Aspen Forest Health Briefs are seasonal, science-based updates from the City of Aspen Forestry Program that explain current tree and forest health issues affecting our community. Topics range from bark beetles and winter stress to drought, storm damage, and emerging pests. Each Brief provides clear context, practical guidance, and information on how the City and community can work together to care for Aspen’s forest over time.
Pests and Pathogens
Insect Pests
Aspen’s urban and wildland-urban interface forests are home to several native insects that can become problematic when trees are stressed. The most significant in our open spaces are the various bark beetles, including Douglas-fir beetle, spruce beetle, and several Ips species. These insects take advantage of drought-weakened trees and can cause rapid decline when populations rise. In the valley’s urban areas,cottonwood leaf beetle is also highly prevalent and defoliates narrowleaf cottonwoods during outbreak years. While many native insects play natural ecological roles, their impacts intensify when drought, heat, or injury reduces tree vigor.
Pathogens
Several fungal pathogens affect trees in and around Aspen, especially those already weakened by age, drought, or mechanical damage. Ganoderma is one of the most important, contributing to internal decay that can compromise structural stability long before external symptoms appear. Cytospora canker is another common pathogen in spruce, aspen, and cottonwoods, often appearing after environmental stress. Needle blights and needle cast diseases also surface intermittently, particularly in ornamental spruces. Most fungal pathogens exploit existing stress rather than causing it, which means early detection and good tree care practices are essential.
Environmental Conditions
Tree health in the City of Aspen is shaped just as much by environmental conditions as by pests or pathogens. Drought stress is one of the strongest predictors of insect and disease activity in our region. Freeze–thaw cycles, winter desiccation, heavy snow loads, and sunscald all weaken trees and create entry points for insects and fungi. Longer-term climate trends may reduce natural mortality of insects like bark beetles and contribute to tree stress across species. Managing these underlying conditions is a major focus of our urban forestry program.