Curious Iowa: Why do leaves change colors?

Iowa’s trees put on a brilliant color show every autumn, turning into shadows of yellow, orange, red, brown and sometimes purple.

While talking with a group of friends, Jenifer Phelan of Cedar Rapids wondered why some maple trees turn bright red in fall while others turn gold and what weather conditions lead to dramatic colors. So, she wrote to Curious Iowa — a Gazette series that answers readers’ questions — to find the answer.

We spoke with Mark Vitosh, a district forester with the Iowa DNR, to find out why leaves change color and what conditions lead to vibrant hues.

The answer lies in the days getting shorter in autumn.

Why do leaves change color?

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In short, when leaves start changing color, the tree is preparing to go dormant for the winter by shutting leaves down.

The color pigments that we see in the fall exist in many deciduous trees all summer, Vitosh said, but at that time they are covered with chlorophyll. When the days start getting shorter, the lack of light sends a signal to the tree to prepare for winter and chemical changes occur.

“And what happens in those leaves, they start to develop what they call an abscission layer,” Vitosh said. “And that’s basically in what we call the petiole of the leaf, that’s the stem of the leaf that connects to the tree. And a corky substance starts to develop there.”

When that happens, fewer carbohydrates and water from the tree get to the leaf. Then, the leaf makes less chlorophyll, which is what pigments the leaf green. Chlorophyll is necessary for plants to use sunlight to make sugars for food. These sugars are stored by the tree during the winter dormant period. Without chlorophyll, plants can’t produce their own food.

Other pigments found in leaves are carotenoids, which produce yellows, browns and oranges, and anthocyanins, which make reds and purples. Both are also found in fruits, flowers and vegetables. Carotenoids are in carrots and daffodils. Anthocyanins are in cranberries and plums.

As the chlorophyll breaks down and gets redistributed back into the tree, yellows and oranges become visible. But anthocyanins are mostly produced in autumn, Vitosh said.

According to the U.S. Forest Service, anthocyanins are formed as a reaction by sugars when the sugar concentration in a leaf increases. The reaction needs light to form the red pigment. This is why one side of an apple may appear red while the opposite side is green.

Looking at the broader picture of changes trees undergo in the fall, when the abscission layer is fully formed, the leaf falls off.

At the same time, deciduous trees are preparing for next season’s growth. The tree’s future growth is predetermined based on this year’s conditions, Vitosh said. Today, if you look closely at a tree without leaves on it, you might be able to see signs of where growth will occur next spring.

“They’re not huge … but if you look close on those branches you’ll see small buds and those buds are what contain, again, depending on the species, you could have buds that contain flowers or you could have buds that contain branches and leaves,” Vitosh said.

Which conditions make the most vibrant fall colors?

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Phelan wondered why some maples turn red, while others turn gold. Vitosh said just as genetics affect a person’s hair and eye color, genetics play a role in tree color. But the health of a tree or environmental factors, like how much light the tree is getting, can impact the vibrancy of pigments.

Typically, fall colors look bland during a drought. Vitosh said that this year — which was wet early, and dry in late summer the fall — he noticed that leaves that normally are yellow turned brown because it was dry.

“But then others … because it didn’t really get a lot of heavy frost, some of those vibrant yellows actually showed up on other species,” Vitosh said. “So, some species, the drought did impact them a little bit and on other species, because we had a longer fall, they actually had fairly vibrant colors. So it can vary.”

Vitosh said that wet and windy conditions can cause leaves to fall faster, and a hard frost or freeze can kill leaves. But this year, the frost Vitosh’s district faced wasn’t hard enough to cause leaf death, so the fall colors last through late October. Vitosh’s district, district 12, covers Benton, Linn, Jasper, Poweshiek, Iowa, Johnson and Muscatine counties.

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Comments: bailey.cichon@thegazette.com