Why do Leaves Shed in Fall?

By JOE LAMP’L

Wouldn’t it be nice to enjoy those brilliant autumn leaf colors all year– rather than for a few short weeks? Maybe so, but the changing leaf color each fall is part of an important and complicated process that ends in their being shed at the end of each growing season. The actual term used to describe this process of leaf drop is known as abscission.

Although some parts of trees like stems and buds can handle freezing temperatures, most leaves cannot. So, in order to protect themselves, trees and plants shed diseased, damaged or dead tissue (namely leaves), while simultaneously sealing the point where the leaf petiole connects to it. Known as the abscission layer, it consists of unique cells that can separate from each other based on certain physiological occurrences. As changing climate and light conditions of autumn evolve, hormones within trees change too. The most notable is auxin. It’s produced in the leaves and body of trees and plants. This balance of auxin levels between leaves and branches is key to determining if and when leaf drop occurs.

During the active growing season, production rates of auxin in leaves are consistent with other parts of the plant or tree. As long as these rates are steady, the cells of the abscission layer remain connected, which in turn, keeps leaves attached. However, as days shorten and temperatures cool, auxin production in leaves starts to decrease in response to changing conditions. As a result, fracture lines develop at the base of the leaf petioles and scarring builds up at the same point to form a protective barrier. Eventually, it’s just a matter of time before wind or rain provides that last nudge and the leaves are released, at least for most trees.

Oaks and beech trees are another story. They hang on to many of their leaves well beyond that of other broadleaf deciduous trees. These brown and tan, dead-looking marcescent leaves cling to branches until newly emerging growth pushes them off in spring.

Although leaves falling in autumn are a predictable event, leaf drop is not only seasonal. Plants and trees can lose their leaves for a number of reasons, namely from drought and other physical or environmental stresses. Although any tree is subject to leaf drop under such conditions, not all trees are considered deciduous or even semi-deciduous. Narrow-leafed evergreens such as fir, hemlock, pine and spruce are able to survive winter without foliage loss for two reasons. First, their leaves develop a protective waxy coating. On top of that, the fluid inside their cells contains a version of nature’s antifreeze. Since the attached foliage remains undamaged, there is no need for it to be shed.

While evergreen trees are beautiful year round, deciduous trees are desirable for another reason. Their leaves provide vital organic matter and build structure and water holding capacity in the soil. So each autumn, enjoy and savor the brilliant display of color. And in winter take pleasure in the evergreens. But be thankful for the deciduous trees too, in spite of all the leaves on the ground. They’re doing more good than you might have ever imagined.

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