Why Do Leaves Change Color?

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Peak foliage in the White Mountains National Forest, NH. (Photo by Chaitram Aklu)

By CHAITRAM AKLU

As summer ends, we see advertisements from various geographic locations enticing visitors to go and experience the spectacular autumn views that the colorful fall foliage presents.

“Fall colors are funny. They’re so bright and intense and beautiful. It’s like nature is trying to fill you up with color, to saturate you so you can stockpile it before winter turns everything muted and dreary,” writes the New York City born novelist, Siobhan Vivian.

We know that leaves change color in the northern hemisphere September through October, from far north proceeding south and in higher elevations first. But why do leaves change color?

The color in leaves which are present all the time, come from molecules of matter called pigments, natural substances that are produced by the cells in leaves.

These pigments are chlorophyll, carotenoid, and anthocyanin. Chlorophyll is green, carotenoid is yellow, and anthocyanin is red.

Chlorophyll is abundant during spring and summer in temperate regions of the world. Chlorophyll enables plants to photosynthesize (make food) absorbing sunlight. There is plenty of sunlight during these two seasons and plants are able to make a lot of chlorophyll – which is why we see green everywhere. The other colors, while present, are not seen because of the abundance of chlorophyll present. In tropical regions where there are long hours of sunlight all twelve months of the year, plants are able to manufacture abundant amounts chlorophyll which in turn allows them to photosynthesize and grow throughout the year.

As summer ends and the days begin to get shorter and shorter (as a result of the earth’s revolution around the sun) and there is less sunlight hours, leaves are not able to make as much chlorophyll and their food-making process slows down. Evergreen trees (trees which do not shed their leaves) on the other hand grow slowly throughout the year. They must conserve energy and nutrients in order to produce new leaves and stay alive under harsh seasonal conditions. As the amount of chlorophyll decreases in deciduous trees (trees which shed their leaves annually) so does the green color. The yellows, oranges, browns, and reds begin to become more prominent and we are able to see them. As the weather continues to get colder, drier, and the duration of sunlight reduced with approaching winter, photosynthesis stops, and the leaves die and fall to the ground. The trees stop growing.

Sometimes we see trees showing only yellow colors while others may have only browns. These differences in color are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf during fall season. The genetic make-up together with the duration of sunlight produces the stunning array of colors that are predicted each year.

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