I will readily admit that fall is my favorite season of the year. The fact that I work at the Polly Hill Arboretum (a tree museum!) makes it possible for me to see the collective fall color of over 1,300 different trees and shrubs. Living in a temperate environment with distinct changes of season, the progression of spring bud break, summer leaf flush to autumn leaf drop is an absolute joy to behold. And when you take a deeper look at the science of fall color you can begin to appreciate the fascinating physiological processes that make it happen. The leaves don’t actually change color. The vibrant reds, oranges, yellows and purples are always present in the leaf. However, they are masked during the growing season by the dominant plant pigment: the green of chlorophyll.
As a plant evangelist I often start out lectures with an ode to chlorophyll. This amazing pigment makes it possible for plants to capture light energy and convert it into food energy. In doing so, plants provide the air we breathe by absorbing carbon and emitting oxygen. We simply cannot live without plants. Chlorophyll is essential for the survival of the plant, but also for the survival of our whole planet. And yet, in the fall chlorophyll takes a breather.
How does this happen? The colors within leaves are contained in tiny structures called plastids. Plastids filled with green chlorophyll are called chloroplasts and those with yellow and orange colors are called chromoplasts. Looking specifically at the pigments, yellow to orange shades are called carotenoids. Not surprisingly, carrots are heavy with this pigment. The vibrant reds and muted purples are from anthocyanins. Here I can cite another common vegetable with intense red pigmentation: beets are rich with anthocyanins.
As the days shorten and nighttime temperatures begin to cool, trees and shrubs begin the great unmasking. They slowly shut down chlorophyll production, and in the process the underlying pigments are revealed. The process, both physical and chemical, is influenced by many factors. The chemical part involves the cessation of chlorophyll production. The physical process involves the shutoff of the internal plumbing system from the petiole (leaf attachment) to the main stem. Here the veins that form the pathway between leaf and stem begin to develop a wax. This waxy substance is called suberin. As a result of shortening days, plants receive a signal to seal off the leaf veins with suberin. Through this slow but steady process the veins are eventually shut off. This area is called the abscission zone — the point of leaf attachment to the stem that develops a barrier that causes the leaf to separate and fall to the ground. The stem remains sealed off by the wax to prevent moisture from escaping but also for protection from what would have been an open wound.
You can bear witness to this miracle of nature by picking leaves off the ground and examining them. Touch the base of the leaf stem and it will appear hardened. Look at it under a microscope (if you have one handy) and you will see the suberin quite clearly.
From an evolutionary perspective there must be some inherent advantage to the deciduous lifestyle. Many scientists suggest that winter-hardy trees shed their leaves to avoid the potential damage of snow and ice to their framework. It is also hypothesized that with decreased sunlight, and therefore the limited photosynthetic potential of winter sun, trees drop leaves to save energy. Whatever the reason, both evergreen trees and deciduous trees are never truly dormant. They are resting up for their next great performance, the verdant emergence of new leaves in the spring as chlorophyll returns to begin the cycle all over again.
Tim Boland is the executive director of the Polly Hill Arboretum in West Tisbury.