Trees Do Their Best Work with Thermostats at 70

Seventy degrees is a lovely, comfortable temperature for most people. And the same turns out to be true for all sorts of tree species. In fact, scientists have found that trees actually have tricks they use to keep their leaves close to that perfect temperature.

The discovery was completely unexpected. Suzanna Richter was a graduate student at the University of Pennsylvania who was trying to wring secrets out of a 45-million-year-old piece of wood. She was hoping the chemistry of that sample would help her learn about the environment in which the tree grew up.

For comparison purposes, she gathered wood samples from many different living trees, growing in all sorts of environments. The modern samples didn't help her solve the ancient mystery, but all her hard work did spark an idea in a professor elsewhere on campus.

Brent Helliker thought maybe he could tease out something useful about today's environment from the samples Richter collected. He discovered that the leaves on these trees did most of their work — that is, capturing solar energy — at 70 degrees Fahrenheit. And it didn't matter whether the trees were growing in the hothouse of the tropics or in the frigid North.

"This was probably my first really jaw-dropping moment as a scientist," Helliker said.

The discovery could help explain why each tree species has a particular range it inhabits. It also has some deeper implications for understanding the fate of trees as a result of climate change. Biologists expect that as the planet warms, cold-adapted tree species could overheat.

Helliker says trees aren't like warm-blooded mammals. And they don't always keep their leaves exactly at 70 degrees. But the trees do seem to run at that average temperature when they're soaking up sunlight and producing wood and sugars.

How do they maintain this temperature? Leaves seem to have evolved all sorts of tricks. For example, a leaf will wilt when it's hot out, and a wilted leaf droops and captures less sunlight.

Leaves can also cool off by losing water, he says, much like the way humans do when they sweat.

"There are some desert plants that it's been shown that just through that process of losing water, they can cool off to 20 degrees below ambient temperature," Helliker says.

Trees in chilly climates also have ways to make their leaves or needles retain more heat from the sun. Pine needles, for example, clump together. Think of gloves and mittens, Helliker says. If you're wearing gloves, wind can easily whip heat away from your individual fingers, leaving you cold. But if your fingers are all together in a mitten, they're going to be warmer.

Richter says the discovery isn't just fascinating science. It gives her a special kinship with trees.

On a recent day in Philadelphia when the mercury was near 100 degrees, she said, "I was staring at a hickory tree and its leaves were down — they had wilted," she says. "And I was thinking, hey, it's hot, I'm hot. They enjoy 70 degrees, and I enjoy 70 degrees, too."

Richter and Helliker's scientific observations about trees and temperature were published Wednesday in the online edition of Nature magazine.

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