Why do only some leaves turn red in the fall? Scientists can’t agree on an answer

This is the time of year when Nicole Hughes gets constantly distracted by the fall foliage as she drives around North Carolina.

“I’m always looking,” says Hughes, a biologist at High Point University. “You know, just seeing what’s turned colors when, and what hasn’t turned.”

Part of her brain is endlessly searching for clues about one of the biggest mysteries of autumn leaves: why the leaves of only some trees turn red.

“We still don’t really know why some species turn red and others don’t,” she says.

She’s made a career out of studying this, even though her interest in red leaves goes back even further. Growing up, she says, “my dad was always talking about his quest for the perfect red leaf.”

Every autumn, trees break down the green chlorophyll in their leaves, so that they can recover precious nitrogen before the leaves fall. (That nitrogen is necessary for photosynthesis, as well as building proteins and DNA.) The loss of this chlorophyll exposes yellow pigments that have been there in the leaves all along. That makes yellow leaves easy to explain.

But the red color is different. It comes from brand-new chemicals that are generated just days before a leaf plunges to the ground.

“The red was not there beforehand, or you would have seen it, because the leaf would have been purple. Because green plus red equals purple,” says Hughes.

Low levels of these same new pigments are also responsible for leaves that look orange, she explains: “Under a microscope, you see lots of red freckles.”

So why are trees bothering to create costly red pigments in leaves that are almost done for? Hughes says scientists don’t have a lot of answers.

Protecting leaves from light

One possible explanation says that the red pigments basically act as a kind of sunscreen for the plant, to help it cope with excess light it suddenly has to deal with when chlorophyll activity is declining. This is known as the photoprotection hypothesis.

“All the physiologists are convinced of the photoprotection hypothesis,” says Susanne Renner at Washington University in St. Louis. “There are a lot of high-tech, biochemical, physiological experimental papers showing that one function [of red pigment] is photoprotection.”

She notes that Europe gets less solar irradiation than the northeastern United States, and has fewer red trees, which she thinks is a suggestive correlation.

And some evidence suggests that what the red pigments are protecting is the tree’s ability to recover nitrogen from the leaf.

One study of yellow mutants of three deciduous species, for example, found that more nitrogen was left behind in their yellow leaves than in the normal red leaves. What’s more, tree species that live with nitrogen-fixing bacteria, which give them abundant nitrogen, generally do not turn red.

But not all researchers are convinced.

Evolutionary biologist Marco Archetti, at Penn State University, points out that a relatively small number of tree species have red leaves. “I think it’s about 15 percent, more or less, if you look at all the trees in the world,” he says. If the red color was for protection against excess light, “why don’t more species need photoprotection?”

Enter the aphids 

He thinks it seems more likely that this trait co-evolved with insects.

It turns out that insects that lay eggs in the fall can tell the difference between leaf colors, and Archetti’s done research showing that on apple trees, aphids avoided trees with red leaves, preferring those with green or yellow leaves.

“There are fewer aphids on leaves with red colors,” he says. And in the spring, newly-hatched aphids were less successful on trees that had been red, suggesting that aphids may have had good reason to avoid putting their eggs there.

He’s also found that reddish colors are found in wild populations of apple trees, which have to contend with more insect invasions, but not in cultivated varieties that get sprayed and tended by farmers.

All of this has led to “hot” debates over the years, according to Simcha Lev-Yadun of the University of Haifa in Israel, who recently laid out the evidence for competing hypotheses about the function of red leaves.

Past discussions of all this were “not even always polite,” he says with a laugh, adding that in science, it’s not unusual that “as you progress, you have more new questions than you had answers.”

He personally is interested in the possibility that the red pigments may have anti-fungal properties.

Parking lot science

While some tree species have leaves that always turn red, and other have leaves that invariably turn yellow, Hughes notes that a number of species are more flexible. Those are the ones she finds particularly compelling, and, these days, she often finds herself pondering the trees in parking lots.

“In a lot of parking lots, you’ll notice that they have trees of a single species, but some are yellow and some are red and some are orange,” she says, “even though they’re in the exact same sunlight, and the exact same temperature.”

Outside one furniture store, she found that willow oaks with red leaves had fewer scale insects, compared to willow oaks that were yellow.

Lately she’s been growing different oak species and feeding the trees different amounts of nitrogen, to see if that affects their color. She’s also been studying sweet gum trees, which can turn red — but also all the other fall colors.

“You’ll have a single tree and some branches will be purple and some will be yellow and some will be green,” she says. “I feel like that species is a fun clue to work on. What’s the difference between those branches?”

She says a lot of her students at the university assume that scientists know everything there is to know about something as common as a red autumn leaf.

“I mean, you might think we know everything,” she says, “but really there’s just not enough people studying these fun mysteries of life.”