What makes leaves change color in the fall?

Every year, the arrival of fall heralds one of nature's most magnificent transformations.
And while many will witness the green leaves of summer foliage transition into a vibrant spectrum of colors before falling from their trees, few will stop to ask why these color changes occur in the first place.

The answer to this question is one we don't yet fully understand, but the basic principles behind the changing of the leaves are actually pretty straightforward. So, where do autumn colors come from?

It all boils down to three important pigments: chlorophyll, carotenoids, and anthocyanins.

Of the three, chlorophyll is probably the one you're most familiar with. Found in the leaves of countless plants, the energy chlorophyll absorbs in the form of red and blue light (it reflects light at green wavelengths, hence its color) fuels the chemical reactions required for photosynthesis, whereby carbon dioxide and water are converted into oxygen and carbohydrates.

What makes leaves change color in the fall?

Chlorophyll is the primary workhorse in leaf cells, so during periods of growth like spring and summer it is the most important, and therefore most abundant, of the pigments.

The thing is, chlorophyll isn't exactly the most stable of compounds. When it decomposes, plants rely on sunlight and warm temperatures to keep their chlorophyll levels up.

As fall approaches, days become shorter, temperatures grow progressively cooler, and less and less chlorophyll is produced. When the rate of chlorophyll production starts lagging behind that of chlorophyll decomposition, the pigment's presence in leaves — and its characteristic green color — begins to wane.

But chlorophyll decomposes much more quickly than carotenoids, which have actually been hiding in the leaves all along. Like chlorophyll, carotenoids absorb energy in the form of light and aid in photosynthesis, they just do it to a much lesser degree, and by absorbing different wavelengths of light; carotenoids absorb light at blue and blue-green wavelengths, so the light that they reflect appears yellow.

As chlorophyll levels begin to fall off, the green colors in leaves begin to fade, leaving behind the yellow hues of the carotenoids. If the carotenoids are also destroyed, their color is sometimes replaced by the brown that results from the oxidation of chemicals known as tannins, as seen in oak leaves, for example.

And finally we have the anthocyanins. Anthocyanins absorb blue, blue-green, and green light. The light they reflect therefore grants them an incredibly striking scarlet, or even purple, hue.

What makes leaves change color in the fall?

As fall sets in, certain trees like red maples, and sumac (pictured here) crank their anthocyanin production up to eleven, causing them to appear particularly red; and some say that anthocyanin production is especially prolific in cold but sunny conditions. But the leaves of many plants will vary significantly — from gold to orange to crimson — depending upon the ratio of carotenoids to anthocyanins in each leaf.

There are, of course, numerous other factors that contribute to the changing colors of the leaves. Temperature, humidity, pH, and even soil conditions have all been shown to influence the hue of autumn foliage. But as is the case with so many other biological and chemical processes, it is ultimately light (or rather its absence) that plays the most significant role, triggering the shutdown of chlorophyll production and upping anthocyanin synthesis amidst relatively stable carotenoid levels to give rise to the beautiful range of colors we witness every fall.

Top & middle images via Clearly Ambiguous
Sumac Leaves by Jim Richardson via National Geographic

This io9 Flashback originally ran October 2011.