Forest ThreatNet

Five consecutive years of drought and warmer than normal temperatures have created prime conditions for bark beetle and wood borer outbreaks in parts of California. The result: an estimated 102 million trees died in 2010 through 2016. This standing fuel is a fire hazard that could threaten human lives, property, water supplies, wildlife habitat, and other forest resources.

Although the number of trees expected to die in California by the end of this summer is less than in 2016, areas of the southern Sierra Nevada are still projected to experience significant new mortality, ranging from 2,000 to 14,000 trees per square mile. This annual forecast, which was delivered to land managers well before fire season, is the product of collaboration between the Western Wildland Environmental Threat Assessment Center, the Pacific Southwest Research Station, and Forest Health Protection.

By having the forecast up to 10 months prior to expected mortality, land managers can better prioritize hazard tree removal, pest suppression activities, fuels reduction treatments, and access to areas with higher than usual wildfire risk. For example, the El Dorado National Forest is using this information to communicate restoration activities needed to reduce fire hazard. Other national forests in the Pacific Southwest Region as well as the California Department of Forestry and Fire Protection (CalFire) are also using the forecast to help protect public health and forest resources.

Pictured: The 2017 tree-mortality forecast for California. Red areas indicate intense levels of tree mortality due to drought, bark beetles, and wood borers; dark blue areas represent a low likelihood of any mortality. Click to enlarge.

The forecast is based on a tree mortality model that uses data available in the fall before the following growing season. Data sources include Forest Service Aerial Detection Surveys, Forest Inventory and Analysis, precipitation and temperature, and fire data from CalFire. For more information, see: Preisler, H.K.; Grulke, N.E.; Heath, Z.; Smith, S.L. 2017. Analysis and out-year forecast of beetle, borer, and drought-induced tree mortality in California. Forest Ecology and Management. 339: 166–178. https://doi.org/10.1016/j.foreco.2017.05.039.