For many years I have enjoyed the fall colors, yet without knowing exactly why the leaves change colors. People have given me a number of one-line explanations, such as: It is because of cooler temperatures. It is caused by lack of sunlight. The fall colors are already in the leaves, but are covered up in the summer by the green color. At some point I became curious enough to look it up. It turns out that I had received a collection of partial answers, and the fall colors are due to more than one cause.
Eastern Canada and the northeastern United States have ideal conditions for brilliant fall colors. The colors of the leaves in deciduous trees turn to variations of yellow, red, orange and brown. The different colors are produced by different pigments in the leaves, which become dominant when the green color fades.
During the growing season the leaves produce chlorophyll, which is responsible for the normal green color. The production of chlorophyll is part of the chemical process of photosynthesis. This is the process that converts sunlight into energy the tree needs to grow. When the fall arrives the daylight hours get shorter and this decreases the production of chlorophyll, until it will eventually stop altogether.
Yellow pigments, called carotenoids, have been produced throughout the growing season, but in smaller amounts than the green pigment. When the green color dissipates the yellow becomes visible. The red color comes to the leaves later in the season, mostly being produced in the autumn. The pigment name for red in trees is anthocyanins. When these pigments are lacking, other pigments called tannins can affect the leaf color. Tannins are mainly responsible for the brown colors.
Variations in Color and Intensity
The 4 pigments (chlorophyll, carotenoids, anthocyanins and tannins) can be present in various amounts. Therefore leaves are not always pure green, yellow, red or brown; they can be a mixture of more than one color. For example, when the yellow and red pigments are dominant the leaves will appear orange; there is no single pigment that will produce orange.
Different species naturally produce specific pigments, which will keep the colors of the same species fairly consistent from tree to tree and year to year. That being said, each autumn the climate is slightly different. Changes in sunlight and temperature will result in varying amounts of pigments, thus affecting the fall colors.
The yellow pigments are always present in the leaves and this will keep the yellow colors fairy consistent. However, red pigment production is whether dependent. Falls that have warm sunny days and cool nights (but above freezing) will result in the most spectacular red and orange colors.
So, if you have noticed that some years the fall colors are more vibrant, it is not your imagination. In those years the conditions were probably ideal. Nevertheless, having some knowledge as to why the leaves change can deepen the enjoyment of the fall colors. Combined with the crisp cool air and generally low humidity, autumn is my favorite time of year. But as it is with many of nature’s spectacles, it does not last for very long. My suggestion is to get out, take a walk or a drive and simply observe.
References: USDA Forest Service, Why Leaves Change Color, http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm, July 7, 2011.
ESF, Why Leaves Change Color, http://www.esf.edu/pubprog/brochure/leaves/leaves.htm, 2015.
Why do Leaves Change Colors in the Fall? https://www.youtube.com/watch?v=AeypaiIoMPI, Uploaded on October 20, 2009.