Drought, pests and overcrowded forests are contributing to a tree mortality epidemic in the Sierra Nevada that’s rapidly spreading, the leader of a state task force says. Aerial surveys by the U.S. Forest Service last year found 36 million more dead trees, bringing the number of trees that have died in California forests since 2010 to more than 102 million, according to the state Tree Mortality Task Force. The mortality epidemic has spread from the Fresno area to Placer County and is continuing to move north, said Gabe Schultz, the task force’s Redding-based chairman.