A U.S. Forest Service aerial survey in April found 20 percent of the trees in a 4.1 million acre area in the Southern Sierra were dead.
"Those areas in particular have been suffering from the most severe drought conditions for the longest period of time and now the effects are really becoming apparent," says Jeff Moore with the U.S. Forest Service.
Coastal redwoods and giant sequoias are losing their foliage in the drought as well says UC Berkeley ecologist Todd Dawson. He says leaves on Blue Oak trees in the Central Valley are only a quarter of the size they should be.
"People are making these observations across the state, so all the way from Southern California even up into the northern parts of the Central Valley, and I think the only place we’re not seeing real severe effects are maybe right up the northwestern part of California," Dawson said.
The U.S. Forest Service plans to conduct a new aerial survey to determine any further mortality, especially among oak trees.