ozone damage to milkweed (8167 bytes)

Air Pollution Affects Local Trees and Shrubs

By Ruth Ann Hales, Colorado State University Cooperative Extension Agent, Horticulture

The brown cloud, so prevalent in Denver during the fall, may cause injury to trees and shrubs. Researchers at universities in smog-prone areas have studied and documented air pollution damage to plants. These areas include the San Gabriel and the San Bernadino mountains of Southern California. Los Angeles smog drifts eastward into the mountains where it damages pine trees.

Symptoms include browning on needles that occurs in a mottled pattern. Plant specialists call this chlorosis. The browning progresses to premature needle drop, and eventually causes premature death in Ponderosa pines. In the meanwhile, the trees are more susceptible to disease and bark beetle attacks.

Extensive air pollution studies also have been undertaken in the Blue Ridge Mountains of Virginia. Eastern white pines have developed chlorotic spotting and premature needle drop.

Much of the Denver area's air pollution comes from automobile exhaust. Sunlight in the presence of exhaust promotes the formation of a gas called ozone that injures plants. 

Symptoms of ozone damage to plants range from slow growth to severe leaf browning, followed by premature leaf drop. On pines, the most frequent symptom is an unusual mottled pattern along the needles. In severe cases, tips turn brown and older needles fall off. Deciduous trees that lose leaves in the fall exhibit a stippling or flecked pattern on the upper leaf surface. Flecks can be white, tan or black depending on the plant species.

Nationally, air pollution injury has been documented on white ash, aspen, Ohio buckeye, Kentucky coffee, Crimean linden, imperial honeylocust, weeping willow and hybrid poplar trees. Other studies have found green ash, silver maple, eastern cottonwood, loblollypine and sycamore trees sensitive to air pollution.

The most likely time to find ozone injury symptoms in the Denver Metro area would be in the fall season. Trees such as linden, maple and green ash may exhibit ozone-injury symptoms. Outer margins of linden tree leaves display a minor black mottling in early August that progresses to severe marginal tissue distruction. Young leaves on maple trees display white flecks much like freckles. Green ash trees lose leaves prematurely and look rather sickly. At Cornell University, researchers have linked ash tree decline to stress from excessive air pollution. How much injury we have in the Denver Metro area is an open question as, at this writing, there is no active research program or expertise on air pollution and plants in Colorado.

Diagnosis from symptoms is difficult because fungal diseases may cause leaf symptoms that mimic air pollution. Local aspen trees have displayed problems that look similar to fungal leaf spots, particularly those of the fungus Marssonina. Colorado State University pathologists, however, have at times been unable to culture Marssonina fungus from aspen samples submitted. This suggests that local aspen trees could be exhibiting ozone damage, not fungal infection.

Local pines and blue spruce may also show signs of air pollution damage. In ozone injury, blue spruce needles are mottled and over-all tree growth is slower than normal. Evergreen trees stressed by ozone and other injuries can become more susceptible to bark beetle infestations and display needle mottling and loss.

Knowing precisely if a tree is suffering from air pollution damage is difficult. Trees of the same species (though of different genetics) growing in identical locations may vary in susceptibility to injury. One tree may show no sign of the problem, while the tree next to it will drop brown needles and die back. It's difficult for tree care professionals to make an accurate diagnosis under these circumstances.

Any injury locally is most likely to be seen from August-October. Older leaves would be the most likely to show symptoms, because they've been exposed to pollutants for longer periods. Fortunately most plants, even those sensitive to damage, are thought to be tolerant of air pollution injury during dormancy. This means that when Denver's brown cloud gets thick in winter, deciduous trees and shrubs that drop their leaves won't suffer further injury.

Note that overall, other causes of stress and injury to plants such as environmental stress (from drought, cold, etc.) and soil conditions (tight clay soils, shallow soils, etc.) are common and the more identifiable causes of Colorado plant growth problems.

Photographs courtesy of United States Forest Service.

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Date last revised: 01/05/2010