SCOTT DETROW, HOST:
Autumn is a big time for trees. Many shed their leaves, and of course, before the leaves fall, they can turn brilliant colors. One of those colors has generated a lot of heated debate among scientists in recent years, and that color is red. To understand why, NPR's Nell Greenfieldboyce went leaf-peeping with a researcher in North Carolina.
NELL GREENFIELDBOYCE, BYLINE: I met up with Niky Hughes - where else? - under some trees on a path next to a reservoir near High Point University, where she works as a biologist. She says these days, she's constantly distracted by fall foliage.
NIKY HUGHES: You know, seeing what's turning colors when and what hasn't turned and just noticing, I guess.
GREENFIELDBOYCE: Her whole life, one color has stood out.
HUGHES: My dad always was talking about his quest for the perfect red leaf.
GREENFIELDBOYCE: She's made a career out of studying leaves that turn red, like this maple leaf at her feet. It's mostly yellow, with mottled red blotches. She says the yellow has been in this leaf all along. It just gets revealed in the fall when the tree breaks down its green chlorophyll to recover and conserve precious nitrogen. The red, however, is a different story. These red pigments get created brand-new in autumn.
HUGHES: The red was not there beforehand or you would have seen it 'cause the leaf would have been purple. 'Cause green plus red equals purple.
GREENFIELDBOYCE: She picks up another leaf from a tupelo tree. This one looks orange. That's also thanks to these newly created red pigments.
HUGHES: Under a microscope, you see lots of red freckles.
GREENFIELDBOYCE: So why are trees going to all the trouble to manufacture red chemicals in leaves that will drop off in a matter of days? What's the red for? The answer to that question depends on who you ask. When I called up Susanne Renner at Washington University in St. Louis, she told me the red is basically sunscreen.
SUSANNE RENNER: There are a lot of high-tech biochemical, physiological experimental papers showing that one function is photoprotection.
GREENFIELDBOYCE: She says leaves need new protection from light as they lose chlorophyll. When chlorophyll is no longer absorbing sunlight, leaves suddenly have this excess, which can be harmful. It may interfere with the tree's effort to recoup that crucial nitrogen. But when I called another researcher, evolutionary biologist Marco Archetti at Pennsylvania State University, he told me that bright red leaves may offer a completely different kind of protection - against insects. He says insects that lay eggs in the fall can tell the difference between leaf colors. In one study of apple trees, aphids preferred green and yellow leaves over red.
MARCO ARCHETTI: There are more aphids on leaves with green color in autumn, and there are fewer aphids on leaves with red colors.
GREENFIELDBOYCE: He suspects that the red color has coevolved with insects. Niky Hughes says there's evidence to support both of these major ideas. And who knows? Red leaves might have other functions, like preventing fungal infections or camouflage.
HUGHES: I mean, you might think we know everything, but really, there's just not enough people studying these fun mysteries of life.
GREENFIELDBOYCE: She notes that while some tree species only make red leaves and some invariably go yellow, others are more flexible. She often sees this in parking lots.
HUGHES: 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, even though they're all in the exact same sunlight and the exact same temperature.
GREENFIELDBOYCE: 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 sweetgums. These trees can turn red, but also all the other fall colors too.
HUGHES: You'll have a single tree, and some branches will be purple and some of them will be yellow and some will be green. And I feel like that species is a fun clue, like, to work on. Like, what's the difference between those branches?
GREENFIELDBOYCE: She thinks there's plenty of experiments that still need to be done to understand that perfect red leaf.
Nell Greenfieldboyce, NPR News.
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