Scientists in California say the evidence is compelling: Air pollution from the fast-growing San Joaquin Valley is responsible for killing thousands of trees in the nearby Sierra Nevadas.

The chief culprit is ozone, a pollutant created when exhaust from power plants or cars mixes with hydrocarbons in the presence of sunlight, say federal researchers and a University of California professor. When a tree is first poisoned, its leaves and needles turn mottled yellow. At later stages, leaves turn brown and drop off, reducing photosynthesis and stunting growth. The trees then become so weakened they can't survive common stresses, such as a bark-beetle epidemic.

"Anyplace where you have a situation like the Sierra, where you have a valley with pollutants that can cook up and make ozone, and a mountain range to the east of it which has strong upslope winds in the afternoon, you should get this effect," says Thomas Cahill, professor of physics and atmospheric sciences at the University of California, Davis.

So far, however, the worst damage in the West has occurred in California: Some forests outside Los Angeles lost half of their Jeffrey and ponderosa pines in the 1970s.

But the studies do have a hopeful side. Researchers found that some trees damaged by ozone compensate by growing foliage with higher photosynthetic rates. Parts of the San Bernardino National Forest outside Los Angeles, for example, have rebounded through such adaptation.

* Martin Forstenzer