Why Do Leaves Shed in Fall?

  • Difficulty Rating: Beginner
Why Do Leaves Shed in Fall?

Although leaves falling in autumn are a predictable event, have you ever stopped to ponder why it happens in the first place?

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.

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. Before you know it, they’re covering your lawn and garden.

If you do nothing, they’ll eventually rot in place or blow away. Yet leaves provide vital organic matter and build structure and water holding capacity in the soil. So although that carpet of leaves is a time consuming job to clean up, be thankful for the deciduous trees in your life. Shred them up with a mulching mower and rake them back into your beds or add them to your compost pile. But whatever you do, keep them on your property. They’re doing more good than you might have ever imagined.