According to University of Illinois Jeffrey O. Dawson, with declining autumn temperatures, tree leaves stop producing chlorophyll, the green pigment of photosynthesis.
He writes for the UI Extension website that as the green color fades, yellow carotenoid pigments already present in the leaves are unmasked. The clear yellow color of such trees as hickory is attributable to carotenoid pigments. During the summer, these pigments, primarily carotene and xanthophyll, function in the leaves to transfer light energy for use in the photosynthetic production of carbohydrates.
While all trees contain the yellow carotenoid pigments, not all possess the genetic potential for producing anthocyanins, which cause the pink, red and purple leaf colors, Dawson writes.
The anthocyanins, which occur in solution in the cell sap, are formed from excess soluble sugars that accumulate in the leaves after the weather turns cool in early fall.
Like the carotenoids, anthocyanins are unmasked when autumn temperatures halt the leaves' production of chlorophyll. If the cell sap is acid, the anthocyanin in solution will be red. If the pH of the cell sap is higher (that is, approaching alkalinity), anthocyanin appears purplish to blue. The color of a tree's fall foliage will depend on the combination of pigments in the leaves.
Poplars, tulip tree, hickories, beech, birches and honeylocust have yellowish hues. The most brilliant displays are seen in trees that form large amounts of anthocyanin: sumac, sassafras and white oaks.
While trees are responding to the cooler temperatures, Dawson writes, they are also reacting to the shorter daylights of autumn. Trees and other plants "sense" day length by means of a substance called phytochrome. Scientists believe that changes in phytochrome brought about by day length may be linked to the production of plant growth regulators.
The effect of day length can be observed in tree leaves that grow close to street lights. These leaves stay green longer and are shed later than other leaves on the same tree, providing the tree species is sensitive to day length.
Source: Jeffrey O. Dawson, emeritus professor of tree physiology