Young trees don't know when to shed

Q: I have two oak trees in my yard that are of the same species. One is big while the other is small (I planted it two years ago). Why, during the fall, does the big tree lose its leaves quickly while the leaves on the young tree hang on for a long time?

A: This is a good question, and you are observant to notice.

Quite simply, young trees are like children: they are not mature enough to know the right things to do. In this case it means not knowing to shed leaves in the fall.

Even older trees sometimes keep a few leaves through winter. You'll find them on a few small, younger branches inside the crown, close to the main stem. This often leaves the tree with a small column of brown leaves snaking through its bare branches. Sometimes the tip of a branch may have a leaf or two that hangs on.

Throughout the growing season, leaves and buds send out a chemical signal. As long as the signal is strong, leaves remain connected to the tree. In return, they receive essential elements, nutrients, and water to make food. In fall, the chemical process becomes inefficient and starts to shut down. Eventually the signal is so weak the tree starts to pull valuable materials from leaves instead of sending it in. Finally, the leaf signal ceases.

The low-level signal or lack of a signal initiates what is known as senescence and compartmentalization. The leaves are shut down, then sealed off from the tree.

Timing differs from species to species. Deciduous trees lose leaves each year in the dormant season, while others shed gradually.

One reason young trees hang onto leaves is because the leaf and bud signals may not be well developed enough for the signal to be understood. As the tree gets older, the signal becomes clear and the abscission layer (an area of cells with thin walls at the base of the leaf stem) forms properly.

Some trees hold onto leaves because they get no help letting go. The dead leaves are still connected to the tree by water-conducting tissue, which is very strong. Wind, rain, or wildlife moving in the treetops must shake loose the leaves and break the water connection.

Because the link to the water transport system is still strong, dead leaves on a tree can still lose water. So, supplemental watering during drought can be very important if you have a young tree that holds onto a lot of dead leaves during the winter. During warm winter days, water loss from the tree can cause a condition known as winter burn.

Sid Mullis is director of the University of Georgia Extension Service office for Richmond County. Call 821-2349, or send e-mail to smullis@uga.edu. The Web page for Extension offices in Richmond and Columbia counties is www.griffin.peachnet.edu/ga/columbia.