Is Air Pollution A Threat to Alaskan Conifers?

One product of civilization is polluted air. Here in Alaska that is seen as arctic haze from Eurasia, ice fog from our cars, and public health warnings due to excessive carbon monoxide. Apparently the trees suffer as well.

Tree damage was first associated with air pollution about ten years ago in West Germany. Air pollution is heavy there, and residents are accustomed to the yellow-brown layer that extends from a few thousand feet down to the ground. Death of spruce and fir trees was traced to the tainted air. Yellow patches are now visible on survivors in Germany's famous Black Forest, particularly on hill tops where pollution effects are greatest. Forest health is now deteriorating all across Europe and some formerly skeptical foresters now believe that air pollution is the prime cause of tree damage.

Pollution-enhanced tree damage may be beginning in Alaska. Near Fairbanks, yellow patches have appeared on some spruce trees, for example those along the north side of Sheep Creek Road near the University. Effects are intensified by direct sunlight, so trees on the shady south side are less affected. Yellowed trees can be found along the railbelt from Fairbanks to Seward as well.

Correlating forest damage with air quality is complicated by the many natural stresses found in the life of a tree: insect damage, nutrient deficiencies, and normal climatic factors such as sudden frosts. These stresses are thought to be enhanced by the action of pollutants. It's like "three strikes and out"; a tree stressed by two insults may be killed by the third.

To contrast observation of forests near urban areas with those more remote, we visited several villages by aircraft and examined the trees. All appeared healthy. Visits with community elders gave no hint of a changing forest -- and these folks are keen observers all.

We did find one exception, but that could be due to natural air pollution. Many spruce trees in a stand at the north edge of Minto Flats near Fairbanks were yellowed or dead. They were downwind from a ground fire that occurred about three years ago, so combustion products could have caused this damage.

Components of air pollution responsible for tree damage vary with the area. In Europe sulfur compounds from oil and coal are a major problem, but in Alaska their levels are low. Both ammonia and ozone do come from various combustion processes here. The ozone is not emitted directly but is produced by the combination of nitrogen dioxide with atmospheric oxygen in the presence of natural or pollution-derived hydrocarbons and sunlight. Once formed, the ozone reacts again with the hydrocarbons to form a group of very reactive compounds called peroxides. Sometimes these reactions occur directly in the leaves of plants where the hydrocarbons changed are a class of plant hormones called auxins. Pollution-associated plant damage has been specifically traced to these altered auxins.

How can pollution-damaged trees be recognized? A healthy spruce tree's needles stay green until they drop; the needles of a spruce injured by pollution will turn yellow while they are still on the branches. These are most visible to an observer looking down from an elevated location. Usually spruce hold needles for five to seven years before dropping them These years can be counted by the nodes that ring the branches every few inches. Damaged trees drop their needles early and flower more profusely. Flowering is also affected in deciduous trees. In addition the leaves are smaller, and the pattern of branching is changed.

Also certain changes in the biochemical composition of conifer foliage can reflect air pollution damage. We found some of these changes in the needles from near Fairbanks and Seward. Needles from the yellowed trees near Minto Flats were not analyzed.

Like water pollution, the chemistry and effects of air quality are relative newcomers to scientific scrutiny. However the fragility of our near arctic environment is a fact, and its ecosystems are slow in forgiving insult. Combustion products in the air are one such insult.