Crews fighting wildfires in 11 Western states are shifting tactics, trying to avoid lifeless, unstable trees killed over the past years by bark beetles, disease and other infestations.

An annual study by the U.S. Forest Service shows an estimated 6.3 billion dead trees still stand. That’s up 8 percent since the beginning of the decade.

The agency estimates the bark beetle is responsible for roughly 20 percent of the dead trees.

Montana has seen the worst of it, losing more than 159 million trees over a five year span. Arizona lost almost five million trees in that same period.