Ask A Scientist

Why do leaves change color in the fall and why don’t they stay green all year long?

Asked by: Jill Stafford
School: Sidney High School
Grade: 11
Teacher: David Pysnik
Hobbies/Interests: Sports, photography and arts.
Career Interest: Teaching

Answer from Dylan Horvath

Steward of Natural Areas, Binghamton University

Research area: Wildlife biology/ecology-wolverines, bats, salamanders and birds. Interests/hobbies: Drawing, photography singing and hiking.

One of the wonderful characteristics of the Northeast is the fall colors.  It is said that ‘the hills come alive’ with fiery shades of red and orange, golden yellows, and even purple and rusty brown. Some people can actually tell which tree species they are seeing, just by the color of the leaves, but how many people know why deciduous leaves change color in fall?

Deciduous trees are 'broadleaf' trees, which drop their leaves in the fall, such as oak and maple trees.  The green color of leaves in the spring and summer comes from a chemical called chlorophyll, which plants use to capture sunlight.  Similar to the way humans can capture sunlight in solar cells and convert sunlight into electricity, plants convert sunlight (along with carbon dioxide, water, and nutrients from the soil) to usable food energy.  Leaves contain other pigments, such as carotenes, the pigment that makes carrots orange, which are masked by the presence of chlorophyll.

With fewer hours of daylight in the winter, the amount of sunlight deciduous trees can capture is not enough to support their leaves through the winter.  Therefore, trees stop maintaining leaves, drop them (hence the season of ‘Fall’), and live off of stored energy.  Conifers, ’evergreen trees’, retain their needle leaves all year long since needles take less energy to maintain.  As deciduous trees cut off nutrients from the leaves, the leaves stop producing chlorophyll.  The other pigments in the leaves become visible as the last chlorophyll breaks down, inadvertently giving us humans our fall color show.

Last Updated: 9/18/13