Short days and cool temperatures are the dual triggers of fall color. They are signals to the plants to begin preparing for their long winter snooze. But some of the glowing colors are not just a part of getting ready for dormancy. Many plants actually expend energy to produce the compounds that make the reds, yellows, and lovely purples, and oranges of fall.
On a basic level, the plants break down some of the more valuable components of their leaves and move them to locations where they can be safely stored during the cold season. As chlorophyll is broken down and its components reabsorbed, the less valuable chemicals remain, giving the leaves a yellow or mahogany hue. The more vivid reds and oranges are another story. These are produced by compounds that the plant manufactures for a specific purpose. As a rule, organisms do not expend energy without some kind of return. People began to wonder what the benefits of foliage color could be.
There is currently much debate in the scientific community as to the exact nature of these benefits. One theory is that the chemicals that generate different colors also protect the plants photosynthetic centers from cold temperatures. This safeguards the plants ability to recover the most biologically costly of the chemicals associated with photosynthesis.
Another interesting theory is that the bright colors are actually a screen against aphids. Worrying about aphids in November may seem a little silly, but this is actually the time when they are sizing up locations to lay their eggs. The eyes of aphids are very sensitive to green, as it is likely to signal a meal, but they have a hard time seeing red. This means that the red leaves which are so bright to our eyes are actually a good disguise against aphids. They will move on to a tree with green or yellow leaves, and deposit their eggs there, thinking they have chosen the best potential food source. Another of natures clever and fascinating ploys!
No one has figured out the definitive answer, and there are many more possibilities. It is a rather interesting conundrum to consider while taking a walk through the bright colors this autumn.