We expect most deciduous trees to drop their leaves in the fall. We wait for this to happen so that we can rake, chop and compost the leaves and prepare our yards for winter. It is nearly Halloween and we are still waiting for many of the leaves to drop and homeowners are impatiently wondering why many trees have not shed their leaves. We expect pin oak, red oak, blue beech and ironwood to hold their leaves through the winter, but what about some of the other species?
To answer this question, we must review some facts about leaf drop in deciduous trees. The cooler nights and shorter days of autumn signal deciduous trees to do two things; shut down the process of photosynthesis and form an abscission layer between the leaf and the twig to which it is attached. The production of chlorophyll, the green pigment necessary for the process of photosynthesis, slows and the yellow, orange, red and purple pigments become visible, causing the beautiful fall colors we enjoy. At the same time an abscission layer which is made up of corky cells forms between the base of the petiole of the leaf and the twig. The abscission layer weakens the connection between cells so that the leaf breaks off as well as forms a corky scar that serves as a barrier to infection and prevents water loss during the winter.
In years with early freezes, as is the case this year, the leaves are killed before developing an abscission layer which results in poorly colored leaves that turn brown and hang on to the trees. Some of these leaves will be removed by strong winds and snow, but many of them may persist until the newly forming leaf buds push them off in the spring.
Although the fact that trees aren’t all dropping their leaves at the same time may be a little frustrating for the homeowner and for the hunter who hears, but doesn’t see his prey, the trees should not be permanently affected. It is possible that the limbs covered with leaves this winter may be weighted down more heavily with snow than limbs from which the leaves are fallen, thereby putting them at greater risk of breakage.
This is another in the long list of anomalies that occurred among plants as a result of an unusual growing season.