Autumnal Equinox 2020: Why Do Leaves Change Colors in the Fall?

by Jon D. B.

Fall is finally here with the arrival of the Autumnal Equinox today. Weather perfect for outdoor activites and nature’s most spectacular display of color are just around the corner. But why does this display – the spectacle of leaves changing their color – happen only once a year?

While it may seem as though the leaves are gradually whithering before their eventual dying off -the science behind it is far more fascinating. Indeed, the biology of leaves is incredibly complex – and every bit as beautiful – as the foliage’s bright display.

To get the science just right, we’re pulling scientific facts straight from leading experts.

Autumnal Equinox brings fascinating biology into play

Close-up of red maple leaves in autumnal ornament. (Photo credit: Roberto Machado Noa via Getty Images)

One such expert is Ed Sharron, a science communication specialist with the National Park Service’s Northeast Temperate Network in Vermont. Sharron recently spoke with CNN on the very subject, breaking down the process. First up? Perhaps the most striking color change: natural green to vibrant red.

In short: the sugars vital to a leaves development begin to “bake” into a new pigment. As a result – we see lovely reds throughout Autumn.

The science behind red fall leaves

“The more favorable warm, sunny day/cool night temperature cycles that occur in early autumn, the more likely that fall season is to experience vibrant colors with lots of reds.

Leaves that get the most sunlight will develop red leaves, as the sugars inside them are “baked” into the red anthocyanin pigments. This is the same process that causes many apples to only be red on the side facing the sun as they grow.”

Ed Sharron

However, the glorious reds of fall are far from the season’s only color on display. Brilliant shades of ambers and orange, golds, yellows, and even purples are noted throughout.

So why do leaves, and other foliage, change to colors?

Why do leaves change to colors other than red?

It’s not just leaves that change color, either. All manners of foliage (such as is photographed brilliantly above) change their colors to signify Autumn’s reign.

A Red Deer stag walks through bracken at sunrise in Richmond Park on a crisp Autumn morning. (Photo credit: Dan Kitwood/Getty Images)

Some factors, such as drought, will have leaves turning brown – and then dropping – prematurely. In a typically wet season, however, several biological factors will have leaves turning more brilliant shades due to how their chlorophyll breaks down.

Chlorophyll is the green pigment present in all green plants. It is how each plant absorbs light from the sun and turns it into usable energy. This process is known as photosynthesis – and as days get shorter – green plants are less able to photosynthesize. As such, they begin to lose their green pigmentation. This process, along with other factors, result in the lovely warm shades we see across fall foilage:

Trees that don’t receive as much sunlight will reveal the orange, yellow and brown colors, caused by the carotenoid and xanthophyll pigments that are already present in the leaves, but are hidden under the green until the chlorophyll breaks down.

Extensive drought or other factors may cause a tree to go dormant for the winter sooner than during a typical year, which could cause the leaves to fall off sooner or be browner than normal.

Ed Sharron

How do leaves know when their time is done?

As mentioned, their tell-tell sign is the length of the days they experience – and the sunlight within. In short: Less sunlight tells a tree it’s time to stop photosynthesizing and start over for a new year’s cycle. This great period of change aligns with the Autumnal Equinox. Then, once a tree experiences this change, they go into a hibernation of sorts for the winter.

As a result, we are treated to the gorgeous colors we all associate with fall. And thanks to the wonders of biology, there’s nothing else quite like it in all of nature’s splendor.

[H/T Ed Sharron, CNN]