Science of…Fall Foliage

For leaf peepers Fall is the time for the best leaves. They turn from bright green to yellow, then orange and if the conditions are right red and purple too. But what makes fall colors so spectacular?

Science can explain.

First just about everybody knows that chlorophyll gives leaves their green color. But chlorophyll is more than just a molecule that gives plants a green pigment. Chlorophyll absorbs sunlight and uses its energy to create carbohydrates from carbon dioxide and water. Trees drink in water through their roots and they breathe in carbon dioxide from the air.

That process we all know is called photosynthesis. It’s what all land-based and many sea-based plants rely on for food. That’s why we call leaves nature’s food factories.

Like all things in nature plants and trees are built for efficiency. As the hours of sunlight decrease during the second half of the year deciduous trees begin to conserve energy so they don’t freeze during the winter. They accomplish this by shedding their leaves. But each tree doesn’t consciously decide to drop the excess baggage. That’s also part of the process which leads to fall foliage being so brilliant.

Before dropping their leaves, trees slow down production in their food factories by producing less chlorophyll. That allows other colors to become visible. First, the yellow pigment which was there all along begins to emerge. That is created by molecules called carotenoids. The two common leaf carotenoids are carotene and xanthophyll.

They are the pigments that gives carrots and sweet potatoes their color. Carotenoids are masked by chlorophyll until fall approaches. When the chlorophyll production slows down carotenoids become visible. They are responsible for yellow and orange leaves.

Ever present but hidden by other colors is another pigment called anthocyanin. Anthocyanin is part of a third pigment family called flavonoids. Lycopene is a flavonoid that gives tomatoes their red color. But anthocyanin acts as the natural sunscreen for plants, blocking harmful ultraviolet rays from the sun. As chlorophyll production shuts down for the year, anthocyanin production ramps up. These give tree leaves their deep red, magenta and purple colors. That increase in anthocyanin is in response to surging sugar concentrations, building up in the leaves.

That glucose glut tends to concentrate where leaf meets stem, making the extra weight a target point for wind or rain to knock the leaves from the trees. After all, that’s the trees’ goal. Get rid of the leaves so they can survive the sunless winter without dying of starvation or freezing to death.

While trees go through this incredibly complicated process to get ready for winter, we marvel at their transformation from vibrant green blowing in the mid-summer sun to yellows, oranges, reds and purples buffeted by fall storms. Perhaps we watch in awe as the slow and steady march of time goes on before our very eyes.

Key Leaf Chemistry

The color of leaf we get is highly dependent on the types and amounts of pigments that are present. Internal plant chemical interactions, especially from variable soil acidity or pH level can have a dramatic impact on leaf color.

With the steadily decreasing length of day and cooler temperatures as fall approaches trees biologically trigger the formation of a corky layer of cells called the abcission layer across the base of the leaf where it attaches to the tree. This formation gradually decreases the supply of water and minerals to the leaf, reduces the manufacture of chlorophyll and traps sugars in the leaf.

In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight during the day and the cool nights of autumn cause the leaves turn this glucose into a red color. With enough sunny days, those anthocyanins turn leaves bright reds and purples.

The muted brown color of oak leaves is made from wastes left in those leaves.

Why is so hard to predict when the leaves will be the best and brightest? In short, it depends on the weather.

Warm wet springs, summers that don’t scorch and bright sunny fall days with cool nights all lend to the brilliance of fall color. The degree of visual splendor and leaf vibrancy requires a lot of variables lining up just right. And when they do, the beauty is unparalleled.

Phenology of Fall Foliage

Phenology is the study of the timing of nature. This ranges from the dates that tulips bloom in the spring to the fall migration of Canadian geese. And it definitely includes watching the leaves turn color in the fall.

But scientists have been struggling to find a direct link between later leaf drops and climate change in the U.S. Europe and Asia have been conducting studies that seem to point to later fall foliage. But it’s been a difficult case to prove in the U.S. Part of that reason could be the wildly fluctuating weather from year to year.

A wet spring, followed by the deluge of Hurricane Irene, a mind-summer drought and normal-ish fall have trees in New England all confused. Some tree leaves are bypassing fall colors, turning brown and dropping early. Some plant biologists believe that’s due to a fungus that relished the wetness and took hold.

Those factors make identifying long term trends difficult at best.

Several organizations are enlisting the help of the public to collect data in hopes of finding the signal for such change in the noise or fluctuating weather.

According to the Wall Street Journal:

“There are signs everywhere that things are changing — how is the question. Some species are being affected while others are not,” said Esperanza Stancioff of the University of Maine cooperative extension and Maine Sea Grant, who has trained 195 citizen scientists to enter data online in her “Signs of the Season” phenology project.

To assist both backyard observers and researchers alike, the National Phenology Network has spent the last four years coming up with standards to be used by observers in reporting foliage color changes. Final tweaks on the uniform reporting standards should be completed in a few weeks, Weltzin said.

Another part of the effort to study climate change through the lens of fall foliage is being conducted from space by the U.S. Geological Survey utilizing satellites from NASA and the National Oceanic and Atmospheric Administration.

Right now, the effort is focused on Shenandoah National Park in Virginia, where scientists are attempting to understand the factors that go into the metrics to ensure proper analysis of the photos taken from above, said John W. Jones, a research geographer with the USGS outside of Washington, D.C.