of small feeder roots also frequently occurs.

The tree also is changed physiologically from defoliation. The production of protective substances that aid in disease resistance may be inhibited. Natural growth regulators also may be modified which may delay dormancy. These changes result in increased susceptibility to certain insects and diseases and reduced winter hardiness.

Factors Influencing Defoliation's Impact

Complete tree recovery following defoliation may take as long as five years. In some cases, trees will not recover and death occurs either immediat ely or within several years following defoliation. The exact effect of defoliation on tree growth and survival is influenced by the interaction of many factors including the severity, timing and frequency of defoliation, initial tree condition and presence of secondary insects and disease organisms. These factors must be considered collectively when evaluating the fate of a defoliated tree.

Severity of Defoliation

Generally the greater the foliage removed, the greater are the adverse effects. However, healthy trees with a full crown can tolerate up to 50% loss in foliage without a significant reduction in vigor. Refoliation usually occurs when more than 50% to 75% of the foliage is removed unless defoliation occurs late in the growing season when the tree is entering dormancy. Refoliation is particularly debilitating to the plant due to the additional depletio n of stored food and the detrimental physiological effects associated with this process.

Timing of Defoliation

Defoliation early in the growing season when leaves just reach full expansion is most detrimental. At this time, considerable energy has been expended in budbreak and leaf development, but food reserves are not replenished by photosynthesis due to the foliage loss. Refoliation usually occurs following heavy defoliation early in the season, which will further weaken the tree. Late season defoliation is seldom injurious because leaves have already manufactured and stored most of the needed carbohydrates. However, if refoliation occurs in the late season, plant tissues would not harden off by winter and considerable injury could result.

Frequency of Defoliation

Most healthy trees can tolerate a single heavy defoliation with only a reduction in vigor resulting. Exceptions are evergreens, which are usually killed by one complete defoliation. Two to three consecutive years of early season defoliation can kill even the healthiest trees.

Tree Condition

The relative vigor of a tree has a large influence on its ability to tolerate defoliation. Vigorous trees that have been cared for by periodic pruning, fertilization, watering, etc. are better able to withstand defoliation than trees weakened by drought, disease, new construction, mineral deficiencies, etc. Older trees are also more sensitive to defoliation than young trees.

Secondary Insects and Diseases

Secondary organisms are those that invade a tree after it has been weakened by a stress-inducing factor such as defoliation, drought, etc. The se condary nature of the organism refers to its sequence in time and does not refer to their importance in causing decline and death of the tree. Common secondary organisms invading defoliated trees include insect borers, bark beetles, root decay and canker (stem disease) fungi. These are usually responsible for the ultimate death of the plant.

Preventative and Corrective Measures

Proper initial selection of tree species that are adapted to a site and resist insect and disease damage will reduce the risk of defoliation. Timely insect and disease control also will help prevent defoliation. Trees that have received periodic care including pruning, fertilization, watering during dry periods, mulching, etc. will better tolerate and survive defoliation.

If a tree is defoliated, watering during dry periods is recommended to aid the refoliation process. Fertilization with a quick release high nitrogen fertilizer will also help encourage rapid refoliation and help replenish nutrients lost due to defoliation. Quick release fertilizers are not recommended after mid-season (approximately July 15) as this could encourage excessively late growth, which may not harden off in time for fall frosts. With defoliation in mid to late season, fall fertilization is recommended after the tree has become dormant.

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