San Mateo County forests show clear signs of a pathogen that’s killing oak trees in such large numbers that even environmentalists have started swinging axes.
Between now and July 28, state workers will remove some 250 bay trees in San Mateo and Santa Clara County parks that could infect about 50 nearby trees with sudden oak death.
Caused by the mold Phytophthora ramorum, the disease is renowned for its ability to turn oaks brown within six weeks and kill mature trees in as little as six months.
The bay trees may or may not carry the pathogen. But they all stand within 15 feet of oak trees thought to be hundreds of years old, so their removal is an important precaution, said Cindy Roessler, a biologist with the Midpeninsula Regional Open Space District.
With trunks up to 14 feet wide, the oaks are the largest trees in the forests and have managed to evade numerous rounds of lumbering, Roessler said. By comparison, she said, the bay trees range from 3 inches to 1.5 feet in diameter and provide less benefit to local wildlife.
Funded by a Proposition 84 grant worth $31,697, the removal will be performed by young workers with the California Conservation Corps, said spokeswoman Susan Levitsky. The district will train the workers how to fell trees in a dense forest and properly dispose of possibly contagious debris.
Phytophthora may have been spawned in the forests of Asia, and spreads its spores via air or water, often jumping from bay trees to oaks. Just climbing an infected tree and later touching your shoe to a coastal oak can transfer the disease, said district spokeswoman Rhea Maze.
The pathogen kills trees by preventing the transport of water to leaves, Roessler said.
Coast live oak, California black oak, Shreve oak and canyon live oak are the oak species most vulnerable to the disease. These trees provide nesting, roosting, hiding and hunting places for 6,000 insect species, numerous plants, and more than 300 species of wildlife, including birds, bats, salamanders, squirrels, raccoons, skunks and rodents.
Over the past 10 years, sudden oak death has caused the most massive oak die-off in known history. Infected trees can be spotted by their wilting or browned leaves, a “bloody” area where portions of the tree have been killed and ooze sap, and by the dust trails beetles leave after boring into the rotten tree.
The disease also can affect bay laurel and huckleberry trees, albeit more slowly, and kills the flowum, the part of the tree that transports food to the leaves and branches, and the cambium, the growth-intensive layer of the tree between the bark and the inner trunk.
First spotted in California in Santa Cruz County back in 2001, sudden oak death has since spread to seven coastal counties. It is most virulent in Sonoma and Marin counties, and in Pfeiffer Big Sur State Park.
Infected trees should be pruned during summer, when the disease is least active, according to the district.
Susceptible oaks: Coast live oak, California black oak, Shreve oak, canyon live oak Immune oaks: White oaks, including valley oaks and blue oaks Local animals that eat acorns: Woodpeckers, jays, turkeys, deer, deer mice, pocket gophers, squirrels, gray foxes, dusky-footed woodrats, insects
Source: Midpeninsula Regional Open Space District