Cool, crisp air, the smell of wood smoke faintly in the distance, that familiar sound of leaves crunching beneath your feet.. Autumn. I’m sure most of you have experienced the beauty and awe of the season out hiking in the woods, or maybe have appreciated a particularly bright colored tree on the commute to work. My favorite picture of autumn is in the mountains; I look forward to it all year. As autumn progresses, it is quite a spectacular sight to watch the color begin to show at the peaks of the mountains and slowly trickle down the elevation until it reaches the valleys. Have you ever wondered what causes the leaves to change color this time of year or maybe why leaves change the colors they do? Believe it or not, when leaves change color from green to hues of yellow, orange, or red, they aren’t necessarily dying immediately. Also, brightness in leaves is determined by cool, above freezing temperatures and dry weather. Here’s a bit of insight to this interesting phenomena, beginning with the key role of light.
Light produces a spectrum of color. Most of us have seen these colors with a naked eye as rainbows. The colors we see produced in a rainbow are caused by various frequencies (from sunlight) bending and reflecting off water particles. A similar idea applies to leaves. When a ray of sunlight reaches a leaf, it takes three courses: some frequencies of light are absorbed, some frequencies of light are transmitted through the leaf, and other frequencies of light are reflected off the leaf. The frequencies of light that are reflected off the leaf result in what we see as leaf color. In short, sunlight interacts with various pigmentation found within the leaf to create the leaf color we see, similar to the way sunlight interacts with water particles to create a rainbow. The three main pigments found in a leaf are chlorophyll, carotenoids, and anthocyanins.
Chlorophyll is the pigment that causes the green color we see in leaves. The role of chlorophyll within the leaf is to absorb light to generate energy for the tree, otherwise known as photosynthesis. With active chlorophyll absorbing sunlight, the frequencies of light reflected off the leaf are what we see as green. Chlorophyll in deciduous leaves begin to break down and die off in autumn with cooler temperatures and shorter days. When the chlorophyll breaks down other pigments to show through, such as carotenoids.
Carotenoids cause the yellow and orange colors we see in leaves. Carotenoids provide an additional means for the leaf to absorb light for photosynthesis, but carotenoids do not depend on chlorophyll to survive within the leaf. Carotenoids absorb different frequencies of light than chlorophyll, allowing the carotenoids to reflect the yellows and orange leaf colors we see. Though carotenoids are present in the leaf the entire year, they are only visible to our eye when the chlorophyll breaks down.
Unlike carotenoids and chlorophyll (which absorb light to make energy for the tree), anthocyanins are a product of a reaction within the leaf. The role of anthocyanins is to act as a sunscreen to protect the leaf from absorbing too much light to avoid leaf fatality. Late summer into autumn, the sugars within a leaf elevate significantly. These raised levels of sugars react with proteins also found in the leaf, as well as other internal and external factors, generating anthocyanins. These anthocyanins absorb different frequencies of light than either carotenoids or chlorophyll, and reflect the red leaf color we see. Anthocyanins are sensitive to the various degrees of acidity of the sugars, causing hues from bright red (more acidic sugars) to purple (less acidic sugars).
Whoa! That was a lot of information. To recap, the colors of leaves are caused by three main pigments: chlorophyll (causing the green color we see), carotenoids (causing the yellow and orange colors we see), and anthocyanins (causing the red colors we see). These three pigments play different roles within a leaf. Chlorophyll and carotenoids help the leaf absorb light to make food for the tree whereas anthocyanins are a product of a reaction within the leaf that protect the leaf from absorbing too much light. In order to see the yellows, oranges, and reds in autumn, the chlorophyll within a leaf ceases to function due to cooler temperatures and shorter periods of light exposure.