The shorter days and cooler temperatures of autumn signal deciduous trees to reduce chlorophyll production, leading to brilliant displays of fall foliage color. During the growing season, trees produce food for themselves by converting water from the soil, carbon dioxide from the air and energy from sunlight into sugars and starches. This chemical reaction, called photosynthesis, occurs in chloroplasts, specialized structures within a plant’s leaves and green stems. It requires the presence of chlorophyll, a pigment which reflects green light. Smaller amounts of other pigments are also present in plant leaves during the growing season, but are primarily masked by chlorophyll. This is why leaves typically appear green to the eye.
In autumn, deciduous trees prepare to lose their leaves and slow chlorophyll production, allowing other pigments to become visible. Yellow and orange leaf colors are due to xanthophyll and carotene pigments, also located in chloroplasts. These pigments help the plant to capture more wavelengths of light for photosynthesis. Many native Colorado trees turn various shades of yellow before the first severe winter frosts. Some tree species produce anthocyanins in response to shorter days and cooler weather. When anthocyanins are present in leaves containing ample plant sugars and exposed to bright sunlight, bright red and purple hues result. Tannins impart a brown color to leaves.
|The fall color of Autumn Purple ash (photo by Bill Monroe, CMG Larimer County)
To learn more about the botany of fall color, check out the following articles:
CMG GardenNotes #141, “Plant Growth Factors: Photosynthesis, Respiration, and Transpiration.” http://www.ext.colostate.edu/mg/gardennotes/141.htmlPlanttalk Colorado #1728, “Why leaves change color in the fall.” http://www.ext.colostate.edu/ptlk/1728.html
University of Tennessee Agricultural Extension Service publication #SP-529, “Changing Colors of Leaves.” https://utextension.tennessee.edu/publications/Documents/SP529.pdfUSDA Forest Service publication, “Why Leaves Change Color.” http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm