In recent decades there have been observations of widespread declines in vigor and dieback of mature forests in many parts of the world. In many cases, pollution may be a factor contributing to forest decline, for example in regions where air quality is poor because of acidic deposition or contamination with ozone, sulfur dioxide, nitrogen compounds, or metals. However, forest decline also occurs in some places where the air is not polluted, and in these cases it has been suggested that the phenomenon is natural.

Forest decline is characterized by a progressive, often rapid deterioration in the vigor of trees of one or several species, sometimes resulting in mass mortality (or dieback) within stands over a large area. Decline often selectively affects mature individuals, and is thought to be triggered by a particular stress or a combination of stressors, such as severe weather, nutrient deficiency, toxic...