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All drawings by Robert Krulwich

This Tree’s Food Weighs 5.5 Quadrillion Tons

There it is, at the edge of my yard, struggling. It’s only one, maybe two feet tall, with a single pair of leaves—a baby maple sapling, so skinny, so fragile, and yet… little plants get help, and not only from the soil below and the water nearby.

The truth is much more startling, and more marvelous, than that. In his classic book The Trees in My Forest emeritus biology professor Bernd Heinrich describes how plants—especially in springtime—are fed, literally, by the whole planet. Think of a giant spoon stretching across continents, spilling nutrients into hopeful little startups.

I’m not crazy. Here’s what Heinrich says is happening.

In April, May, and early June, if you look very closely, you’ll notice that the branches of any little sapling are getting a bit longer—a smidgen of an inch higher—as it climbs toward the sun. It’s adding new wood cells to itself, long, narrow cells called tracheids. The cells vary in size depending on the species of tree, but they look roughly like this:

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Wood cells form long chains that sit side by side … like this …

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Copyright 2011 Cornell University Plant Anatomy Collection (CUPAC)

It’s the pace of the cell-building that made my jaw drop. Springtime is the growing season, and a plant gets taller like a tower does, by adding units, brick by brick, or, in our sapling’s case, wood cell by wood cell. Wood cells are small, and parts of a wood cell are even smaller, so they have to be manufactured fast—and furiously.

There are ten million bricks in the Empire State Building. Plants use sugar molecules instead, strung together in chains. There are about two billion cellulose (sugar) chains in a single wood cell, each composed of a thousand sugar units. That means every wood cell has (two billion times a thousand) two trillion sugar molecules. That’s a lot of bricks to make—and the plant has to work fast. Very fast. All around it, other trees are about to leaf out, blocking out the sun. Spring is its big, short chance for light, so the pace turns frantic.

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Photograph by Cultura RM, Alamy

In a 30-day period one wood cell will add an average of 2 x 10,000,000,000,000 sugar units, which works out to 771,600 sugar units per second (2,592,000 seconds/30 days) per wood cell.

Let me say that again: 771,600 units, every single second.

Eating Air

Now let’s get even smaller—let’s count atoms. How does a plant make sugar? Basically, it eats air.

Our atmosphere is rich with carbon dioxide. So imagine a bunch of carbon dioxide molecules floating by a leaf. The leaf opens its pores, the CO2 drifts in, and then, powered by sunshine, it re-engineers a bunch of molecules, splitting water, rearranging atoms, building new sugars, and spitting oxygen back into the air.

So our plant is harvesting raw carbon straight out of the air, then building carbon molecules. Each one requires six atoms. Count them if you like.

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So, mathematically, if our plant is building 771,600 carbon molecules per second, six atoms per molecule, that means that the little sapling at the edge of my yard in springtime is chomping down 4.6 million carbon dioxide molecules every second! For 30 straight days!

“These staggering numbers suggest a story,” Heinrich says in his book.

The carbon dioxide wafting in the air outside your window right now could come, literally, from anywhere on the planet. The total mass of the Earth’s atmosphere is about 5.5 quadrillion tons, and it’s all in motion, sweeping, Heinrich says, “across the continents in a matter of days.” The breeze on your cheek right now might include atoms that two days ago were crazily distant. Just take a look at this beautifully rendered animated wind map from NASA’s “Dynamic Earth: Exploring Earth’s Climate Engine.”

With air moving so freely and so swiftly, Heinrich imagined that the newest wood cells in his garden (“say in a twig of a maple seedling next to my cabin”) might have been built from molecules that only a day or two earlier had been in exotically different places, like say, from “a decaying log in the Amazon … ”

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Photograph by John Sullivan, Alamy

… or from a car on a distant freeway …

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Photograph by Prashanth Vishwanathan, Bloomberg via Getty

… or from a coal-burning plant far away …

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Photograph by Patrik Stollarz, AFP, Getty

… or from a hornbill exhaling somewhere in Asia …

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Photograph by Tim Laman, National Geographic Creative

… or from a baboon exhaling through it’s magnificent (and pipelike) nose in Africa …

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Photograph by Carsten Peter, National Geographic Creative

And now, entirely by chance, they find themselves plucked from the moving air and strung together in a conga line made by a baby maple in a North American garden, hanging at the far end of a freshly growing twig.

And there they will stay for, well, maybe 100, 150 years, surrounded by more plucked atoms, stuck in place as the branch of a maple tree, until one day, the tree topples over or the branch drops to the ground, the beetles and fungi move in and munch those atoms apart so they become soil or beetle droppings and then, after a long while, take to the air again. It’s a slow dance, but as Heinrich says, every tree in every garden is a soup of earthly air frozen into wood cells. “Each wood cell of every tree in my forest is in a give-and-take with the rest of the world.”

Or as I put it: Every plant breathes in the whole planet.

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Bernd Heinrich has written of woods, trees, buds, seasons, and the animals that live around his cabin in Maine. The Trees in My Forest, published in 1997, is now a classic. The math in this column comes from Heinrich. He has written many books since then.

Wood cells, or tracheids when they form, are thick with sugars, but when the cells mature, they hollow out, leaving only the cell walls. These are aligned, end to end, forming continuous tubes for fluids to flow through. I wrote not so long ago about how forests scrub our planet’s atmosphere every year of much excess CO2, which is a very different and equally beautiful planetary dance, and you can find that blogpost here.