1. Leaves contain a green pigment called chlorophyll. Chlorophyll is instrumental in photosynthesis, the process in which energy from sunlight (along with water and carbon dioxide) is used to make sugars that feed the plant. Over time, chlorophyll breaks down, but during the growing season it is continually replaced.
2. In autumn, the environment is less conducive to growth (primarily as a result of less sunlight and colder weather), and so photosynthesis is suppressed. Chlorophyll continues to deteriorate, but now not as much is replaced. After the first frost, the rate of breakdown increases dramatically.
3. Yellow and orange pigments called carotenoids, which have been in the leaf all along, are unmasked as chlorophyll levels decline.
4. In some tree species (red and sugar maples, for example), red and purple pigments called anthocyanins develop in late summer and fall, largely in response to external stress — such as the first frost.
5. This is fall. Leaves with a variety of chlorophyll levels, many with so little that all we see are the carotenoid and anthocyanin pigments, create a “mosaic of color on the hillsides.”
6. The more anthocyanins produced, the more brilliant the red. Shades of color are also influenced by how the pigments interact with other factors in the leaf, including the pH level and what minerals are present.
Paul G. Schaberg, Ph.D., Research Plant Physiologist, USDA Forest Service, Northeastern Research Station.