Findings released this week by the U.S. Forest Service show that approximately 6.3 million trees are dead in the Sierra foothills.
The drought and bark beetle infestations are to blame.
A similar survey in April found more than 12 million dead trees in portions of the Sierra. The new report looked specifically at the foothills between Placerville and Porterville.
Researchers flew over the entire survey area in a small plane in July.
The trees in the southern portion of the survey were the hardest hit. In some cases, large groves of trees are completely dying off, leaving what is known as a "ghost forest," where seeds are unable to sprout and grow into a new forest.
The forests of the high Sierra are generally doing better than the foothills, said Jeffrey Moore, one of the aerial survey managers.
"They're usually happier further up slope where it's cooler," explained Moore. "They get a little more moisture. Things aren't as extreme."
Dead trees are of great concern to firefighters because they burn easily.