The World’s Largest Organism Is a Tree, and It’s In Trouble

To a driver passing Fish Lake on State Route 25, in Utah, an aspen grove would look just like a forest of pale trunks. But this is isn’t your average forest. Rather, it’s a single quaking aspen masquerading as a forest; one organism, spread across 106 acres (43,000 square meters), joined underground by one living root system.

This organism, called Pando, is almost certainly the heaviest organism on Earth. It might be the biggest organism on Earth, or even the biggest organism ever to live on Earth. It’s only a vast honey fungus in Oregon’s Malheur National Forest that may outweigh it, depending on how scientists estimate its hidden mass; but let’s not get ahead of ourselves.

A Tree Made of Many Trunks

Quaking aspens can reproduce in two ways. They make seeds, as other flowering plants do. But they also clone themselves, sending new shoots from spreading roots. So pando is a clonal organism: a group of genetically identical individuals.

In an aspen grove, those shoots are sometimes called ramets. Each looks like a separate tree. Together, they can be part of one larger genetic individual, known as a clone. This sounds bizarre, and it took researchers a while to piece things together, too.

Pando’s unity was first recognized in the 1970s by researchers Jerry Kemperman and Burton Barnes, who studied traits such as leaf shape, color and the timing of seasonal change. Later genetic work confirmed that the “forest” was one clone.

It’s hard to say just how old Pando is. The popular claim is that it’s 80,000 years old, and that’s plausible, but not exactly confirmed. The age of a single tree is easy to figure out, but the . Tree rings can date individual trunks, but they cannot reveal when the underground organism began. Living stems in Pando are generally a little over a century old, but Utah State researchers say there is no good method for assigning a precise age to the entire clone.

Recent genetic work has suggested Pando may be many thousands of years old, perhaps dating to the long environmental changes that followed the last ice age. Even the more cautious estimates make it old enough to have persisted through major shifts in climate, fire and animal life around Fish Lake.

Measurements indicated that the Trembling Giant, as Pando is sometimes called, covers 43 hectares (106 acres), weighs nearly 5900 tonnes (13,000,000 pounds), and has over 40,000 stems (trunks). Individual trees might often come and go, but the colony survives and endures.

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But even if Pando is 80,000 years old, or even more, its survival is far from guaranteed.

An Ancient Organism in a Modern Trap

Pando’s been through a lot.

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It’s seen climat change, extreme weather events, and plenty of animals come and go. But in recent times, things are pretty tough.

The most important pressure appears to be browsing. Mule deer, and in some areas cattle, eat young aspen shoots before they can grow beyond reach. A 2018 study by Paul Rogers and Darren McAvoy found that mule deer strongly limited successful regeneration in Pando.

The result is an organism with thousands of old trunks and not enough young ones. Pando is not dying all at once, but it is thinning by attrition. Managers have tried to help by fencing portions of the clone. In some places, that has worked. Where young shoots are protected from hungry animals, they can grow into saplings and begin rebuilding the grove.

Pando may be the biggest organism on Earth, but it’s far from an immortal creature. It lies beside a road, near recreation sites, in a working western landscape shared by wildlife, cattle, visitors and land managers. Its future depends on keeping enough young stems alive, monitoring the results and adjusting when protection fails.

It would be a shame not to take care of it.

This article was originally published in 2015 and has since been edited to include additional information.