The number of dead trees in California's drought-stricken forests has risen dramatically to more than 102 million in what officials described as an unparalleled ecological disaster that heightens the danger of massive wildfires and damaging erosion.

Officials said they were alarmed by the increase in the number of dead trees, which they estimated to have risen by 36 million since the government's last aerial survey in May. The US Forest Service, which performs such surveys of forest land, says that 62 million trees have died this year alone.

Crews run controlled burnings to contain a wildfire in Sheep Ranch, California earlier this year. Credit:AP

"The scale of die-off in California is unprecedented in our modern history," said Randy Moore, forester for the region of the US Forest Service that includes California. Trees are dying "at a rate much quicker than we thought".

Scientists say five years of drought are to blame for much of the destruction. The lack of rain has put California's trees under considerable stress, making them more susceptible to the organisms that can kill them, such as bark beetles.