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The heat wave along the Eastern seaboard has finally eased up a bit, and people aren't the only ones feeling relief. A new study finds that trees are happiest at a comfortable 70 degrees. In fact, scientists were surprised to discover trees actually control the temperature of their leaves - and that's whether they live in a steamy tropics of boreal forests of the far north.

NPR's Richard Harris has the story.

RICHARD HARRIS: Suzanna Richter didn't start out trying to understand how trees managed to live in so many different environments. As a graduate student, she was trying to see what secrets she could wring out of ancient forest samples.

Ms. SUZANNA RICHTER (Botanist): The original project started with 45-million-year-old wood that was amazingly well preserved. So well preserved that we wanted to know if the chemistry of the wood could tell us something about the climate in which the trees were growing 45 million years ago.

HARRIS: For comparison purposes, she gathered wood samples from many different living trees growing in all sorts of environments. It turns out 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 the University of Pennsylvania campus. Brent Helliker thought maybe he could tease out something useful about today's environment from all those samples.

Professor BRENT HELLIKER (University of Pennsylvania): This was actually my - probably my first ever jaw-dropping moment as a scientist.

HARRIS: It turns out the leaves on those 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 hot house of the tropics or in the frigid north.

Prof. HELLIKER: We're simply not saying that leaves are always at 70 degrees. There's quite a lot of variation throughout the day and there's quite a lot of variation throughout the season. But on average, we're saying, when plants are assimilating most of their carbon, they are operating at this temperature somewhere around 70 degrees Fahrenheit.

HARRIS: Somebody listening to this might think, well, gee, it almost sounds like trees are warm-blooded. They seem to be regulating their temperature.

Prof. HELLIKER: That is, well, that is absolutely not true.

HARRIS: But Helliker and Richter say trees appear to have evolved all sorts of ways to adjust the temperature of their leaves.

Prof. HELLIKER: The most common example, and what we see a lot of just outside today, because it's so incredibly hot, is that a leaf will wilt.

HARRIS: A wilted leaf droops and captures less sunlight. Leaves can also cool off by losing water. Think of sweat.

Prof. HELLIKER: There are some desert plants that it's been shown that just through that process of losing water, they can cool off to as much as 20 degrees below ambient temperature.

HARRIS: And 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. Helliker says think of gloves and mittens. If you're wearing gloves, wind can easily whip heat away from your individual fingers, leaving you cold.

Prof. HELLIKER: Whereas if you're wearing a mitten, your fingers are close together and the wind cannot as easily whip away the heat from your fingers. Well, it's a similar concept with the leaves.

HARRIS: Their 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.

Prof. HELLIKER: What we have offered is a potential mechanism for that. We've given a reason for what they might fail.

HARRIS: Suzanna Richter, whose research project on ancient trees started this all, says that the discovery also makes her feel more of a kinship with trees. She was thinking about that as she came into the studio for this interview on Tuesday.

Ms. RICHTER: You know, it's close to 100 degrees in Philadelphia, and I was staring at some of the trees there. There's hickory tree and its leaves were down, they had wilted. And so I was thinking that, hey, it's hot, I'm hot - we're all hot. They enjoy 70 degrees and I enjoy 70 degrees too.

HARRIS: Richter and Helliker's more scientific observations about trees and temperature are published online by Nature Magazine.

Richard Harris, NPR News.

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