In the Autumn, northern forests put on spectacular displays of color. The shades of reds, oranges, yellows, purples, greens, and brown are both genetically and environmentally determined for each species: each having a range of colors that develop in response to temperature and moisture changes.
Accessory Pigments
Autumn colors are provided by photosynthetic accessory pigments: carotenes (yellows and oranges) and anthocyanins (reds and purples). An archaic explanation for fall colors was that the pigments responsible for them are always in the leaves and are merely uncovered when the more dominant chlorophyll breaks down in autumn. Recent studies show that concentrations of carotenes and anthocyanins increase dramatically as autumn progresses.
Providing Ultraviolet Protection
When leaves age, they still contain large quantities of proteins, starch, vitamins, and minerals. Converting these nutrients into mobile forms and moving them to storage areas for reuse takes time. New evidence shows the increased levels of carotenes and anthocyanins absorb ultraviolet light (UV) and reduce sun damage while the photosynthetic machinery is disassembled. These same pigments in food protect our cells against free radicals that cause cell damage in a manner similar to that of UV.
Color Production Is Lower For Some Species
Whereas maples (Acer spp.) and others produce outstanding color displays in the fall, oaks (Quercus spp.) tend to undergo less dramatic changes as their leaves prepare to drop. Except for some red oak species, the leaves of most oaks rapidly transform from green through yellows to brown while still on the tree. This would imply these leaves are not easily damaged by ultraviolet light and/or they can break down and remove their photosynthetic apparatus more rapidly than can trees with more delicate leaves like the maples, sassafras (Sassafras albidum) , and sumacs (Rhus spp.).
Oak leaves have high concentrations of tannins (tannic acid) that increase through the growing season, reaching their maximum levels in the fall. Tannins absorb ultraviolet light much like carotenes and anthocyanins, and provide the same protection from UV damage that accessory pigments do.
Insect Repelling Pigments
Recent research on fall pigments in tree species groups that are typically infested with aphids in the spring suggests that aphids lay fewer eggs on the stems of species that produce more anthocyanins. Amotz Zahavi, in his book The Handicap Principle, and this research suggest investing energy to produce these pigments may represent a form of aposematic warning to aphids that these trees that have the energy to produce an abundance of pigments in the fall will also be able to produce toxic chemicals in the spring that will inhibit growth of the aphids and restrict production of their offspring. If so, aphids should avoid brightly colored trees and lay their eggs on ones with duller colors that might indicate plants with poorer defenses.
Red and sugar maples produce brilliant red and orange foliage in the fall, while Norway and silver maples are paler yellow and orange. In the spring, aphids feeding on Norway and silver maples produce vast quantities of honeydew that covers the windshields of autos, while red and sugar maples, with brighter colors in the fall, have fewer aphids in the spring.
What About Dull Oak Leaves?
In addition to absorbing UV light, tannins also block the digestive activity of leaf eaters, but not of aphids that attack stems. Many species in the red oak complex produce acorns and leaves with higher tannin levels than are found in most other groups of oaks, but some red oak species also produce large amounts of anthocyanins. Since absorption of UV light by tannins protects the aging leaves, large concentrations of anthocyanins would appear to be superfluous in red oaks unless these plants were subject to attack by stem feeding aphids. Thus, there should be some other reason for these oaks to generate superfluous anthocyanins.
 |
 |
 |
