As winter approaches the fall colors signal the hibernation of the deciduous plants. The diurnal (daylight) cycles are an ancient influence on many forms of life. The pattern of decreasing sunlight from June 22 to December 21 and increasing sunlight from December 22 to June 21 regulates many activities, such as the dormancy of deciduous plants and the onset of fall colors. In Pacific Northwest the maples, aspen, oaks, dogwoods, hazelnut, and poison-oak produce beautiful fall colors. Dr. C. Ricthie Bell, a botanist from North Carolina, in a recent interview gives a clear and good-natured account of the development of fall colors. fall color starts the 22nd of June, that (photoperiod change) is the one thing thats been going on for millions of years. Thats the trigger for so many things. As a matter of fact, I try to point out, usually about Christmas, 4 or 5 days after the shortest day of the year, the tufted titmice, boy, their hormones change and theyre singing it! And that triggers it - that little extra daylight. We cant even tell the difference, but the birds can. Its really a neat clock that things are set to."
What color are those leaves right now? "Well theyre green." But if you take away the chlorophyll what color are they? "Theyre bright yellow because the xanthophylls are the yellow pigments. People didnt even know what they did (until recently), but it turns out the xanthophylls are the actual light energy receptors, not the chlorophyll. So the xanthophylls catch it (light energy) and hand it over to the chlorophyll. The chlorophyll makes it into chemical energy.
You see this beautifully in tropical stuff and in rhododendron, evergreens holly, when the chlorophyll dies, which it does - its being continually replaced. (They turn bright yellow). And cool nights just stop that replacement. So as the replacement of the chlorophyll in the leaf stops or slows - well looky there - that leafs turning yellow, right before your eyes. Over a period of about three days, as the chlorophylls gone, that leaf turns yellow.
Now, if the leaf was really productive in photosynthesis and was making a lot of sugars and stuff, well, just before the leaf falls something called an abscission layer forms between the leaf stalk and the trunk of the tree so the tree doesnt bleed. It prevents bleeding when the leaf falls. Well thats great, but if your a sugar molecule and youre in the leaf and somebody closes the gate, what do you do? Well, youre stuck. And if theres a lot of sugar molecules in there, the chemical changes that then go on in the leaf convert it into anthocyanins, the red pigment
The really interesting thing that I like to tell people is that there is no known biological importance or value to color change. It is a gratuitous by-product of the metabolic seasonal change. And its free!