From state highways, foothill campgrounds and aerial surveys, it’s easy to see the catastrophic tree die-off in California forests. What isn’t as easily grasped is the scale of rapidly expanding tree mortality in the state’s 40 million acres of forestland—and what to do about it.

The U.S. Forest Service said in June that its survey showed more than 66 million trees, mostly pine species, have died in the southern Sierra Nevada alone, and more are dying. Forestry experts say the scale of the die-off is beyond anything ever observed.