Enormous numbers and varieties of insects, fungi, bacteria, and viruses occur in forests and are adapted to live on or around trees. Many of these are beneficial, and even the destructive ones are usually held in check by their natural enemies or an unfavourable environment. The normal population levels of pest organisms result in limited reduction in tree growth or the total destruction of only a small number of trees in the forest. The losses are generally accepted by foresters as unavoidable and are tolerated as long as the annual destruction does not seriously affect the net annual increase in wood production.

Every part of a growing tree—root, trunk, bark, leaf, flowers, and seeds—is potentially subject throughout every stage of its life to attack by some harmful insect or fungus. Insects actually destroy more standing timber than does any other agent. Bark beetles, including species of Dendroctonus and Ips, are among the most destructive insects. They bore into the tree and feed just below the bark, where they create tiny channels that disrupt the flow of food to the roots, often killing the tree. Diseases frequently retard growth of trees and are less of a factor in mortality. A particularly destructive disease is caused by fungi that decay the wood of trees. The heartrot fungi gain entrance through any wound resulting from fire scars, broken limbs, or anything else that damages the tree’s protective tissue. Were it not for heartrot, a large number of conifers and broad-leaved trees could be left to grow for many more years.

Insect and disease organisms accidentally introduced to forests from other parts of the world, known as invasive species, often develop serious epidemic conditions because of the lack of any natural control. Because of rapid global transportation, insects and fungal spores can be spread easily throughout the world and arrive in a healthy condition. The seriousness of the situation cannot be overestimated, and the enforcement and improvement of plant quarantine laws is essential. Typically disasters have arisen where quarantine has failed or has been imposed too late. The American chestnut, Castanea dentata, has been virtually wiped out by the chestnut blight fungus, Endothia parasitica, which does little harm to related trees in its native China. Elms have suffered severely, both in Europe and in the United States, from the elm disease fungus, Ceratocystis ulmi, which was first detected in the Netherlands and is carried from tree to tree by flying beetles. Minute aphids, probably introduced on living plants from Asia, now make it impossible to raise commercial crops of two conifers once valued in Britain, namely, the white pine, Pinus strobus, from New England, and the European silver fir, Abies alba, native to Switzerland.

Generally the healthier the forest, the more resistant it is to widespread pest attack. Overmature, weak, wind-thrown, and lightning- or fire-killed trees have little or no defense against infestation and are a factor in the buildup of pest populations. Selective cutting of susceptible trees, thinning that accelerates growth, and other similar long-range forest management practices that stimulate vigorous tree growth are good methods for indirect control of insects and diseases. These practices reduce the host material and breeding grounds of pests that may spread to healthy trees. In regions with a high incidence of a known pest, foresters attempt to avoid serious trouble by planting only trees known to resist existing pests in the regions where the trees are grown. Many forest genetic programs have as a major goal the selection and breeding of trees with insect and disease resistances.

Occasionally the natural conditions that suppress the population of pest organisms change, and outbreaks in forests may reach epidemic proportions. Even-aged stands and plantations with trees of the same species and of uniform size and age often create perfect conditions for the rapid spread of insects and diseases. Even the more complex uneven-aged forests with their inherent check-and-balance systems can develop devastating populations of pests. At this point the forester must consider direct control measures.

Because effective direct control of insects and diseases of standing timber is generally expensive, it is employed only when the potential mortality or loss in growth is extreme. Routine monitoring of insects and diseases allows foresters to schedule timely harvests of infested trees and to limit the spread of the problem to uninfested trees or areas. These sanitation and salvage harvests, coupled with piling and burning the limbs and branches left after logging, reduce the material and conditions that allow pest populations to develop. Traps baited with sex-attractant chemicals, or pheromones, are a promising method to reduce breeding populations of certain insects. Application of insecticidal or fungicidal sprays from the ground or from low-flying aircraft offer a short-term measure to check sudden plagues of insects or outbreaks of fungal diseases. Action has most frequently been taken against exceptional outbreaks of defoliating caterpillars, including those of the spongy moth in the United States, the nun moth in central Europe, and the pine looper moth in England. At the time of year when feeding caterpillars are most vulnerable, light aircraft fly across the forest on carefully planned courses, distributing pesticides.

A disadvantage of these blanket treatments by potent, broad-spectrum chemicals is that they also eliminate parasitic and predatory insects that serve as natural controls on the pest’s numbers. They may also adversely affect birdlife and can contaminate waterways and affect aquatic invertebrates. In practice, large-scale chemical treatments of forests are infrequent and are restricted to a small proportion of the areas at risk. Generally, natural control through predatory organisms, which also cycle opportunistically in a slightly delayed sequence with the pest populations, combined with physical factors like cold winters, provides adequate checks. Biological control involving the release of predators or diseases of pests is promising in some situations.

Less spectacular preventive measures are commonly taken as routine steps in practical forestry to lessen anticipated losses. Nursery stock, easily reached and handled, may be grown in fumigated seedbeds and sprayed during production so that uninfested and vigorous seedlings are planted in the forests. Prompt removal of logs from the forest to distant processing mills transfers beetles that may emerge from beneath the bark to areas where they can do no harm. Stumps of freshly felled conifers can be easily and cheaply treated by brushing on a fungicide to check the white root-rot fungus, Fomes annosus. This is a serious agent of decay that spreads underground through root grafts after gaining entry via the exposed surface of a felled stump.

Agroforestry

Agroforestry is a practice that has been utilized for many years, particularly in developing countries, and is now widely promoted as a sustainable land-use approach that yields both wood products and crops. Trees and crops may be grown together on the same tract of land in various patterns and cycles. The trees may be planted around the perimeter of a small farm to provide fuelwood and to serve as a windbreak. The limbs and foliage may be removed periodically for livestock fodder. Trees also may be planted in rows that alternate with crops or they may be planted more densely with interplanting of crops until crown closure of the trees precludes further crop production. These practices are most extensively used as a part of subsistence agriculture, but their use in large-scale production systems is becoming more common.