How many trees are there in the Amazon?

A couple of weeks ago some colleagues and I wrote a paper in Science reporting some new findings on Amazonian forests. Some of the findings are actually just numbers, and one of those numbers is really big. It's the number of trees we think probably grow in the Amazon, and it's 390 billion.

One advantage of writing for scientists is that you can let numbers like that drop without much in the way of apology. Three point nine times ten to the eleventh: it is, as they say, what it is. When you write for a broader audience, though, you can't type '390 billion' without pausing for a minute and casting about for something neighborly to say. Cast about for long enough and eventually you'll hear a voice—it's that booming, prepare-to-have-your-mind-blown voice you hear in movie trailers—say something like:

About as many trees…

as there are stars…

in… the Milky Way….

Or:

Four Amazonian trees…

for every neuron…

in… your brain….

And so you pick up your pen and add one of those after '390 billion,' satisfied that the reader has been suitably instructed and astonished, and you go onto the next sentence.

That rhetorical device has its value, I guess, and with smaller numbers you often can find a neat comparison that does the trick. But for me, and I suspect for many other people as well, any number that's larger than a few thousand is fundamentally impossible to imagine. The number of cells in my body? The number of liters in Lake Michigan? The number of mosquitoes in Alaska? They're all one number, basically. It's like that Amazonian tribe has it: after three, the next biggest number is many.

This is a problem for me, because as a researcher I don't just like to know things—I also like to have some feeling for the things I know. Over the last decade the area of the Field Museum I work in has helped governments in South America protect about 23 million acres of Andean and Amazonian rainforest. On the one hand, that number is part of what makes it so great to come into the museum every morning. On the other hand, 23 million? Also makes me sleepy. I mean, what is 23 million? The number of bacteria in my gut? Or is that 57 billion? And this is the problem. They're all the same, these numbers: big enough to imagine, but too big to actually feel.

Anyhow, over the last few days I have been thinking about the number of trees in the Amazon and wondering if the movie-trailer-guy will ever say anything smart about it—something that doesn't involve the distance between Earth and Jupiter, please. He hasn't said anything smart yet, and I've been thinking about that, too, thinking and daydreaming.

I like to think about the animals that live in those trees. There are thousands of trillions of them, of course. The larger ones spend their days moving from one tree to another—bee-lining from this inflorescence to the next, scampering from trunk to trunk, throwing themselves across the gap between two canopies in search of a new territory, or a gum that tastes especially good, or a dry spot where they can hunker down in a thunderstorm—one tree after another after another, from dawn to dusk, every day of their lives.

The smaller animals are even more numerous, and many of them spend their whole life on a single tree. These are rotifers, or termites, or water bears, or beetles, or some other creature that is living out its life in the root mat of an epiphyte garden high in the canopy and that no one will ever see. Pretty much everything these animals see or touch or taste is an Amazonian tree. And even though the lives of these animals are deeply foreign to me, thinking about them has given me some small notion of how a number as big as 390 billion might actually feel.

Because look around the room you're in. Imagine for a minute that every one of the things you see there—every book, light switch, floor tile, potted plant, flip-flop—is an Amazonian tree. Well, all right. But don't stop there. Everything in the next room you go into? Also Amazonian trees. Every single object in every single room you've ever seen in your life? They're Amazonian trees. It turns out that every person in your family is a tree, that all your friends are trees, that everyone you've ever Googled or passed on the street is a tree.

And there's more. Every person you've ever read about or seen on TV is a tree. Every word you've ever spoken is a tree, every word you've ever read is a tree, every thought or emotion you've ever had—your superstitions, your disappointments, your political convictions—they have leaves and branches. Trees are every penny you've spent, every shoelace you've tied, every building you've ever walked past, every cloud and bird you've ever seen, every song you've ever listened to, all the stars you've ever seen in the sky—every star you ever could see in the sky.

Count them all, sum them up, and you're left with a number.

That number? The sum of all the things that make up your experience of the world? How about we put it like this:

That's the number of trees in the Amazon.

To learn more about the study mentioned in this post, see:

The Field Museum Science Newsflash describing the article.

The article itself: ter Steege, H., et al. 2013. Hyper-dominance in the Amazonian tree flora. Science 342: 1243092.