The Science behind Autumn Leaf Colors

Special post by: Nika Reininger, Master Gardener in Larimer County (uploaded by Alison O'Connor)

Colorado's fall color: Oak leaves (upper left) appear green while continuing to produce chlorophyll during the growing season. When chlorophyll production stops in autumn, yellow carotenoids in this maple leaf (center) become visible. An ash leaf (right) growing in dappled sunlight develops vivid red hues on parts of the leaf exposed to bright sunlight, as a result of anthocyanins. Shaded leaf surfaces remain yellow. Frost has turned a linden leaf (lower left) from a brief autumnal yellow to brown, as leaf tissues succumb to necrosis. (Photo by Nika Reininger, CMG in Larimer County)

     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)

     Why is autumn color variable from one year to the next? A number of factors determine the timing and intensity of leaf color changes, including plant genetics, moisture, temperature and sunlight. Periods of late summer drought may postpone the start of autumn color changes for a few weeks. Warm, dry, sunny days, followed by cool nights, favors development of water-soluble anthocyanins.  During warm fall days, sugars are produced in the leaves, but cool nights prevent normal flow of sugars and water through the petiole (where the leaf attaches to the branch). Increased concentrations of sugars trapped within leaves results in increased anthocyanin production. Because carotenoids are always present in leaf tissue, yellow autumn foliage remains more predictable from year to year than anthocyanin-based red coloration. While bright, sunny days do favor development of dramatic red and purple colors, excessive drought during a tree’s cessation of chlorophyll production can lead to premature brown coloration and leaf drop. Freezing temperatures result in death of the leaf tissue. Genetics can also influence fall color displays. Within a species or local population of trees, individual trees may develop more dramatic foliage colors than their neighbors. Recently developed cultivars of many tree species offer attractive fall foliage, extending seasonal interest in the garden.
     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.html

Planttalk 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.pdf

USDA Forest Service publication, “Why Leaves Change Color.” http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm