Trees died throughout the state but especially in the southern Sierra Nevada. Trees weakened by drought could not defend themselves from beetles, who burrowed under the bark and laid eggs. The larvae hatched and their burrow killed and girdled the trees. Other trees died directly as a result to drought.
University of California and other researchers have been collaborating to measure tree mortality in the Sierra Nevada and identify the causes and future trajectory of affected stands. For information including data summaries and events, please see the Tree Mortality Data Collection Network publication.
Bark beetles have had massive outbreaks throughout western North America in recent years. More information on the causes, control, and consequences of bark beetle outbreaks can be found in this August 2017 webinar by Dr. Jodi Axelson, UC Cooperative Extension forest health specialist.
Resources
What to do with dead and dying trees?
What to Plant After the Trees Die - Article
What to Plant After Tree Loss - Brochure
California Forest Stewardship Helpline
Tree Mortality: Master Gardener Tool Kit
County Resources
Amador County Tree Mortality Task Force
Calaveras County Tree Mortality
Calaveras County Tree Mortality Program