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LIANE HANSEN, host:

This is the season when trees take their big bow, flashing brilliant yellows and reds, revealing their hidden colors. But NPR's science correspondent Robert Krulwich says there's something you should know about trees. They're hiding a secret.

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ROBERT KRULWICH: Back in the 1950s, the Americans and the Russians and the British and the French tested - that's what they called it - blew up atomic bombs in the atmosphere. It was a Cold War competition, but it had a curious biological effect. The explosions released neutrons, and neutrons hit carbon atoms floating through the air, and when a regular carbon atom called carbon-12 picks up two extra neutrons.

(Soundbite of bumping)

There's one. (Soundbite of squishing)

That's two. It becomes, says Professor Nalini Nadkarni, carbon-14.

Dr. NALINI NADKARNI (Department of Biology, Evergreen State College): And so what happened was, during these atmospheric testings of the atomic bombs, we were getting a tremendous spike of carbon-14, actually 100 percent more carbon-14 coming into the atmosphere than what we had had previous to those tests.

KRULWICH: And all that carbon-14 stayed up in the air...

Dr. NADKARNI: This cloud of carbon-14 went round and round and round the Earth and was persisted for quite awhile.

KRULWICH: Until President Kennedy signed a treaty with the Russians...

Former President JOHN F. KENNEDY: To ban all nuclear tests in the atmosphere...

KRULWICH: After which, levels of carbon-14 went down. But for a little while, starting around 1954, all over the world, trees - woodland trees, equatorial trees, mountain trees, trees everywhere sucked in not just regular carbon because that's what all trees do...

Dr. NADKARNI: They take in carbon dioxide.

KRULWICH: And turn it into tree food. But because there was more carbon-14 in the air, they absorbed more carbon-14 into their trunks. If you look inside trees, and botanists have looked...

Dr. NADKARNI: You can find studies in Thailand, you can find studies in Brazil, you can find studies all over the world where you see this pulse that happened right at that time. When you measure for C-14, you'll see it there. So it's like a notch that's been struck in every tree that was born ever since 1953.

KRULWICH: So if you're trying to figure out how old a tree is in, say, the Amazon, where trees do not have tree rings - equatorial trees generally don't have rings - you can still look at the wood, says Professor Nadkarni, and think...

Dr. NADKARNI: Hmm. If I know that these trees took up carbon-14 in large amounts, and I find that spike in where ever it is in the wood...

KRULWICH: Then she knows that this tree was alive in 1954 or the years just after. It's like a biological birthday candle, a calendar marker.

Dr. NADKARNI: It's a calendar marker. That's right.

KRULWICH: Stamped into every tree in the world. So, here's a manmade activity.

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KRULWICH: In this case, blowing up atomic bombs that changed the biology of our planet. But it turns out those bombs didn't just change trees. Those bombs changed us. A Swedish biologist, Jonas Frisen, who studied pine trees and carbon-14, decided to take a look at humans, and sure enough, brain cells in 1950s babies had a little extra carbon-14 in their DNA, right there in their brain cells. But the 60s babies? Much less.

So just like the trees took in extra carbon-14 in the '50s, moms and dads took in apples and pears, bananas, almonds, maple syrup, plants, and the extra carbon-14 turns up in their babies' DNA for a few years. So, even today, you can spot a tree or a person created in the mid 1950s by measuring their, I don't know, is there a technical word for this? The atomic bomb thing?

Dr. NADKARNI: Yes, it's called the atomic bomb thing.

(Soundbite of laughter)

Dr. NADKARNI: I'm just kidding. No, it's just that we know that...

KRULWICH: That bombs that went of 50 years ago left a deep mark in nature, deeper than we knew. Robert Krulwich, NPR News.

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HANSEN: Professor Nalini Nadkarni has a new book about trees. It's called "Between Earth and Sky." She teaches at Evergreen State College in Washington.

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HANSEN: You're listening to Weekend Edition from NPR News.

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