Stumped by Trees

By Emily Ashbolt, Biomedical Physics, 2017

How many trees are there in on your road? In Boston? In the US? Could you estimate? Where would you go to find out?

If you answered “I don’t know” to any or all of the above questions, you are not alone. As it turns out, only a small group of people even thought they knew. Data from a fall 2015 survey reveals that it’s likely that even this select group of people were very, very wrong.

A few years ago, there was a number floating around claiming that there were about 400 billion trees on earth. That seemed like a big number, breaking down to about 61 trees per person. This number was based on satellite images from NASA, taking photos of parks and forests and estimating based on average trees that could be seen per square mile calculated from that image.

However, soon after that number was published, someone actually counted the trees per square mile for a parcel of land in the Amazon basin- and their math calculated to show over 390 billion trees in that Amazon basin alone.

Obviously, there was a disconnect somewhere. But what could be better than satellite images when it came to optimizing time and efficiency? Enter Thomas Crowther.

Crowther was a Postdoctoral Research Fellow at the Yale School of Forestry and Environmental Studies when he began pondering this question. He had a friend who was trying to combat climate change by planting a billion trees. But was a billion trees even that many? “They didn’t know if planting a billion trees was going to add 1 percent of the world’s trees, [or] add 50 percent of the world’s trees,” said Crowther to NPR in September of 2015. The data just wasn’t there.

Crowther’s solution was the one that brought such unexpected results from the Amazon basin: people. His team contacted and sent out people all over the world, nearly half a million, to count trees in various climates, hundreds of thousands of them, and then used this data to calculate the number of trees in all the different parks, forests and jungles on this earth.

The number they came up with? Over 3 trillion. This is 7.5 times more trees than previously estimated — billions and billions more trees. Crowther and his team were astonished, but they were also concerned.

While this figure, and the fact that all those trees were hiding in plain site is amazing, not all of Crowther’s discoveries were happy. Estimates from his study put the number of trees being cut down each year at 15 billion. Since human civilization began, Crowther’s team suspects the earth has lost 46 percent of its tree cover. This is bad news, since trees, and their multitude of species, are essential to biogeochemical and water quality cycles, as well as being the backbone of most ecosystems.

What this means is that while there are more trees in this world than we ever imagined, it is still very important to take care of the ones that we have. You might never have guessed the sheer number of tree around you, but there’s no denying we would all miss them if they were gone.

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