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
"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,
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
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.