Tree and Forest Health Guide

A Primer for Foresters


Forestry involves promoting and protecting the health of selected trees. Information on this page is intended to help Virginia foresters provide good advice about tree and forest health to landowners. Many excellent references exist to help identify and treat specific pest problems. Conditions that are common or important enough in Virginia to gain frequent attention are covered in this section. General guidelines are also given for the diagnosis and treatment of less familiar conditions.

Health is an abstract concept and has no definition that applies universally. Tree health is quite different from forest health. Generally speaking, a tree is considered healthy if it exhibits no symptoms or signs of injury or disease. Beyond this there is no commonly accepted basis for characterizing tree health. It is meaningless to ask how healthy a tree is. In reality, what we usually address is not health, but "sickness" because it is more concrete and we have some experience in dealing with it.

Guidelines for Promoting Tree and Forest Health

The following guidelines should be considered when making forest management decisions:

  1. Tree species survive and grow best within their natural ranges. Artificially extending these ranges involves risk. Occasionally it's worth the risk; many species perform well even on foreign continents. Experience is the only reliable guide. Trees do not always grow best on the sites where they normally occur; they just compete best in those places. When competition is not a factor, most species tend to grow best on deep, moist, well drained, fertile soils.
  2. Mix species. Mixed stands tend to be less susceptible to attack and less vulnerable to damage from pest organisms. If management objectives don't require pure stands, encourage a mixture of species.
  3. A full crown is necessary for optimum health and growth. Give crowns all the light they can use. The crown uses light energy to produce cellulose (a complex carbohydrate) from carbon, hydrogen and oxygen; roots only provide water and nutrients to support the process. A full crown will also ensure that roots have adequate space.
  4. A tree's apparent health does not always reflect current conditions. Trees usually respond quite slowly to environmental changes. They may decline over a period of several years before succumbing to prolonged stress; and it may take many years of favorable conditions before they recover fully from a weakened state.
  5. Injuries to boles and branches of hardwoods often lead to defect, degrade and decline. Thinnings and other partial cuts should be planned so that injuries are minimized and damaged trees can be removed as cutting progresses. This usually means beginning in the least accessible parts of a tract.
  6. Tree decline and mortality following significant soil disturbance. Roots can be damaged by soil compaction, grade change and mechanical injuries. Design access for partial cuts and construction sites so that soil disturbance is minimized around residual trees.
  7. Trees can live longer than people, but they don't live forever. Vigor decreases with advanced age. Harvest trees when they mature, or expect them to deteriorate.
  8. Well-designed and properly constructed roads help ensure long term forest health. If properly installed the first time, good roads help protect trees against injury, protect water quality and minimize site degradation from management activities.
  9. Unusual habitats tend to include species and environmental conditions that can be difficult to re-establish if lost. Protecting unique areas from disturbance is an easy and effective way to help maintain or enhance diversity.

Diagnosing Tree Problems

Most tree problems result from combinations of factors. When the cause of a health problem is not obvious, there are always clues that can help with diagnosis. However, every condition will not result in a satisfactory explanation. Often some abiotic (nonliving) influence, such as drought, makes trees more susceptible to invasion by biotic (living) agents such as fungi and insects.

The following guidelines should be helpful when diagnosing tree porblems:

  1. Problems caused by physical, chemical and environmental factors usually affect most or all plant species present, whereas problems caused by organisms seldom affect more than a few species, and often just one.
  2. Symptoms caused by organisms usually vary in space and develop over time; symptoms that appear suddenly, are relatively uniform and stabilize quickly, are probably not caused by an organism.
  3. Healthy buds support a prognosis of recovery; dead or unhealthy buds suggest that recovery is unlikely.
  4. Wilting indicates that water is not moving through the tree fast enough. The most common causes of wilting are root disease, vascular disease and drought. Vascular disease is usually caused by microorganisms; root disease can stem from physical or chemical injury, excess moisture, infection by microorganisms, and feeding by various animals.
  5. Symptoms often result from the effects of secondary agents, not the primary agent. Trees weakened by adverse weather, unfavorable site conditions, injury, competition or advanced age become more susceptible to infections and infestations by secondary organisms. Treatment related to these secondary agents will provide temporary benefits at best, unless the primary problem is also addressed.
  6. Agents that affect only foliage are unlikely, in themselves, to result in tree mortality. However, they can reduce growth and predispose trees to other problems. Agents that affect only heartwood (e.g., some decay fungi) can increase the likelihood of stem breakage, and can make trees unmerchantable, but they might have little effect on tree life span.
  7. Symptoms that seem to be associated with aspect, exposure, drainage or disturbance are very likely to involve an important environmental component; but organisms could still be the primary agent(s).
  8. Check with a magnifying glass before ruling out organisms. Look for frass, silk, eggs, shed skins, holes, or life stages of mites and small insects. If you suspect fungus infection look for fruiting bodies, lesions, cankers, resin or sap flow, resin soaking of stems or roots, or sapwood stain near the transition between healthy and diseased tissue.
  9. Genetic factors can have a noticeable effect on tree response to adverse conditions. Symptoms of ozone injury and needle cast infection, for example, can vary greatly among trees of the same species growing right next to each other.

Making Recommendations

The potential causes of tree problems are countless and complex, and practical treatment options are relatively limited and simple. The following guidelines should help you evaluate what is practical to do in most cases.

Yard Trees and Forest Trees

Landowners expect foresters to know something about yard trees as well as forest trees. In both cases, good advice hinges on knowing ownership objectives. Yard trees present such different circumstances from forests that they will be discussed separately.

