Fall approaches, so I thought I’d explore the reasons behind the annual burst of color. We in the south won’t see real changes to our trees for a few months yet, but folks in Northern climes should start seeing variety in their verdure by the end of this month. Dwindling daylight triggers the transition, with moderate fall temperatures producing the best color.
Of the three pigments that determine a leaf’s color, green-hued chlorophyll dominates during the long summer days. It soaks in sunlight and converts the solar energy to sugars that feed the tree–the process known as photosynthesis. (This process also produces oxygen. In fact, photosynthesis creates nearly all the oxygen on the planet, with about one-third coming from land plants and two-thirds from ocean plants like plankton and kelp.) Photosynthesis depletes chlorophyll, but sunshine creates new chlorophyll to perpetuate the process.
As days shorten, chlorophyll production in the leaves of deciduous trees and bushes sputters and finally collapses. The other two pigments in the leaves seize the opportunity and assert themselves. These are carotenoid pigments, which produce reds, yellows, and oranges; and anthocyanin pigments that contribute reds, purples and blues.
As the long-suppressed pigments emerge, the splashiness of their display depends on the weather. Warm, sunny days allow the leaves to produce as much sugar as possible before they fall off. Too many autumn clouds, or an early freeze, will short-circuit sugar production and with it the season’s color.
Finally, we can’t forget evergreens. It’s not true that they never shed their foliage. Evergreen conifers, which include pine trees, replace older needles, but not necessarily every year. Those that drop change colors first, turning deep copper or light gold before dropping The difference between deciduous trees and evergreens is that evergreens always retain a healthy supply of green to carry on photosynthesis, while deciduous varieties give us brilliant colors to enjoy every fall.
“Autumn Leaf Color,” Wikipedia.
“Autumn Leaves: How Plants Prepare for Winter,” Science Made Simple.
Garcia, Julie, “Houston is Having a Vibrant Fall Foliage Moment. Here’s the Science Behind Why,” Houston Chronicle. 26 Nov 2019.
“Save the Plankton, Breathe Freely,” National Geographic.
“Science of Fall Colors,” U.S. Forest Service.
Taylor, Lindsey, “Photosynthesis in Pine Trees,” Sciencing. 22 Nov 2019.