The infection arrives with the rains, splashing down from a neighbor’s thick leaves. It takes hold one winter and persists silently for a year or two. Ambrosia beetles and bark beetles colonize the weakened tanoak, and the bark oozes and spots with dark-red liquid. Then, in a matter of weeks, the leaves brown and decay, and the tree dies.
Sudden oak death, a disease caused by the pathogen phyophthora ramorum, first appeared in an area near Mount Tamalpais in 1995. Though today it has devastated many parts of coastal California and Oregon, Marin County was one of the first places to receive attention from academic institutions studying the disease.
Since 2015, the University of California, Davis, the Marin Municipal Water District and the United States Forest Service have collaborated on a study of management strategies that contain the disease. The effort, called the Resilient Forests Project, has been focused on around 25 acres at three sites along the Bolinas Ridge, Laurel Dell and the San Geronimo Ridge.
This fall, the study will expand onto 10 to 30 new acres near the intersection of the Bolinas-Fairfax Road, Bolinas Ridge Road and Skyline Boulevard, with some new goals in mind.
“The severity of the disease is unfortunately exceptional on Bolinas Ridge, which has one of the highest mortality rates of tanoaks in the state,” Susan Frankel, a biologist from the forest service, said.
Richard Cobb, a researcher who began work on the project while at Davis and who now works at California Polytechnic State University, San Luis Obispo, explained the process. “The disease causes the tanoaks to die above ground, but then they regrow from the roots, leaving behind smaller, infected trees that then also die. The resulting dense understory is a fire threat, and also depletes the soil-water reservoir,” he said.
Mr. Cobb said collaboration with local land managers to find a working solution is critical.
Marin Municipal is funding the water retention aspect of the study and supplying the crew for the vegetation work, which is primarily focused on clearing tanoaks with excavators and pulverizing them with chippers.
U.C. Davis researchers have received funding through the California Department of Forestry and Fire Protection from a greenhouse gas reduction fund—which is generated by California’s cap and trade program—to focus on research related to the effect of carbon storage in the diseased forests. In preliminary findings, they’ve seen carbon storage levels diminish as the forest converts from older-growth, healthy trees to constantly dying, small undergrowth.
“Our hope is to identify techniques that are optimal in terms of suppressing the pathogen and also increasing carbon sequestration. We are still collecting data about the short-term trade-offs between, for example, reducing fuel levels and increasing the total amount of carbon stored,” Mr. Cobb said.
Before it was logged at the turn of the 20th century, the Bolinas Ridge consisted primarily of redwoods. Douglas firs, tanoaks and second-growth redwoods replaced the native habitat. But today, there are acre-wide gaps in the canopy, where hundreds of thousands of smaller tanoaks have sprung up.
Though the disease kills coast live oaks, California black oaks, Shreve’s oaks and canyon live oaks, among other tree species, tanoaks are especially susceptible. Sudden oak is known to spread through spores carried by air or water.
“It’s basically a brush field out there,” Janet Klein, Marin Municipal Water District’s natural resource program manager, said. An estimated 10,000 to 12,000 acres of the district’s total 22,000 acres are infected by the disease.
Ms. Klein said about 2,000 acres of the infected area are readily accessible. The district hopes to address about 100 acres per year, funding permitting. So far, the project has been expensive, with management costs at around $12,000 per acre. The next year is funded, Ms. Klein said, but she is still looking for support for two to three years out.
The team has some new goals for the Bolinas Ridge site, including testing the success of more efficient and cheaper methods. In the past, they “saw an increase in carbon sequestration and soil moisture, and also improved fire safety, but it was really expensive,” Ms. Klein said. “Our next attempt will focus on working faster and more cheaply. We are hoping, for instance, to not chip the wood so finely. We’re hoping we can still realize the same benefits but leave coarser material behind.”
New data also shows that native species such as redwoods were not regrowing in the areas cleared of tanoaks, as was hoped would happen. The next phase of the project will include both re-vegetation efforts and controlled burns.
“The time to eradicate the disease? Well, that was over two decades ago,” Mr. Cobb said. “Now the disease is part of the Mount Tam ecosystem. It is part of the mountain and there’s nothing we can do with it. That’s not cause for cynicism; rather, it helps us focus on what needs to be done now.”