While relaxing in my hammock under the shade of our mature cherry bark oak trees, I realized that my heritage river birch tree, growing in front on my house, was expressing air pollution symptoms. The older leaves were turning yellow and beginning to fall. I noticed similar symptoms on the magnolia and crape myrtle.
In the Garden this Week
Make the Right Pruning Cuts
After reading last week’s timely tip on summer-pruning trees and shrubs, you’ll no doubt go to work as soon as temperatures drop. Here’s how to make the right cut.
Those symptoms are not drought-related. New immature leaves are more sensitive to drought than older leaves. Under drought conditions, the young immature leaves turn yellow and crisp along the margins.
A different villain was at work.
Air pollutants such as ozone, sulfur dioxide, nitric oxide, peroxyacetyl nitrate — aka just plain smog — are poisonous gases that, when they enter the leaves, kill leaf tissues. Older leaves discolor and fall from the branches first because their guard cells are sluggish due to age. Guard cells surround each stomata, the microscopic holes that allow carbon dioxide to enter leaves and oxygen from photosynthesis to exit.
In young leaves, the guard cells rapidly close each stomata opening, thus preventing the poisonous gases from entering. This is why the older leaves are the first to exhibit air pollution symptoms.
If air pollution occurred only at night, there would be no symptoms because then, regardless of age, guard cells close the stomata openings.
The native species most sensitive to air pollutants are white pine trees. However, their symptoms of air pollution will not occur until mid- to late-September and again next spring. White pines affected by air pollution suddenly drop last year’s needles in just a few days, while with trees growing in clean air, needles drop over several weeks.
Next year the needles of affected white pine trees will be shorter and yellow-green in color. If you drive through downtown Baltimore or Washington, D.C., you will see very few white pine trees in landscapes because of their sensitivity. A few flourish because not all white pines are equally sensitive.
Summer squash plants are also good indicators of polluted air. The leaf surfaces develop a silvery sheen, and the leaves tend to have a smaller leaf surface.
During the late 1960s and early ’70s, I used summer squash plants in Garrett County to demonstrate high levels of sulfur dioxide in the air. The squash were planted in existing Christmas tree groves that were deteriorating due to air pollutants. The pollution was caused by the burning of high-sulfur coal in the region.
This research helped lead to abatement hearings and the establishment of air-quality levels.
Air pollutants are unhealthy not only for us humans but also for the plants that supply us with oxygen. Healthier plants mean healthier lungs and a healthier you. That’s another reason to strive to improve air quality.