As sudden oak death continues to ravage Sonoma County woodlands, a secluded creek near Glen Ellen bears the impact of a malady that experts say is likely unstoppable and has just begun its onslaught.
Coast live oak trees felled by the disease along Stuart Creek leave holes in the forest canopy, admitting direct sunlight along a cool, damp waterway that harbors amphibians, spiders and woodpeckers.
"It definitely changes the habitat," said Jeanne Wirka, resident biologist at the 535-acre Bouverie Preserve off Highway 12 near Glen Ellen.
The creek runs through a protected area of the preserve managed by the Audubon Canyon Ranch and open to the public by reservation on select dates.
But it is squarely in the path of sudden oak death, which has been killing trees in Sonoma County for more than a decade — largely in the west county and Sonoma Valley — and got a boost from the rainy spring of 2011.
Sonoma County has the third-highest rate of infection among the 12 San Francisco Bay Area locations surveyed in the fifth annual Sudden Oak Death Blitz organized by UC Berkeley's forest pathology laboratory.
The county's 46 percent infection rate among bay laurel trees sampled in May by blitz volunteers trailed the 59 percent rate in Saratoga and 51 percent in Marin, where sudden oak death was discovered in 1995.
Matteo Garbelotto, head of the UC lab, said he was not surprised that Sonoma's infection rate was up from 35 percent last year and 13 percent in 2010.
"Spring rains are really what favor the spread," he said, noting that the wet spring of 2011 boosted sudden oak death's prevalence that year and this year.
West County forests and the hills flanking Sonoma Valley have been hardest hit by the disease, according to surveys taken since 2008, a trend attributed to the wetter conditions in those areas.
But this year's survey found sudden oak death was gaining a foothold in drier areas, such as the Santa Rosa Plain and Sonoma Valley.
The pathogen that causes the disease — <i>Phytophthora ramorum</i> — is "definitely better established" in the flatlands and is "more and more an issue" in suburban residential areas, Garbelotto said.
An estimated 105,000 acres in Sonoma County are infected, according to aerial surveys by the U.S. Forest Service since 2005.
Sudden oak death kills kills tanoaks and four types of oak trees, including coast live oak and black oak common to Sonoma County. Tanoak is not a true oak but is a member of the same botanical family.
Volunteers sample leaves primarily from bay laurel trees, which host the pathogen but are not harmed by it.
The disease now infects 14 counties from Humboldt to Monterey along the coast, as well as Lake, Napa, Solano, Contra Costa, Alameda and Santa Clara counties.
Garbelotto said he is convinced the hardy pathogen is "here to stay" and will, over the next 20 years, change the appearance of coastal forests by thinning out oaks and tanoaks.
So far, it has not spread inland to the much drier Central Valley or to the abundant Sierra forests farther east, he said.
The pathogen seems unlikely to adapt to the drier climates, but Garbelotto said he isn't making any long-range forecasts.