nat 88

In California, oaks suddenly 'bleed'

By John Ritter, USA TODAY

 

KENTFIELD, Calif. Strange yellows, browns and grays pepper the dense forests of Mount Tamalpais, intruders in the natural green abundance of this scenic knob across the Golden Gate from San Francisco.

They are the colors of thousands of dead and dying oaks, stricken in unprecedented numbers by a mysterious pathogen, a killer that's ravaging one of California's signature native trees from Mendocino to Santa Barbara.

The malady's cause has proved elusive, and researchers and homeowners are stunned by its lethal speed. Oaks and tanoaks, indigenous to 10 million acres of the Pacific Coast as far north as Oregon, are cascading from healthy green to dead brown in a few weeks.

Scientists fear that a foreign invader might be at work, an interloper like the fungi that wiped out America's chestnuts and elms a generation ago.

"We're trying to rule that out, but that's what I'm most afraid it is," says Rick Standiford, who is associate dean of forestry at the University of California-Berkeley. "We're worried that it's an introduced organism the oaks have no built-in immunity to."

Last week, Marin County declared an emergency and asked the state for $3 million to fight what has been dubbed "sudden oak death." Homeowners across the county are struggling to save the trees, many more than 100 years old, and are paying thousands of dollars to have dead ones cut down and disposed of.

The county fire marshal is planning for a potentially devastating fire season because dead oaks can become a dangerous fuel source on a burning hillside.

Water district officials worry that with hillsides thinned from dying oaks, accelerated runoff from winter storms will degrade watersheds and hasten the buildup of sediment in reservoirs.

Ecologists say other species in the forest are at risk because they depend on the dominant oaks, prolific producers of the acorns that are a key food source for many birds and mammals. Aggressive, non-native weeds also could invade the ground where oaks grew.

"Oaks are the pillar, the cornerstone of the food chain in this part of California and the coast ranges," says Dennis Odion, a vegetation ecologist with the Marin Municipal Water District.

Horticulturists and foresters have only recently become alarmed. When sudden oak death appeared in Marin in 1995, it seemed to spread slowly. Experts thought it was a short-term phenomenon that nature would overcome in due course.

But during the past two years, oaks died in great numbers. Homeowners began to lose trees as scores of dead oaks stood out on the hillsides.

The range of the problem has spread, and significant losses have been reported in the Santa Cruz Mountains and the Big Sur area south of Monterey.

"It's incredible, the numbers are staggering now," says Pavel Svihra, <FZ,1,0>a horticulturist with the University of California Cooperative Extension. "It got out of hand. We somehow underestimated the whole syndrome."

Teams of plant pathologists, horticulturists and entomologists are racing to find a cause. They're using aerial remote-sensing technology to map the syndrome's progression and studying tissue samples in the lab. They're monitoring 20 plots containing more than 500 trees, but the prognosis is grim: 40% to 50% of them are expected to die this year.

Homeowners are asked to inspect their trees regularly. Live oaks are evergreens with rounded, leathery leaves. Unlike the ramrod straight deciduous varieties, these oak trees are crooked and commercially worthless.

Often, the first visible symptom is "bleeding" -- thick globules of sap emerging from the bark around the base of the trunk. Then a fine sawdust-like froth trickles down the bark, the waste from thousands of Ambrosia beetles that have bored inside the tree.

The beetles, each no bigger than a grain of rice, host a fungus that enables them to digest wood. Soon, the oak's internal tissue layer that transports water up from the roots is choked off, and the tree begins to die. The presence of a second, naturally occurring fungus is seen in the form of dull-green, dome-like structures on the bark.

What confounds scientists is that this common scenario is by no means universal.

Sometimes, they find trees with bleeding but no external fungus, sometimes with beetles but no bleeding, sometimes with just the fungus.

They know that the beetles will finish off the tree, but they don't know why beetles suddenly appear. One theory is that they only attack trees that have been weakened by another, unknown agent.

The role the external fungus plays is not known.

Healthy trees appear to escape attack, though scientists worry that with so many dead trees in the forest, even healthy specimens might be vulnerable.

Stressed urban trees, whose roots often have been cut and altered in the construction of streets and buildings, might be the most inviting targets.

Mature live oaks, trees that architects design around, can boost a property's value 10% or more. "They're not only something we enjoy, that the critters enjoy, but they're one of the reasons we're here," says Patricia Burton, whose hillside home here had 19 oaks on three-fifths of an acre. "They're a real asset."

Burton has spent $6,000 to have six trees taken out. Homeowners who don't remove trees might contribute to the spread of the syndrome, experts say.

Logs from cut trees don't simply go into the fireplace. The county wants the wood stacked outside and covered tightly with clear plastic for six months so the sun can generate enough heat and humidity to kill the beetles.

Homeowners who spot bleeding might be able to save their trees. Early insecticide spraying appears to keep the beetles at bay. "But we're not sure if we're saving them, or just keeping them alive awhile longer," horticulturist Svihra says.

As for the true source of the killer, at this point scientists have little more than theories. Air pollution and weather changes, such as drought in the early 1990s followed by several wetter-than-normal years, are potential factors. Evidence now seems to dispel the idea that the syndrome kills quickly.

Marin Fire Marshal Keith Parker says he's looking at a decade of elevated fire risk before dead trees fall and decay. And maybe longer if the oaks, which are more resistant to fire, are replaced by other trees that burn more readily.

The last big fire in Marin, in 1995, burned 12,000 acres around Point Reyes and destroyed 48 homes. "It's tough to protect structures even under good conditions," Parker says. "This is just another major problem in an already complex situation."

The tree lovers and forest protectors who won battles for laws to protect live oaks now see a catastrophe looming on the coastal landscape.

"Live oaks are the kingpins, widespread, much revered and very, very beautiful," says David Chipping, vice president of the California Native Plant Society. "Losing them would essentially mean deforestation

 


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