Why do all the leaves fall off the trees when winter is coming?

Austin Crane, 7
Ogletree Elementary

Dear Austin,

Like people, the leaves on trees age, but do so much more rapidly. Each spring the young leaves develop from buds and then rapidly mature to full size to carry on photosynthesis, the process that provides food needed for the tree to grow and maintain itself. By autumn, the leaves of many hardwood trees have reached old age and do not function well. At this time a tree must also prepare for the harsher weather conditions of winter. Signals to the tree indicating it is time to prepare for winter include shorter days, less intense sunlight and cooler temperatures. Prior to the leaves falling, nutrients like nitrogen and phosphorus move from the leaf into the adjacent stem to be reused during the next growing season. The following spring a new crop of leaves are produced and the cycle is repeated.

For some trees not all leaves fall off prior to winter. Pine tree leaves, called needles, are evergreen meaning their leaves remain green for more than one growing season. There are some hardwoods like southern magnolia, American holly and live oak that are also evergreen. Leaves on these trees can live for two or more years, however the oldest leaves that no longer function well are shed each year.

In other hardwoods like oaks and American beech, leaves die in the fall but are not shed until the following spring. This occurs in younger trees typically on the lower branches where leaves are retained throughout the winter months. In these same trees, leaves are shed in the autumn from the upper branches. Thus, in the winter, the upper branches will be bare while the lower branches will retain dead leaves.

Trees will also shed leaves early in the summer when they experience a severe stress like a drought. Under such conditions, the shedding of leaves will lessen potential harm to the tree by minimizing water loss.

Thus while trees can live for decades and even centuries, some parts of the tree like the leaves, live for only a few months or years.

Thanks for your question,
Aubie and Dr. Gjerstad