The health of forests and trees is important for many reasons and can mean many things to different people in different situations. The most widely used definitions of forest health tend to incorporate two aspects:

The forest’s ability to sustain unique species composition and processes, and
The forest’s ability to accommodate present and future needs for a variety of values, products, and services.

Included within those aspects is the forest’s ability to renew itself and be resilient against disturbances.

Disturbances

All forests experience disturbances. Disturbances are any activity that disrupts forests; injures, weakens, or kills trees; or changes the structure or composition of the ecosystem. Disturbances are natural and necessary in forests. Disturbances allow forests to renew, return nutrients to the soil, and can provide important habitat for wildlife. Types of disturbance can include:

insect activity,
disease-causing pathogens,
fire,
weather or climate events,
animal activity, and
human activity, among others.

While some disturbance is natural and part of a healthy forest, too much disturbance or changes in frequency, timing, or severity of disturbances can have negative impacts on our views of healthy trees and healthy forests.

In Alaskan forests, insect outbreaks, pathogens, wind events, and fires are common disturbance types. These disturbances can be normal and healthy in the forests; however, large-scale events such as the spruce beetle outbreak of the 1990’s on the Kenai Peninsula and the Copper River Valley, can be devastating.

Spruce mortality as a result of spruce bark beetle infestation. Photo by A. Steven Munson, USDA Forest Service, Bugwood.org

A fire damaged forest. Photo by Doug Page, USFS/BLM, Bugwood.org

Downed trees after a wind storm. Milan Zubrik, Forest Research Institute-Slovakia, Bugwood.org

Invasive species are also disturbance agents in our forests and urban trees and can be devastating for forest health since invasive species pose challenges that native pests do not.