• Witter, John A.
  School of Natural Resources, The University of Michigan, Ann Arbor
  
• Lynch, Ann M.
  School of Natural Resources, The University of Michigan, Ann Arbor
  

Numerous studies in Eastern North America during the last 40 years have recorded the amount of tree mortality in individual spruce fir stands during outbreaks of the spruce budworm (Witter er all 1984). Mortality in mature balsam fir stands often ranges from 70 to 100 percent; in immature stands, mortality often ranges from 30 to 70 percent (MacLean 1980). However, mortality in an individual stand can be less. High mortality is more likely in stands with the following characteristics:

1. Fifty percent or more of the stand composed of balsam fir, white spruce, and/or red spruce.

2. Mature fir stands, 50 or more years old.

3. High basal area of balsam fir, white spruce, and/or red spruce.

4. Open stands in which spiked tops of host species protrude from the forest canopy.

5. Stands on poorly drained soils that are abnormally dry or wet.

6. Extensive stands of mature spruce-fir type.

7. Stands located downwind of the current outbreak.

8. Stands growing at elevations lower than 2,300 ft (701 m) and south of the 50 degree latitude (a line extending from the north shore of the Gulf of St. Lawrence to Winnipeg, MB).

A number of rating systems have been developed to assist land managers in predicting the vulnerability of spruce-fir stands to the spruce budworm. This handbook describes the objectives and uses of rating systems in spruce0fir management and the most recently developed rating systems.