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The Role of Forest Insects and Diseases in Aspen Ecology
John Guyon, Forest Pathologist, Forest Health Protection, USDA Forest Service, Ogden, Utah
A forest decline in is often described as a syndrome involving multiple abiotic and biotic factors leading to the death of trees. Many agents can cause stress contributing to aspen forest decline including abiotic agents such as drought or frost, and a wide range of biotic agents including grazing animals, humans, and forest insects and diseases. Insect and disease incidence in aspen the Intermountain Region ranges from 18 to 85% in trees over 5” DBH from surveys conducted on several national forests, but the actual rate of mortality caused by these native organisms has only been examined in a few areas by repeated measurements over time. Where they have been reported, average annual aspen mortality rates varied from 1.1 to 5.2 percent on trees between 2.5 and 30 cm DBH. Insects and diseases can play a wide range of roles in aspen forest ecology, and the roles they play are different in young suckers versus large stems. They can cause outright mortality or contribute to decline or cause insignificant minor damage. For example, the impact of an outbreak of defoliating insects or foliar fungi can stimulate a clone to produce new sucker sprouts or contribute to mortality if additional stresses are present. Recognition of the roles that all contributing agents play in aspen decline is critical to future successful aspen management.