Trees in California are dying at the highest rate in at least 15 years, raising the risk of faster-moving and more-intense forest fires, according to the U.S. Forest Service.

Aerial surveys conducted by the service last year estimated a tenfold jump in tree mortality since 2014. According to results released last month, an estimated 27.6 million dead trees were found in the forest landscape last year. In 2014, an estimated 3.3 million dead trees were identified. The statewide aerial surveys date back to 2001.