Researchers at the U.S. Forest Service (USFS) have completed an aerial survey of the Sierra Nevada and certain parts of Southern California, covering 8.2 million acres of forest, and the view from the air is alarming. Thanks to the worst drought in over a 1,000 years, trees in California are dying by the millions. The USFS estimates that about 12.5 million have already died in the state's national forests, and the massive die-off is not over, as millions more are expected to die over the summer.
All these dead trees and dry conditions are making California's forests dangerously flammable. This means that any small fire can turn into a huge blaze, potentially destroying many more trees as well as killing wildlife and damaging other property.
The drought isn't necessarily killing the trees directly, but it has made them more vulnerable to insects like the Bark Beetle. Just last year, it's estimated that the tiny pest has ravaged 800,000 acres of trees in the state.
The bark beetle preys on California pines, feasting on thirsty, defenseless trees weakened from years of drought.
“They’re very small, very tiny beetles, about the size of a grain of rice,” said Cal Fire Regional Manager Glenn Barley. The beetles eat a path through the bark, and then lay their eggs.
One mating pair can reproduce more than 12 million beetles a year. At that rate, it’s hard for forestry experts to keep pace with the problem. Tree removal expert Noah Whitney said he has been cutting trees for 17 years and has never seen it this bad. (source)
What weakens the trees against the beetles is that without sufficient water, trees can't produce enough sticky resin that acts as a defense mechanism.