What is the heaviest living thing on Earth?

Walk into a certain patch of forest in Utah and you will see tens of thousands of pale aspen trees. Here is the twist: you are not looking at a forest. You are looking at one tree. One very, very heavy tree.

The short answer

The heaviest living organism is usually said to be Pando, a clonal colony of quaking aspen (Populus tremuloides) in Fishlake National Forest in south-central Utah. Its trunks are all genetically identical and connected underground by a single root system. Together they are estimated to weigh around 6,000 metric tonnes, roughly 13 million pounds.

The real story

Quaking aspens have a party trick. As well as making seeds, they can send up new shoots from their roots. Each shoot becomes what looks like an independent tree, but genetically it is the same individual as its neighbour. Given enough time and space, one aspen can spread into an entire grove.

Pando took that idea and ran with it. The colony covers about 43 hectares, or roughly 106 acres, and has more than 47,000 stems. Botanists Burton Barnes and Jerry Kemperman identified it as a single organism in the 1970s by studying aerial photos and mapping the land. In 1992 it was re-examined and given its name, Pando, which is Latin for I spread. A genetic study led by Karen Mock in 2008 confirmed it is one individual, and a male one at that.

How old is it? Nobody can say precisely. The average trunk is only around 130 years old, but the root system keeps replacing them. Published estimates for the whole organism range from several thousand years to as much as 80,000 years.

The bad news is that Pando is struggling. Over recent decades young shoots have been eaten by mule deer and elk before they can grow into trunks, so old stems are dying without replacements. Drought, disease and changing land use add to the pressure. Fencing off sections has helped saplings recover in places, although deer have been known to jump the fences.

A small footnote for the pedants: the so-called Humongous Fungus in Oregon spreads over far more ground, and some estimates of its weight, including the one used by Guinness World Records, put it ahead of Pando. Fungi are hard to weigh, so Pando is the heavyweight most scientists point to, but the title is not settled.

Why people get it wrong

The blue whale is the undisputed heavyweight of the animal kingdom and the biggest animal known to have ever lived. Because we tend to think of living things as animals first, the whale automatically gets the crown.

The catch is in the definition of an individual. We look at a grove of trees and our brains say forest. Biology says that if every trunk carries the same DNA and shares the same roots, it is one organism. Once you accept that, a single whale, however magnificent, is simply outweighed by a hillside of identical trees.

“Forty-seven thousand trunks and every single one of them is the same bloke. That’s not a forest, that’s a family reunion with one guest.”