Plants Need Clean Air, Too
A Bay Weekly reader asked me why his Heritage birch was dropping its leaves despite the fact that it was under irrigation. The answer was simple: air pollution.
The Heritage birch is a clone of river birch, and river birch trees are extremely sensitive to ozone and sulfur dioxide. Since the middle of June, we have experienced several days of Code Orange, and in early July we have also experienced Code Red. This means that air pollutants are sufficiently high to affect humans, and the foliage of river birch trees is even more sensitive to air pollutants than humans.
If the air pollution had occurred at night, the plants would not have been exposed. At night, the stomata — openings in the leaves that allow for the exchange of carbon dioxide and oxygen — are closed, preventing polluted air from entering and damaging the tissues within. Once inside the leaves, the polluted air kills the conducting tissues that allow water to enter the leaves. Thus the leaves turn yellow and die, then fall to the ground.
If you examine an affected tree closely, you will notice that it is the older leaves that are primarily affected. The stomata in younger leaves close quickly, thus preventing the pollutants from entering. The stomata of older leaves are sluggish, thus allowing the pollutants to enter and damage tissues.
Vegetable gardeners may notice that the leaves of summer squash have a silvery sheen. This is also a symptom of air pollution injury, especially if the air pollutant is sulfur dioxide.