Below is an excerpt on how air pollution in California affects nearby forests from the book:
Johnston, Verna R. 1994. California Forests and Woodlands: A Natural History. University of California Press.
It was in the Mixed Conifer Forests of the San Bernardino Mountains east of Los Angeles that air pollution damage to Pacific Ponderosa Pines first became recognized in the early 1960s. For nearly a decade a mysterious blight known as “Disease X” had been turning the needles of mature Ponderosas a mottled yellow before destroying the trees. No one guessed, at that time, that the smog from the Los Angeles Basin 60 miles away could be the killer.
As trees died by the thousands, Paul Miller and colleagues traced the lethal agent to its source. The brown smog that blankets Los Angeles comes largely from millions of automobiles. When hydrocarbons and nitrogen oxides from car exhausts combine in sunshine, one of their major products is ozone (O3 ). An invisible, eye-stinging, caustic gas, ozone can crack rubber, deteriorate fabrics, scar lungs, and cause coughing, shortness of breath, pain, and fatigue. And, it kills trees!
Ozone, along with all the other air pollutants generated in Los Angeles, ordinarily remains trapped there during much of the day by inversion layers of warm air over the mountains rimming the basin. When afternoon offshore breezes blow the smog-laden air upslope into the mountains, ridges in direct line with the air currents suffer devastating tree losses.
Ozone acts quickly. Ponderosa Pines usually keep their needles 3 to 4 years. Ozone-struck trees shed all but the current crop, leaving a sparse, stripped tree. The remaining needles soon show yellow mottling as ozone destroys the chlorophyll. As the needles fall, the root system deteriorates and resin flow in the trunks slows down, opening the way for bark beetles.
Fortunately, not all kinds of trees succumb in the same degree and, even among the most sensitive Ponderosa and Jeffrey Pines, some specimens show an inborn resistance. Sugar Pine, luckily, seems relatively immune. But the list of vulnerable conifers grows with each decade of bad air exposure. And this disaster is not confined to southern California.
Ozone damage, now known as ozone mottle, became visible in the southern Sierra in the mid-1970s and continues to increase ominously. ON countless days a brown layer of pollution hangs over the San Joaquin Valley west of Sequoia/Kings Canyon National Parks. It shows up plainly from Moro Rock. Afternoon upslope breezes blow the ravaging pollutants into the 5,000 to 7,000-foot levels (1,500 to 2,100 m) of the parks, where ozone works ruinous havoc on the pines and oaks and on some Giant Sequoia seedlings. Sequoia National Park has recorded the highest cumulative levels of ozone over a one-day period for all of the national parks. Levels in Sequoia regularly climb higher than those of Los Angeles.
Yosemite National Park, farther north, suffered a fivefold increase in ozone damage between 1985 and 1990. Thirty percent of its Jeffrey and Ponderosa Pines show yellow needle mottling, and no area in Yosemite’s mixed conifer belt stands free from ozone’s relentless scourge. Adjacent national forests display steadily growing numbers of Ponderosa and Jeffrey Pines with the thinning crowns and mottled needles that are ozone’s trademark.
California’s Great Central Valley sits in the midst of an even larger basin than Los Angeles, bordered by mountains and hemmed in by the same inversion layer that traps smog beneath. As populations of valley cities boom, more and more of their polluted air follows its daily, deadly flow uphill to the Mixed Conifer Forests of the western Sierra Nevada.
The most diverse coniferous forests on this earth, still very beautiful, face all the natural ecological challenges of forest life-fire, drought, insects, fungi, winds-with adaptations built in over centuries. Their genetic resistance to poisonous air is now being sorely tested. In both the short and the long run, air pollution of human derivation will require a solution for humans and trees, for both are dependent on the same air for survival. Pages 109-110