We know summer is at an end because the days get colder and shorter. Although many people dread the end of summer, there is one feature of autumn that continues to amaze us: the beautifully changing colors of the leaves. But why do the leaves change colors? And why do they fall from the trees?
Let’s start with the basics. Like all living organisms, trees need energy from food to survive. For trees, the food of choice is glucose, a simple carbohydrate or sugar. However, trees do not consume the carbohydrates they need; they have to make it themselves. They do so through the process of photosynthesis, in which water and carbon dioxide react to form glucose and oxygen. But this process does not occur spontaneously; there needs to be a catalyst that gives the reaction a kick start. This catalyst is light energy, which is readily available from the sun during the summer. Light is composed of different wavelengths of different colors, each of which has a different amount of energy.
Photosynthesis needs the energy from red and blue wavelengths to complete its reaction. In order to collect this energy from light, trees use a pigment called chlorophyll to absorb light for use in photosynthesis. Electron delocalization in a portion of the chlorophyll molecule allows for the absorption of red and blue wavelengths of light, but the electrons of the light receptor complex are not excited by some wavelengths of light, most notably green. Therefore, the unabsorbed green wavelengths are reflected off of the pigment, causing them to appear green.
Photosynthesis occurs at the area of a plant that receives the most sunlight; for trees, this would be the leaves. Photosynthesis occurs in the leaves, so the leaves appear green from the chlorophyll.
As noted before, autumn brings colder and shorter days. The tilt of the Earth causes the Northern hemisphere to receive less direct rays than during the summer, so the amount and intensity of sunlight reaching Earth diminishes. Because sunlight is so important to trees in the process of photosynthesis, they cannot produce as much glucose when they receive less energy from sunlight.
As less sunlight strikes the plant, red and blue wavelength absorption decreases, which means green wavelength reflection increases. This gives rise to colors from other pigments found in leaves, such as carotenoids. Carotenoids are pigments that give the yellow, orange and brown colors found in many fruits and vegetables.
This pigment also contributes to the photosynthetic process, but not through sunlight. Therefore, the amount of sunlight that reaches the leaves does not affect carotenoids and the colors they produce.
During the summer when sunlight is plentiful, chlorophyll does most of the work in photosynthesis, so its green color overpowers the carotenoids. When light energy is scarce in autumn, plants break down the chlorophyll, which allows the yellow, orange and brown colors of the carotenoids to emerge.
So why do leaves fall when they have changed colors? Leaves contain many veins that they use to transport water and glucose throughout the entire tree. In order to keep the entire tree alive, these veins must keep the water and food flowing. During autumn, the colder temperatures cause these veins to shrivel up. As a result, water and food cannot properly flow throughout the entire tree. If the tree’s vascular system cannot keep a continuous flow, the entire tree will die. Therefore, the tree naturally closes off connection with the leaves so that water and food can flow easily through the branches, trunk and roots, keeping the tree alive. Completely sealed off from the tree, the leaves simply fall off.
A slightly rarer autumn event is the leaves turning red and sometimes purple. This is caused by warm days and cool nights during autumn.
During the warm day, the leaves are still able to produce some glucose through photosynthesis. But when the night turns cold, the shriveled veins are not able to transport the glucose into the trunk. Not ready to give up just yet, the tree uses another pigment called anthocyanin to help keep things flowing before the leaves fall off. The abundance of anthocyanin creates the red and purple color sometimes found in autumn leaves.