Air Alert for Heritage Birch
If your Heritage birch is dropping yellow leaves, blame it on the Orange Alert of early August. Heritage birch is a clone of river birch, which is highly sensitive to both ozone and sulfur dioxide. Both of these gasses are present in an Orange Alert.
An Orange Alert is announced to warn the elderly and people with pulmonary disorders to remain indoors in air-conditioning and minimize outdoor activities until the alert is lifted. Heritage birch, the deciduous trees most sensitive to air pollutants, have no choice but to remain in place and try to survive.
Maple, oak, cherry, apple, dogwood and other tree species are not affected.
Only older leaves are yellowing and dropping. Younger leaves closer to the ends of the branches are remaining green, and the trees are producing new leaves at the ends of the branches.
Age is the cause of the leaf drop. The spongy layer of plant cells in leaves converts carbon dioxide into oxygen by absorbing air through small openings called stomata on the underside of birch tree leaves. These stomata are surrounded by guard cells that open and close depending on moisture, time of day and the presence of air pollutants.
In younger leaves, the guard cells remain very flexible. As soon as they detect air pollutants entering the leaves they close, thus preventing damage to the spongy leaf tissues that absorb carbon dioxide and release oxygen. However, as leaves age, the guard cells become sluggish and sometimes stop functioning, thus allowing the polluted air to enter and kill the spongy leaf tissues. In other words, the guard cells are not as spry as they once were.
Once the spongy leaf tissues are killed by the air pollutants, the older leaves react as if they had been damaged by an early frost.
If the air pollution were to occur at night, it is unlikely the problem would be as severe because the guard cells close at about the same time the sun sets. The damage would be limited to only those leaves where the guard cells are stuck in the open position.
Ask Dr. Gouin your questions at [email protected]. Please include your name and address.