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Impact of Pests on Southern Forests

Insects and diseases can negatively impact forests in several ways. They can kill trees; reduce their growth; degrade wood and other products; cause dieback, decline and deformity; change the composition of the forest; reduce biological diversity; affect water quality and quantity; create safety hazards; increase fire risk; reduce the quality of the landscape; and cause other kinds of damage. Some of these types of damage may not be significant if they are not detrimental to the intended use of the forests.

It is important to note that pest outbreaks do not respect ownership boundaries. While the management strategies discussed below may lead to a measure of protection of forest lands from destructive insect or disease activity, failure of a landowner or land-manager to control pest outbreaks can (and often does) affect other owners lands. Passive management of forests can easily lead to pest population spillover and negatively affect forest resources of adjacent landowners.

Although impact can be expressed in many ways, it is usually measured in relation to number of trees killed, volume of timber lost, area of defoliation, or amount of growth loss resulting from pest activity. It has been estimated that forest insects account for 20 percent of the total negative growth impact on forest trees, while diseases account for 45 percent of it (Tainter and Baker 1996). Recently foresters have tried to express impact using values, such as quality of the landscape, water quality, biological diversity, and other values, that refer to the intended use of the forest ecosystem but are very difficult to assess objectively.

Native disease-causing organisms and insects are natural components of ecosystems. They often have a positive impact by contributing to biodiversity, improving habitat for various flora and fauna, and hastening decomposition and ecological succession of the forest (Coulson and Witter 1984).

Whether the effects of insects or diseases are perceived as positive or negative depends on the intended use of the forest. In a “natural” forest native insects and diseases are simply part of the ecological processes that maintain a mosaic of ages and stand conditions. Dead and dying trees contribute to the health of natural forests by contributing to the crucial processes that recycle elements from dead or downed trees. They also are among the mechanisms driving removal of the weakest and favoring the healthiest trees in any stand.

In an industrial plantation, where profit from wood is the primary objective, the presence of dead and dying trees is not generally considered a healthy condition. The more intensive the forest management, the more forest pests become potential threats for the intended use of the forest. However, with more intensive management this potential damage is generally precluded by management practices designed to forestall pest-caused damage. Impacts of insects and diseases can be even greater in urban forests, where buildings and other structures and peoples’ lives are threatened by falling trees or branches.