Yard Trees

Trees in yards are usually valued for:

  • beauty
  • shade
  • screening
  • wildlife habitat
  • fruit
  • real estate enhancement
  • some combination of these.

Landowners are concerned most with the appearance and expected life span of their yard trees. Defect and degrade are often unimportant except when they create unacceptable hazards.

Regardless of the cause or nature of yard tree problems, the only practical treatment alternatives available to most homeowners are: mulching, fertilization, sanitation, watering and pesticide application.

Appropriate treatment(s) can be chosen without specific information about causal agents. Yard tree problems resulting from environmental stress or moderate site disturbance can usually be alleviated by improving soil conditions. Even healthy trees benefit from attention to soil quality. Important soil characteristics include aeration, moisture retention, fertility and drainage.

Mulching is the simplest way to improve and maintain soil characteristics. Apply a layer roughly two inches thick over as large an area as suits the landscaping scheme. Mulching also helps reduce injuries from lawnmowers and other equipment. Organic mulch is preferable; avoid piling it against the bole. Where soil compaction is already severe, aerate before mulching. Watering during drought helps, but is often impractical except for small trees and new transplants because of the large volumes required; occasional thorough soaking is best. Nitrogen fertilization is usually beneficial for trees in decline. Avoid changing grade level or drainage characteristics around established trees. Select species adapted to poorly drained soils or install drainage and condition the soil before planting in wet areas. See also How To Fertilize Shade Trees.

Infestation or infection by organisms can sometimes be ignored or reduced through sanitation. Removal of dead, dying or fallen twigs and foliage is usually harmless and often helpful. When removing infected twigs, cut well back into healthy tissue; sanitize pruning instruments between cuts if transmission of microorganisms is likely. Use of registered pesticides should be considered only after the landowner is aware of alternatives, consequences, costs and benefits and when unacceptable damage can be prevented through pesticide application. For large trees, pesticide applications should be left to companies that have the specialized knowledge and equipment required for such treatments. The cost per tree is usually high. See also How To Prune.

Forest Trees

High timber value is an asset even when it is not an ownership objective.Private forests are usually valued for:

  • recreation (including wildlife benefits)
  • screening
  • environmental protection
  • income
  • investment
  • financial reserve
  • inheritance
  • some combination of these.

The benefits of preventing or treating forest tree problems depend largely on the perspective and disposition of the landowner. Prevention through proper thinning, sanitation and protection is usually most practical.

These tenets will help you establish and maintain healthy forests:

  • match species to site
  • favor species mixes where practical
  • protect unusual habitats
  • give desired trees plenty of light and growing space
  • prevent or avoid unnecessary site disturbance and tree injury
  • remove undesirable trees
  • harvest rees before their quality begins to decline.

Pesticide applications and other special treatments are expensive and should be subjected to cost/benefit analysis. Sometimes, the value of a single, high quality tree is enough to cover the management costs for several acres.

Tree Problems: Diagnosis and Treatment

  • Several to many unrelated species affected.
  • Only a few or related species, or only a single tree affected.
    1. Signs of feeding or oviposition present: generalist feeders such as gypsy moth, fall cankerworms, sapsuckers, beaver and deer; oviposition injury by cicadas; small area clearing by woodchucks or mound ants. Defoliation of healthy trees can usually be ignored; protection of unhealthy trees, if desirable, should be accomplished before significant feeding has occurred. Appropriate barriers can protect individual trees against birds, mammals and cicadas.
    2. No such signs: weather damage; air pollution; chemical injury; site disturbance; fire; flooding. Such conditions are usually beyond practical treatment; trees often recover on their own.
  • Symptoms relatively uniform within tree and among trees so that affected parts (or whole tree) look very much the same wherever they occur.
  • Symptoms vary within or among trees and may change or progress noticeably over a period of a few to many days.
    1. Whole tree involved to some extent (i.e., dead, discolored, wilted, missing, debarked).
    2. Some tissues or parts of tissues not involved: chemical injury; air pollution; heat or cold injury; lightning, hail or squirrel injury; some insects, mites or microorganisms. Such conditions are usually beyond practical treatment.
  • No above ground evidence of biotic agents; some foliage or twig distortion possible: herbicide, salt or other chemical poisoning; root problem from infection by microorganisms, site disturbance, flooding, girdling by insects or mammals, drought, transplanting shock, shallow soil, winter injury or suppression; vascular disease from microorganisms, including nematodes; storm damage; lightning. Such conditions are usually beyond practical treatment.
  • Above ground symptoms of infestation, infection or other injury present: mammal damage; bark beetles; severe foliage injury or defoliation by caterpillars, mites, lacebugs, miners, beetles, scales or fungi. Sanitation often helps; pest suppression is sometimes appropriate.
    1. Symptoms include one or more of the following: feeding injury, frass, silk, galleries, holes, shed skins, waxy secretions, honeydew, galls, slits, stage of causal agent (use hand lens; dissect galls, affected twigs, bark): insects, mites, mammals, birds. Pest suppression is sometimes appropriate on Christmas trees and ornamentals, especially for scale insects and mites.
    2. Symptoms include one or more of the following: cracks, lesions, stains, fruiting bodies, resin flow, cankers, galls, leaf spots or blisters, scorch, bleaching, browning, premature abscission, chlorosis, mottling, epicormic sprouting, necrosis, decay: microorganisms, weather, soil conditions. Sanitation often helps.

Last modified: Thursday, 06-Nov-2014 10:23:26 EST