Walk into a stand of quaking aspen in central Utah and you will think you are among tens of thousands of separate trees, their white trunks and trembling leaves stretching across a hillside. You are not. You are standing inside a single living organism — one root system, one genetic individual, sending up trunk after trunk after trunk. Its name is Pando, Latin for "I spread," and by some measures it is the heaviest living thing on Earth.
One tree wearing forty thousand trunks
Quaking aspen have an unusual habit. A single tree can reproduce not only by seed but by cloning: it sends out lateral roots that push up new shoots, each of which grows into what looks like an independent tree but is in fact genetically identical to its parent and physically connected underground. Over long spans of time a single successful seedling can spread across a hillside this way, until an entire grove is one plant — a clonal colony — sharing a common root network. Each visible trunk is technically a stem, or ramet, of the same individual.
Pando is the most celebrated example. It covers around forty-three hectares — roughly a hundred acres — in the Fishlake National Forest, and comprises an estimated forty-seven thousand genetically identical stems. Weighing the whole thing is guesswork, but estimates put its mass at around six thousand tonnes, which would make it the heaviest known organism, heavier than any blue whale or giant sequoia. The individual trunks are not especially old — a given stem lives perhaps a hundred and thirty years before dying and being replaced — but the organism as a whole, the root system continually renewing itself, is ancient. Age estimates are highly uncertain and range wildly, from several thousand years to, in some speculative figures, many tens of thousands.
The giant that cannot replace itself
Here is the poignant part, and the part quizzes increasingly ask about: Pando is dying, and the cause is deer. For a clonal organism to persist, old trunks that die must be replaced by young shoots growing up from the roots. In much of Pando that renewal has stalled. The reason is over-browsing. Mule deer and cattle, whose natural predators have been reduced and whose numbers are inflated in the area, eat the tender new aspen shoots as fast as they emerge. The result is a strange, top-heavy forest: a canopy of ageing mature trunks with almost no young ones coming up beneath to replace them. When the old stems fall, there is nothing to take their place.
Ecologists studying Pando have found that the sections of the colony fenced off from grazing animals are regenerating healthily, sprouting the young stems the organism needs, while the unfenced areas are thinning out and failing to recover. In other words, one of the oldest and most massive living things on the planet — an organism that may have survived ice ages and millennia of climate shifts — is being undone not by fire or drought or logging but by too many hungry deer nibbling its seedlings, a consequence of human interference with the predators that once kept those herds in check.
Pando is a useful corrective to how we picture individuals and lifespans. We instinctively count trees by trunks and measure age by rings, but nature does not always oblige. The largest organisms on Earth are not always the ones that look large; a vast fungal network in Oregon and Pando in Utah both make the point that a single life can be spread thin across a landscape and endure for an almost unimaginable span. And the heaviest, oldest of them can still be brought low by something as mundane as an unbalanced deer population. Conservationists have taken to fencing sections of the colony and culling or deterring the browsers, in a slow effort to let Pando raise a new generation of stems before its ageing canopy gives out.
There is a quiet lesson in the trembling leaves. The aspen's shimmering — the "quaking" that gives the tree its name, caused by flattened leaf stalks that let the foliage flutter in the faintest breeze — has whispered over that Utah hillside for a length of time no human civilisation can match. Whether it continues now depends largely on whether we can persuade a few thousand deer to eat somewhere else.
Quiz nuggets
- Pando is a single clonal colony of quaking aspen in Utah's Fishlake National Forest, made of an estimated 47,000 genetically identical stems.
- Its name is Latin for "I spread"; it is often called the heaviest known organism, at an estimated mass around 6,000 tonnes.
- Each aspen trunk is a stem (ramet) of one individual, all connected by a shared root system.
- Pando is failing to regenerate mainly because mule deer and cattle over-browse its young shoots; fenced areas recover while unfenced ones decline.
- Individual aspen stems live only around 130 years, but the root system as a whole is thousands of years old.