The following description appeared in an article written by Rich Norby that was originally published in the UT Arboretum Society Journal, The Leaflet. To view the entire article click here.

Why do some trees dazzle us so much more than others? It’s all a matter of chemistry, and the chemistry reflects a range of strategies of how trees interact with their environment. The red color of scarlet oak leaves comes from compounds called anthocyanins. Anthocyanins form in leaf cells by reactions between sugars and other ring-like compounds. They are soluble in the cell sap, and their color depends on the acidity of the sap—they are red in acid solution and become blue as the acidity decreases (the same story as with hydrangeas). The color from anthocyanins often mix with that from other compounds such as carotenoids and xanthophylls that are responsible for the bright yellow color of some species. The color of many oaks is dominated by the brown of tannin. Although new anthocyanins may be produced in the fall if there are enough sugars in the leaf, many of these compounds are present all summer long. They can play an important role in protecting the leaf from the damaging effects of too much light or ultraviolet radiation (natural sunscreen). Their colors, however, are masked in the summer by the strong green color of chlorophyll. In the fall, chlorophyll breaks down and no more is synthesized as the need for photosynthesis declines. Various proteins associated with chlorophyll and the process of photosynthesis also are broken down, enabling the valuable nitrogen-containing compounds to be reabsorbed into perennial tissue. (The leaves of alder and black locust trees, you might notice, stay green right up until frost. These species are nitrogen-fixers and do not need to conserve nitrogen as much as other trees.) So the characteristic fall color of different species results from various combinations of the formation of red pigments (anthocyanin), the unmasking of yellow pigments (xanthophyll and carotenoids), and the disintegration of the green chlorophyll. Scarlet oak leaves, as well as red and sugar maple, sumac, sweetgum, sassafras, and a few others, form large amounts of anthocyanin, and thereby make the top ten list for fall color.

Every year the local weather man, and occasionally a tree physiologist, is asked whether it will be a good year for fall color. Any environmental factor that favors the production of sugars or the conversion of insoluble carbohydrates to soluble one will favor the production of anthocyanin and bright autumn colors. Low temperatures (but above freezing) stimulate anthocyanin, but severe early frosts will retard it. Bright sunny days will also favor bright colors, and leaves that are hidden from the sun will not develop bright colors (try this as an experiment!). People often think that lots of rainfall in the autumn should be good for color, but this is not so. Drought actually stimulates anthocyanin production, and lots of rain means not very much sunshine. When leaves are damaged, by an insect for example, the process of resorption [movement] of nitrogen compounds out of the leaf is disrupted, and a portion of the leaf may stay green, resulting in a splotchy appearance (another experiment you could try!).

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