Root rot to become bigger problem for Douglas firs, study suggests

A fungus that devours the roots of Douglas fir trees costs the timber industry millions of dollars each year, and it’s likely to become a bigger killer as the climate changes, a new study says.

Laminated root rot, a native pest, is found in Douglas fir stands throughout the Northwest. If the disease doesn’t kill the firs outright, it leaves them weakened and susceptible to bark beetle attacks and uprooting during wind storms.

“Tree stress plays a big role in how vulnerable these stands are to root fungus,” said Karen Ripley, the Washington Department of Natural Resources’ forest health program manager.

Climate change forecasts for warmer temperatures and more frequent droughts means the firs will be more susceptible to the fungus, she said.

Foresters have known about laminated root rot since the 1940s, but much about the disease remains a mystery. Ripley was part of a panel that recommended a deeper look at the molecular biology and genetics of the fungi-caused root rot and its interaction with host trees. Their study was published last month.

Other panel members included scientists from the University of Washington, Washington State University, Weyerhaeuser Corp., the U.S. Forest Service, Canada and the Pacific Forestry Center.

Due to retirements, Washington’s universities don’t currently have researchers studying forest pathology, Ripley said. That’s an issue that panel members want to bring to the Washington Legislature’s attention, she said.

Douglas firs are a keystone species, valuable both economically and ecologically. Laminated root rot occurs across the species’ range, from Montana to the Pacific coast, and from British Columbia to Northern California.

The root rot lowers the Department of Natural Resources’ timber harvest yields in Western Washington by 5 to 15 percent annually. Over a two-year period, that reduced revenues to the state’s public school and university trust fund by $10 million, according to agency estimates.

The disease also affects recreation and biodiversity, Ripley said. Big, old trees had to be removed from Lake Wenatchee’s campground because they were infected with root rot and had become hazards, she said.

In addition, root rot is closely affiliated with bark beetle outbreaks. In the Colville and Wenatchee national forests, many tree stands with high bark beetle mortality were first attacked by root rot.

Beetles can sense which trees are in poor health and choose them for host trees, Ripley said.