Ask A Scientist
How or why do the leaves change color in the fall and why do they fall down in the winter?
Asked by: Ariana Mariah Selby
School: Maine-Endwell Middle School
Teacher: Kevin Wagstaff
Hobbies/Interests: Swimming, playing tennis and basketball
Career Interest: Nurse
Answer from Stanley N. Salthe
Visiting scientist, Binghamton University
Research Area: Natural philosophy Interests/hobbies: ecology, evolutionary biology, semiotics, systems science, and thermodynamics. Woodland gardening, nature walks, all of the arts Ph.D: Columbia University Family: wife Barbara, two children Becky and Eric
The fall colors of deciduous trees are caused by the presence of two chemicals and left over sugar that was formed in the leaf by photosynthesis. The reds and violet colors are produced by anthocyanins and the sugar, glucose, while the yellows result from xanthophylls. Orange comes from mixtures of reds and yellows. As the green color, chlorophyll, used in photosynthesis, fades these other colors become revealed.
Curiously, leaf color change happens only in eastern North America and northeastern Asia, for reasons unknown. Trees that come from other geographic areas and planted in these colorful places do not color up, and so we know that the ability to develop color is inherited. But conditions are important as well. Some years the colors are magnificent, while in others they are less so. The combination of temperature changes and humidity are important in coloring. The reds in particular seem to require special conditions to be just right, and red leaves often drop earlier than yellow ones. Hickories will get a nice yellow only in shady conditions, just turning brown in the open. The brown colors seen after the bright colors fade come from a mixture of waste products. The dropping of leaves is way that deciduous trees get rid of waste products, and also sets up recycling for next season.
In our area, the reds are seen, for example, in sumacs, red maple, pepperidge, and, later on, in many of the oaks. The imported Asian shrub called burning bush is especially outstanding, being brilliant crimson in the open and a striking pink in the shade. Some ash trees display a beautiful purple color, but the leaves drop quickly after turning. Our sugar maples develop a striking golden to orange color resulting from a mix of yellow and red pigments. Most other trees in our area take on various shades of yellow. South and east of our area, a champion colorist is the sweetgum, which turns just about every color you can imagine. Some trees, like the tulip poplar, turn one leaf at a time, yellow to brown, dropping them individually. Others, mostly imports from the Midwest like the catalpa and the black locust, don’t turn color at all.