The Reason for Fall

Perhaps the most iconic aspect of autumn is the shift from vivid green tree canopies to waves of yellow, red, and orange. Following that, leaves begin to drop and leave behind the skeletal form of the trees they once adorned. But why exactly do trees do this? Why not just hold onto their leaves year-round?

Leaf color is linked to various chemicals stored within a tree. The green color associated with trees and plants is the result of chlorophyll. Chlorophyll allows a tree to absorb light and conduct photosynthesis, converting light into energy. It is crucial for a tree to generate as much energy as possible during the spring and summer.

As summer reaches its end, so does a tree’s need to produce energy. Photosynthesis begins to slow as the tree reduces the amount of chlorophyll in its leaves. This in turn begins to reduce the green color within a leaf. As the amount of chlorophyll is reduced, it allows for other chemicals to fill that space. Trees with yellow foliage in the fall have a higher level of carotenes. Carotenes are always in leaves, but the yellow color is superseded by green from chlorophyll. If temperatures stay above freezing, chemicals called anthocyanins are produced, which result in red and pink colors.

What causes leaves to fall?

Trees shed their leaves for a number of reasons, but the main reason is that they’re trying to conserve energy through the colder months. By shedding their leaves, they can focus on making it to next spring.

In the spring months, a layer of tree cells called the abscission layer forms where a leaf grows from a branch. Throughout the growing season hormones and chemicals stay fairly constant, and energy can pass through this layer into the branch and then the trunk. As days begin to get shorter and the temperature starts to drop, the tree starts to produce less of a growth hormone known as auxin. As this happens, the abscission layer begins to weaken, causing the leaf to wither, die off, and eventually be knocked off by the wind. Some trees have stronger bonds at the abscission zone than others, such as White Oak. Oftentimes the following year’s new leaf growth can also push off the previous year’s old leaves.

What about evergreens?

Up until this point we’ve been talking about deciduous trees (trees that shed their foliage each year), but what about evergreen trees? Most commonly, evergreens refer to conifer trees (Pine, Spruce, Fir, etc), but there are evergreen broadleaf trees as well, such as the Live Oak. While it may look like these trees simply never lose their foliage, evergreens typically just shed their needles or leaves at a slower rate and hold onto their canopy for longer. This gives the appearance of being constantly green. Evergreen trees achieve this by having foliage better adapted to harsher weather and can conserve water for longer, which in turn extends their growing season.

One more note about leaves

When possible, it’s best to keep the leaves your trees drop in your yard. While they may be a nuisance to some, leaves are a great source of organic nutrients that can help keep your trees healthy. For more information on why it’s important to leave the leaves, take a look at some of our previous articles on the subject.