Integrating the ecological roles of phytophagous insects, plant pathogens, and mycorrhizae in managed forests

Insects, fungi, other arthropods, and bacteria oftenare overlooked, both in catalogs of forest dwellersand in discussions of forest ecosystem function.Though small, these organisms are numerous and di-verse. In most forests, they comprise the greatest por-tion of biological diversity and even a surprising por-tion of the biomass, especially belowground. Theyimpact many ecosystem functions, sometimes con-trolling them, and their populations can fluctuatedramatically, responding quickly to changes in re-source availability and environment. Some are considered pests when they interfere with managementobjectives, usually by killing or damaging valuabletrees, and others are valued for their essential contri-bution to survival and growth of planted trees.
Much has been written about the economic lossescaused by tree-killing insects and plant pathogenicfungi in forests. Similarly, the benefits of mvcorrhizalfungi to tree growth are well documented. In thischapter, we aim to broaden the traditional view offorest pests and symbionts in order to consider themas part of functioning ecosystems, and thus to provide a better basis for predicting their responses tochanges in forest management and other distur-bances. To reach this goal, we provide a very brief re-view of insect and fungus biology as background to adiscussion of the contributions of insects and fungi,especially herbivores, pathogens, and mycorrhizae, toecosystem function. From this base, we discuss theresponses of phytophagous insects, tree pathogens,and mycorrhizal fungi to changes in forest conditionsas a basis for integrating these organisms into the"new" practices of ecosystem-based forest manage-ment. One of the keys to successful forestry is under-standing and accepting the species and site speci-ficity of the knowledge base. Our examples comelargely from the forests of the western United Stateswith which we are most familiar. Different organismsin different environments will respond differently,but we hope to establish underlying principles toallow predictions in new situations.