Three indicators of forest health are being studied: disturbances, air pollution, and diminished biological components.

Disturbances: Historically, large fires caused the greatest disturbance to forests. More recently, timber harvest has replaced fire in this role. Harvest units are much smaller than historic fires like the Tillamook burns of the 1930s;
on the other hand, we tend to harvest forests more frequently than they were burned, on average, by large fires.

Other disturbances have had a significant impact on the health of Oregon's forests, including: Swiss needle cast, a fungus that attacks Douglas-firs growing near the coast; and bark beetles and spruce budworm, which have killed millions of trees in central and northeastern Oregon.

Air pollution: Fortunately, this is not much of a factor in Oregon, where air quality is much higher than in the eastern United States or in Europe (where acid rain has significantly damaged forests).

Diminished biological components: Some scientists are concerned that timber harvesting may reduce key features of the ecosystem important for wildlife:

• large trees,
• snags,
• riparian areas
• down logs,
• shrubs,
• hardwoods

Over time, with increased understanding of the importance of key biological components, the Oregon Forest Practice rules have been changed to provide better protection, including retention of snags and down logs after harvest.