Why do leaves change colors?
The colors of autumn are one of the best parts of the season.
Autumn is my favorite season of the year. The hot, dry summers in Eastern Washington are great for swimming and trips to the lake but I am always ready when the mornings become crisp and the first signs of fall colors appear on the trees. As my children have reached the age of inquisitiveness they have each asked, "Why do leaves change colors?" Each time I answer, I am able to experience the wonder of autumn from their young eyes as I explain the phenomenon.
Spring and Summer
In deciduous trees, there are pigment cells in the leaves. Chlorophyll pigment cells produce the vibrant greens displayed by the leaves during the growing season. Chlorophyll captures the rays of the sun and changes its energy into food for the plant. During the growing seasons of spring and summer chlorophyll is so abundant in the plant that the green masks any other pigments present.
Leaves contain veins, not unlike our own body's veins, that carry fluid and nutrients through the leaves. In the late summer the bases of each vein become closed off by cork cells, which causes less minerals and fluid to be carried throughout the leaf. This also decreases the amount of chlorophyll which allows other pigments and their characteristic colors to show through.
Carotenoids are pigments that produce the yellows, oranges, and browns we see in autumn leaves. These pigment cells are always present in the leaf but during spring and summer they are masked by the prominent greens of the chlorophyll. When the amount of chlorophyll is reduced these other colors begin to show through. Carotenoids are also present in yellow vegetables and fruits, giving them their color.
Anthocyanin pigment cells are produced in the sap of the leaves during the late summer. When these pigments are allowed to show through in autumn, they give the leaves of some trees the deep reds and purples we see. Anthocyanin is also present in red fruits and vegetables, such as beets and berries. When the carotenoids and anthocyanins combine in the foliage of some trees, the leaves display fiery oranges and reds and bronzes.
The Purpose Behind the Change
Chlorophyll, carotenoid and anthocyanin pigments explain how leaves change color in the autumn, but our original question was, "Why do leaves change colors?" There is not a definitive answer to this question, unfortunately. Scientists disagree as to what purpose the changing of colors has. Most agree that deciduous trees lose their leaves because the amount of energy needed to continue photosynthesis during the low light of the winter months is just not worth it, and dropping its leaves allows a tree to conserve its energy.
The changing of color prior to leaf fall is another story – scientists have many guesses and no real answers. The theories include the protection of the leaves from light at lower temperatures which can be harmful to the tree, and the warning off of insects to protect the tree from heavy parasite loads during the winter month.
So even science can't answer every question for us. I like to think that leaves change colors in the fall simply for the pleasure of those who observe them. If you want to enjoy some of the most beautiful fall foliage, visit New England or Eastern Canada during the 'leaf-peeping' season.