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Something else is killing the oak trees in Cienega Canyon, a San Timoteo Canyon offshoot in the hills south of Interstate 10 and east of Beaumont. Most of the deaths are on 357 acres the conservancy acquired in 2003 to be part of a 10,000-acre state or county park. About 75 coast live oak trees have died in or near the Riverside Land Conservancy's San Timoteo Canyon area preserve, and many more are sick.

"It started over there, where the trees are falling down," said Easton, the conservancy's biologist and land manager told a gathering of about 30 forest, conservation and fire officials. "You can see how the mortality now extends on the ridge."

Whatever is killing the trees may be the newest threat to the state's oaks, which have been falling to mold and beetles for more than a decade.

Since the mid-1990s, nearly 2 million oak trees along the coast from Monterey County and into Oregon have been killed by a type of mold that thrives in the humid conditions near the shore.

In the more arid inland environments of San Diego County, a bark-boring beetle native to Arizona has killed tens of thousands of oak trees in the past several years. Forest, fire and conservation experts have formed a task force to try to stop the spread of the goldspotted oak borer. The insect's larvae feed under the bark and cut off the flow of water and nutrients within the tree.

Easton said something else is killing the oak trees in Cienega Canyon, a San Timoteo Canyon offshoot in the hills south of Interstate 10 and east of Beaumont. Most of the deaths are on 357 acres the conservancy acquired in 2003 to be part of a 10,000-acre state or county park.

The initial fear was that the gold-spotted beetle had jumped counties.

UC Riverside plant pathologist Akif Eskalen has taken soil and tree-tissue samples that ruled out the goldspotted oak borer as the culprit. At the gathering in the hills last week, Eskalen said he is still working to identify a pathogen, or microscopic organism, that might have killed the trees.

Easton said one possibility is that a pathogen has been carried to the trees by another type of insect, the western oak bark beetle, which is common in the San Bernardino National Forest and has caused the branch tips of some trees to die.

By analyzing aerial photographs, Easton has determined that the Cienega Canyon trees started dying as early as 2003, and the damage appears to be spreading to the east, west and south. The deaths don't appear to coincide with recent dry years, although drought does weaken trees, he said.

The trees die a slow death, Easton said. A sick tree may linger with several dead branches for years, he said. The reasons aren't clear, but it might the influx of non-native weeds, such as the foxtail that covers the hillsides, and the dropping water tables that result from farming and urbanization, Smith said.

Easton said the Riverside Land Conservancy intends to plant oak trees from seedlings sprouted in greenhouses.

But the higher priority, he said, is to learn what's killing the old trees.