Autumn has arrived in the Northern Hemisphere, and you know what that means – the temperatures are dropping, every conceivable food and drink now comes in pumpkin flavor, and the trees are all decked out in fancy colors. But why do they do that? Science! Let’s take a closer look.
Of course, not all trees change color in the fall. Conifers retain their needles all year round, but for deciduous trees, it’s time to start preparing for the winter. During the spring and summer, deciduous trees and shrubs have green leaves due to the constant manufacture of chlorophyll, a chemical that traps energy from sunlight so that it can be converted to carbohydrates and give energy for the plant to grow. These plants are very sensitive to the amount of sunlight they absorb, so as the days get shorter after the autumnal equinox, they have to start storing energy to get through the winter months. Each species has its own tipping point as to when to start this process and takes a different amount of time to complete it, which is why in some areas the foliage season can last for well over a month, while in less bio-diverse regions, the trees can go from green to bare in about two weeks. In areas where aspen is prevalent, an entire hillside can change color in a single day since the trees are genetic clones grown from a single, shared root system.
The shorter days and cooler weather actually signal a growth process in deciduous plants. The cells at the base of the leaf (known as the abscission layer) start to divide internally and clog up the passages that previously allowed for the exchange of minerals between the leaf and the stem. Chlorophyll breaks down in sunlight, and without a fresh supply of nutrients from the roots to enable the leaf to manufacture more, it disappears entirely. This disappearance allows other chemicals that are usually masked by chlorophyll’s green pigment to become visible. Xanthophylls and carotenoids are chemicals that aid in the harvesting of energy from sunlight, and have yellow and orange pigments, respectively. In some species, the leaves continue to produce carbohydrates known as anthocyanins, which are red or purple in color. Scientists aren’t yet sure why trees expend energy producing carbohydrates that can’t be stored to help get through the winter, but they suspect it may have to do with either signaling to insects and other potential pests that the tree is still healthy so they’ll lay their eggs elsewhere, or that these compounds somehow protect the tree from frost damage or excess water loss.
As autumn progresses, the abscission layer becomes drier and more brittle, eventually allowing the leaves to break off. Wind, rain, or an early frost can cause leaves to fall off sooner than they otherwise would, and some species keep their leaves throughout much of the winter. The colorful pigments fade, leaving behind only brown tannins. Losing leaves allows the trees and shrubs to conserve energy in the winter months, so when the days begin to lengthen and get warmer in the spring, new leaves can bud to start the growing cycle anew.