Why Leaves Change Color and Fall
 | Author:
Allison Kosto, MSU Broadwater Extension Agent
MSU Broadwater County Extension Agent |
Fall is in the air! And with fall comes many changes, some welcomed and others not, often depending on your thoughts. One more enjoyable change is the changing of the leaves in the fall. However, not all trees change color in the fall. Some seem to simply dry up and fall off. What causes leaves to change color in the fall? For many years, scientists believe that the coloring of fall leaves was caused by waste products that accumulated in the tree throughout the growing season. However, we now know that is not true.
Trees respond to changes in the length of daylight in addition to temperature changes in the fall. At some point, the changes are great enough to signal the tree to stop the food-making process and initiate leaf senescence. As a result, chlorophyll, which makes the leaves green, starts to break down and the green color can often disappear. As the green color disappears, it reveals yellow or orange pigment that is usually hidden by the abundance of chlorophyll throughout the summer. Sometimes a mixture and varying amounts of chlorophyll with produce a less exciting brown color.
Another cause of brown or copper leaves can be an abundance of tannins, often seen in oaks and beech trees. Tannins make the leaf taste bitter and are considered a waste product of certain plant processes. Leaves store them up as a defense mechanism against disease or anything that may feed on the tree. This pigment is present throughout the year but is often hidden until chlorophyll fades.
At the same time, chlorophyll stops or slows, and other chemical changes are happening during leaf senescence. These chemical changes produce red anthocyanin pigments. This produces the red and purple fall colors that you often see in maples, dogwoods and sumacs. Temperature, light and water supply also influence the intensity and duration of fall color. An early freezing spell can weaken fall color. A warm gradual fall will often intensify color.
Another process that happens during leaf senescence is the formation of a special layer of cells where the leaf stem is attached to the tree. Once this layer is complete, it severs the flow of materials between the leaf and tree and the leaf is ready to fall. When the leaf falls, the tree seals the location where the leaf was attached to prevent any diseases and insects from entering the tree. What is left behind is a leaf scar which can often be visible for a few months to several years after a leaf falls.
Even though most trees lose their leaves in the fall, there are a few exceptions, most notably oak trees. The dead brown leaves of oak trees and a few other species may stay on the tree until growth starts again in the spring.
Most conifers or evergreen trees with needles do not lose their needles or scales. They remain green year around. However, they may experience seasonal needle drops. This is the process of trees shedding needles that are 2 to 4 or more years old. They start yellowing or browning beginning with the older inner needles and then they will fall. This is often most visible on pine trees and can cause some alarm. However, it is a natural process.
Trees really are amazing! They can take a lot of work but can also be a lot of fun. Sit back this fall and enjoy the beauty of it all! If you have any questions about tree care, you can reach the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu.
