Understanding Precisely Why Leaves Change Colors in the Fall

'Tis the season for the leaves to begin the process of changing colors. Some of the northern reaches of the country and the higher elevations are already beginning to see the leaves start to gradually change hues. What is the science behind the leaves changing and when can you expect the burst of color in your neck of the woods? Read on for all of the details.

How Science Explains the Annual Fall Foliage Season

The annual changing of the leaves demonstrates the unique synergy between the weather, specific tree species, growth cycles, and the science of chemistry. The beginning of the fall season brings cooler temperatures and shorter days, ushering in the necessary ingredients for the leaves to begin to change colors and eventually fall from their branches.

Mother Nature is the most significant driver of when the leaves will start this process. The part of the leaf known as the abscission layer is where the transition begins. Scientists detail that this layer of cells, located between the leaf stem and the tree branch, gradually stops sending crucial nutrients and water to the core of the leaf during the early fall. The stoppage of this flow of nutrients is what causes the trees to lose their leaves.

The natural change in the weather then accelerates the process by ushering in the fall season with cooler temperatures paired with less sunshine. The end result is that the leaves do not produce as much green chlorophyll, paving the way for the colors to emerge.

The abscission layer also serves to guard the vulnerable parts of the plant from the elements of winter. This includes colder temperatures, windy conditions, and increasing dryness.

As a result of the decrease in chlorophyll production, the other chemicals housed in the leaves begin to emerge. For instance, the pigment known as anthocyanins exudes a red color. Carotenoids are responsible for leaves with an orange hue, while xanthophyll imparts a yellow color.

Warmer temperatures in the overnight hours during the early fall can mitigate the production of the anthocyanins, resulting in duller colors for the season. Conversely, an early-season frost or freeze can also halt the ability of the leaves to generate red and purple. A harsh enough cold weather event can stop the turning of the leaves completely, sending them to the ground.