Sustain Our Nation's Forests and Grasslands

Predictor map forecasts reduced likelihood of tree mortality in 2018

CALIFORNIA — The odds are in Californian forests’ favor that they may be getting a reprieve from the recent tree mortality epidemic, at least statistically speaking. For the third consecutive year, a scientist with the U.S. Forest Service’s Pacific Southwest Research Station and her collaborators have created a map representing the projected number of trees likely to suffer from beetle- or drought-related mortality.

Research statistician Haiganoush Preisler reports that between 3 to 26 million trees are projected to die in 2018 from drought or beetles, down from 27 million in 2017 and a peak of 62 million in 2016.

“Our past projections have accurately identified more than 95 percent of the areas in the state which have suffered high levels of mortality,” Preisler said. “While it’s difficult to determine precisely how many specific trees might die at a location, just knowing the areas most likely to experience mortality is helpful information for land managers.”

The maps are generated by combining aerial survey data with variables known to influence bark beetle-induced tree mortality, such as low precipitation and high beetle pressure (as indicated by high levels of tree mortality in prior years). The methodology for the forecasting model can be read in the September 2017 issue of the peer-reviewed journal Forest Ecology and Management.

The predictor map is a valuable tool for land managers to adapt management strategies or prioritize treatment locations to coincide with areas most likely to experience mortality within the next two years. Since 2010, prolonged drought coupled with a bark beetle outbreak has resulted in a record 129 million dead trees across 8.9 million acres in California.

Graphic: four copies of the map of California, each higlihgted with different colors to correspond with tree mortality progression from 2015 to 2018
This series of maps shows the progression of tree mortality severity across California since 2015.

This color chart represents the risk levels or potential number of trees that are likely to succumb to bark beetle- and drought-induced mortality, ranging from one tree per square mile per year to more than 10,000 trees per square mile per year.