Why do Leaves Change Color in the Autumn?

With the coming of another Autumn, with its cooler temperatures and glorious gold, orange and red leaves, we all marvel at how beautiful the leaves are as they change color from the vibrant greens of Spring and Summer.

And yes, many of us do know that the chlorophyll produced by leaves gradually lessens, giving way to the oranges and yellows of their Autumn garb. But what about the vibrant reds that we see in leaves in the Fall? That poses much more of a mystery — and it might be one of those conundrums that is never fully explained, no matter how much we learn about the natural world.

For some people, they want an instantaneous answer to all things scientific — or at least to how these colors appear at the end of every Summer.

For Owen Reiser, who is a mathematics and biology student at Southern Illinois University, the process was all worth it, as he compiled more than 6,000 close-up photos of leaves into a short video documenting their change over the seasons.

“I was taking a field biology class and we were learning about deciduous trees,” Reiser explains. “I’ve been getting into wildlife photography and time-lapse for a while, and I couldn’t find a time-lapse of leaves changing color, so I just went for it.”

Although it took six weeks, using a macro lens on his camera, along with an LED light and a battery that allowed the camera to run continuously, the effort resulted in the first time-lapse video of individual leaves changing color.

Reiser used leaves from eight different deciduous trees, including sassafras and sugar maple, which shed their foliage every Fall, and took a photograph of each leaf every 30 to 60 seconds for up to three days, as reported in Smithsonian Magazine.

Editing the thousands of images together into a single video showing the dramatic changes in each leaf, he created a timeline of the chemical and visual changes that occur in the lifetime of the leaves.

The chlorophyll can be seen ebbing away over time, showing the myriad chemical changes that are taking place there in each cell.

David Lee, professor emeritus in biological sciences at Florida International University and author of the book “Nature’s Palette: The Science of Plant Color,” states that he had never seen a video like Reiser’s before. “The color even on an individual leaf varies dramatically, and this shows that change over time,” he says.

Even though most people know that chlorophyll ebbs and colors like yellows and golds are already in the leaves, there is even more science behind the changing of the leaves than that.

“Every fall, people write about color change, and typically the articles are full of all kinds of mistakes,” Lee states to Smithsonian. Many people think all the Autumn colors are produced by the leaves in the same way — but that’s not true, with the one exception being the most vibrant and striking of them all, the reds.

Yellow leaves that are seen on trees like witch hazel do indeed go through a breakdown of the green photosynthetic chemical called chlorophyll, which exposes the yellow pigments, or carotenoids, hiding underneath.

And just like you would imagine, carotenoids are the very same chemicals that give pumpkins and carrots their golden colors. As the leaves continue to lose chlorophyll, they carotenoids eventually produce tannins and become brown.

But then there are the reds

Most red tones in Autumn leaves are produced by a pigment called anthocyanin which the leaf produces as it dies. “People argue that the red color is (also) an unmasking from the breakdown of chlorophyll, and that’s simply wrong,” Lee explains, adding “The red color is actually made when the chlorophyll is beginning to break down — there’s a synthesis of those pigments, so it’s quite a different thing.”

So why would a leaf use up all its energy like this? Lee says there are two main theories on that.

William Hamilton, an evolutionary biologist, stated that the color is used to protect plants from being eaten, because red might trick insects into believing that a leaf is toxic or unhealthy. This would have the great advantage of discouraging bugs from feeding on it, or even laying eggs on such a surface.

However, most experts believe the theory propounded by horticulturist Bill Hoch that red pigments offer protection from excess light when the leaf is most vulnerable — and the would be the case in the Fall, when the light is very bright and clear and the trees and other plants are not photosynthesizing nearly as efficiently as they did when they had plenty of chlorophyll.

These anthocyanins actually help shield the leaf by absorbing excess light at wavelengths that cannot be used for photosynthesis, including the green part of the visible spectrum of light.

In what might be another crucial finding, these red pigments also act as antioxidants, protecting leaves from the toxic byproducts that are formed from the breaking down of chlorophyll when it ages.

Spotty coloring on leaves may be evidence of microenvironment of each leaf

Reiser’s unique time lapse of leaf colors may be a key cog in proving this theory since the making of anthocyanins could also explain the spotty patterns of color on the leaves — since of course light exposure and even temperature can vary a great deal over the surface of a leaf, making the pigment appear only in small areas.

Such an intricate process might seem to belie the fact that all this takes place over just six weeks or so, just before the leaf dies and falls from the tree. Why such an intense effort on the part of leaves to survive a couple more weeks?

“The advantage to the plant is that the leaves that are breaking down can more efficiently remove nitrogen from the proteins that are breaking down, and transport the nitrogen back into the plant, either in the big limbs or even in the root system,” Lee says in explanation.

Of course, nitrogen is one of the essentials needed by all plants to grow, so giving as much back to the tree before the death of all its leaves can only help it survive in the long term, adding to its store of nutrients that will be used during the next year.

And it’s also possible that there are even more mysterious factors at work as we view the beauty of Autumn leaves every year and wonder at their glorious displays of hues.

“It’s like our panda,” Lee says regarding how peoples’ fascination with the changing of the leaves never seems to abate. “It’s the thing that really grabs a lot of attention for the plant world compared to the animal world,” he says.  “An odd color is something that we all notice.”

Now, with tools such as Reiser’s painstakingly-made video, we can perhaps appreciate the forces at work even more, making the beauty of Nature’s colors and patterns even more remarkable.