Kathie Fiveash, Island Naturalist columnist for the Island Ad-Vantages
Walking the roads and trails of coastal Maine in the crisp fall weather, with the shadow of winter looming, is a pleasure tinged with sadness. The low autumn sunshine lights the leaves from the side, making them shine. The red maples blaze, the birches flutter yellow, the huckleberries stand deep red under the pitch pines, the oaks turn russet. The spruce trees that usually dominate the landscape seem to stand aside for the other trees, which step forward into their glory.
What causes the amazing changes in the colors of leaves? The deep green of summer leaves is produced by the pigment chlorophyll. This is the compound found in all green plants that activates photosynthesis. In photosynthesis, plants capture light energy from the sun and transform it into chemical energy, which is used to produce simple sugars and oxygen from water and carbon dioxide. Chlorophyll is the molecule that makes this energy transformation happen. The Greek word photosynthesis means, literally, “putting together with light.” Without chlorophyll, most life on earth would be impossible. There would be no plants and no animals, because all animals rely directly or indirectly on plants for their food.
In midsummer, leaves are busy making food in the long hours of sunshine, and chlorophyll is the dominant pigment.
The powerful color of chlorophyll conceals the presence of other pigments in the leaves. Many leaves are also rich in carotenoids, the yellow and orange pigments that give carrots, bananas, corn, daffodils, buttercups and many other plants their colors. In autumn, as the days grow shorter, trees respond by shutting down the production of chlorophyll. Leaves are vulnerable to damage by frost, and the tree needs to jettison them to prepare for the dormancy of winter. As the chlorophyll dies away, the yellows and oranges that have been hidden under the green are unmasked, and the leaves change color. Meanwhile, the tree reabsorbs some of the nutrients held in the leaves, and weakens the attachment of the leaves to the stems. In some leaves, particularly maples with their sweet sap, sugar is trapped in the leaves as the leaf-to-stem connection breaks down. When these sugary leaves are exposed to bright sunlight, they produce anthocyanins, the pigments that cause the brilliant scarlets and oranges that light up our forests in autumn. Then the leaves begin to fall, filling the air and carpeting the earth with their colors.
Soon I will be walking down roads looking at bare branches against the sky. With the leaves mostly down, I still enjoy scuffing through them, kicking them up ahead of me, listening to their rustle. After the next hard rains, the leaves will be sodden and deteriorating underfoot. While the trees burn with autumn colors, it seems that this last flare of beauty must be designed to lift the human spirit. But really, it is the expression of a process of change that ends with leaf fall. For the forest, this is a crucial part of the cycle of life. The leaves are proceeding with the real work they still have to do. They are decomposing and adding their sustenance to the poor soils of our rockbound island, providing next summer’s nourishment for the trees that gave them life in the first place.