A brilliant and ephemeral season is upon us: fall foliage. As the temperatures drop and the days get shorter, a corky layer of cells – the abscission layer – begins to form at the end of deciduous leaf stems. This layer prevents the transport of materials between the leaf and the rest of the tree. Chlorophyll, the green pigment that absorbs the sun’s energy to transform carbon dioxide and water into carbohydrates, is no longer able to be replaced and slowly breaks down and disappears.
Without that green chlorophyll, the yellow and orange pigments – xanthophylls and carotenoids – that were concealed in the warmer months are revealed. Some trees (such as birches, aspens, elms, beeches, hickories, american basswoods) merely unmask these shades of lemon and honey.
Other trees produce red pigments – anthocyanins. These mix with yellows to create flaming crimsons and purples. A succession of warm, sunny days and cool, crisp but not freezing nights brings about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. Lots of sugar and light spur production of the brilliant anthocyanin pigments.
Why is there so much variation in color between trees and even leaves on the same tree? Each maple leaf produces its own combination of green, yellow, orange, and red depending on how much sugar is trapped in the leaf, how close to dying it is, and how much light it is getting. On any one maple, there should be leaves in all stages of coloration and if you watch one leaf from day to day, you will see it change.
How many shades of leaf color can you find?!
