In the fall, the leaves of deciduous trees turn yellow, orange, red or mixes thereof. The vivid yellow and orange colours that we see in the fall are present throughout the spring and summer, but are not readily visible to our eye.
Tree leaves contain three primary pigments: carotenenoids, anthocyanin and the photosynthetic pigment chlorophyll, which captures the sun’s energy to turn water and carbon dioxide into glucose and oxygen. As the most abundant pigment, chlorophyll gives leaves their green hues in the spring and summer, while masking the other pigments.
As the hours of sunlight decrease in the fall, the veins that carry sap into and out of a leaf gradually close. With the approach of cooler weather and daylight hours getting shorter, the tree’s growth processes turn to preserving food for the winter months. The tree stores the available nutrients and food in the tree’s roots, branches, stems and trunk until next spring when they are recycled to re-leaf the tree.
With the loss of chlorophyll, other pigments present in the leaves become visible to the human eye. Carotenoids, already in the leaves, shine through as the yellows of maples, birches and aspens; the browns in autumn leaves are the result of tannins, a chemical that exists in many leaves, especially oaks.
The yellow, gold and orange colours created by carotenoids remain fairly constant from year to year. That’s because carotenoids are always present in leaves and the amount does not change in response to weather.
In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose into red pigment bearing anthocyanins. Anthocyanins provide the oranges and reds of maples, sumac and oaks. Most anthocyanins are produced in the fall, and only under certain conditions. Not all trees can make anthocyanins.
When a number of warm, sunny autumn days and cool nights come one after the other, with low moisture levels, we can expect vibrant fall colours with an abundance of reds. The frequency and amount of rain during the year also affects the autumn leaf colour. If there’s more cloudy days with less sun, anthocyanin isn’t as chemically active and the maple leaves are more yellow or orange than red. Colours are muted, subdued.
Before they completely die, the leaves take the nitrogen, nutrients and sugar… and move it back to the trunk for storage. A cork-like barrier forms at the base of the leaf stem, separating the leaf from the tissue that connects the leaf to the branch. The leaf is expendable and falls.
Credits:
- “by EarthSky in Earth” – September 2016
- Trees Inside Out – virtual museum – Montreal
- Science Daily – UW – October 2007
- Globe and Mail – Nicole Mortillaro – September, 2015
- Wisconsin Dept. Nat. Resources – Why do Leaves Change Color?