During an outbreak, forest insects and diseases can cause moderate to high rates of mortality (>1% of forested area) in short periods. This indicator is a comparison of mortality rates as area or volume compared to a 1% threshold. The reporting units for this indicator may include ownership, species or plant type, type of insect/disease, and by exotic/native insect/disease type. It may also be reported as a percentage of area or volume.

Forested areas may experience rapid mortality rates during an outbreak of an insect or diseases. This indicator describes risk of mortality as area of forest, or volume of trees, using a 1% mortality rate in 15 years threshold. Reporting units may include ownership, plant species or groups, type of insect/disease, and exotic /native insects/disease as a group.

Ozone is a airborne pollutant produced by industrial activity that can damage human and ecosystem health. This indicator reports on the area of forest and/or volume of trees affected by ozone. The reporting units could be ownership type, forest type, or bioregion.

As more and more trees die in an area from insects or disease, the risk of fire ignition and spread increases. If swaths of trees die near forest communities, it could put the community at risk from fires.

Area and volume of forest pest damaged trees above the 1% mortality threshold. This evaluates current extent and damage from forest pests.

Estimate Area and volume of forestland at risk of experiencing greater than 1% mortality in the next 15 years. This evaluates risks to forestland from future forest pest outbreaks.

A parallel means of tracking level of agency and program commitment to stopping the spread of forest pathogens and insect pests. Dollars spent or appropriated by state and federal for programs to combat the spread of forest pathogens and pests.

The historically high levels of mortality seen in the early 1990s in the Sierra and Modoc bioregions have declined in recent years, although new pests are beginning to become established that threaten forest and rangeland resources. Several pests and diseases are of particular interest including sudden oak death (Phytophthora ramorum), eucalyptus borer (Phoracantha sp.), white pine blister rust (Cronartium ribicola), and pitch canker (Fusarium subglutinans). Sudden oak death is spreading through a variety of tree and shrub species in 12 coastal counties of California and is continuing to be found in new hosts. Eucalyptus borer and related exotic Australian defoliators cause significant damage to urban southern California eucalyptus trees. White pine blister rust, a disease with a long history in California, continues to threaten sugar and other pine species by affecting regeneration and size class distributions. Pitch canker, which affects coastal pine species, is in decline although no remedy for eradication of the disease has been identified. (from the 2003 Forest and Rangeland Assessment)

There are many sources of stress on forests, including climatic, human, and biotic. Naturally occurring and introduced insect pests and diseases can kill large numbers of trees in contiguous areas of forest until natural limits curtail the insects or diseases. Introduced (weeds) or naturally-occurring plants can stress trees, for example, mistletoe is a parasitic plant that can create structural and nutritional pressure on individual trees. Over-browsing or grazing by animals can reduce succession or seedling recruitment rates in forests (e.g., cattle consumption of seedling oak trees).

Trees die for a variety of reasons, creating habitat, contributing to soil carbon and nutrients, and creating openings. Rates of mortality can indicate natural die-off of old or sick/damaged trees or forest and trees that are overly-stressed because of climatic, artificial, and pest sources of stress.