Every tree in the Pando is genetically identical.

The largest living organism in the world isn't a blue whale or a giant California redwood.

Key points:

• Huge underground singular root system of aspen trees that covers more than 100 acres is dying off
• New research suggests deer and cattle are to blame
• Researcher is arguing to build temporary fence to give Pando chance to regenerate

It's a huge underground singular root system that sends up tens of thousands of clone aspen trees, each one genetically identical to the next, over an area of more than 100 acres.

Pando aspen clone, also known as "the trembling giant", lives on a hillside in the Fishlake National Forest in central Utah.

Literally translating from Latin as "I spread out", Pando is collapsing; the forest is ageing, but there aren't enough new recruits to replace the dying trees.

Now research published in PLOS ONE today has found that browsing animals, such as deer and cattle, are most likely to blame, according to lead author Paul Rogers from Utah State University and the Western Aspen Alliance.

"Basically in a single sentence, Pando is failing and humans are at the centre of the failure, because humans control the browsers," Dr Rogers said.

"The problem is the forest is ageing but every young shoot that comes up is immediately eaten because the browsers have no predators."

The researchers compared sample plots in Pando between an area that was fenced off from deer and cattle in 2013, another area that was fenced in 2014 but was breached by mule deer, and another entirely unfenced area.

They found a significantly higher amount of new and young shoots in the area that had been fenced off from browsers since 2013.

Their observations are consistent with the known impacts of deer in other regions.

Even in Australia, where introduced deer are less common than in the US, they have an impact on forests, according to Associate Professor Mathew Crowther from the University of Sydney.

"We've looked at areas where you don't have deer, and where you do have deer, and you get different vegetation communities," said Dr Crowther, who wasn't involved in the study.

"And that can lead to impacts on the smaller mammals that depend on that habitat."

'Conservative' Utah not likely to bring back wolves

 

Mule deer jumped a 2.4-metre fence to reach Pando.

The question of how to control the deer population has become political, according to Dr Rogers.

Although deer are native to Utah, their natural predators like wolves and bears have been suppressed, so deer have been able to expand beyond that region's previous carrying capacity.

Wolves were reintroduced to Yellowstone National Park in neighbouring Wyoming to deplete an out-of-control population of deer with great success, but Dr Rogers said that won't happen in Utah.

"Reintroducing predators is a very uphill battle. We live in a pretty conservative state, so that's a no-go in the short term."

Their only other threat is from hunters, but shooting is considered too dangerous in the tourist-friendly park.

And while fencing the 43-hectare clone forest seems like an obvious solution, it doesn't sit well with many in the region.

"[Fencing] is a quick fix but it's not a long-term fix. Nobody … would like to see fencing any more than there already is. It's as if we're putting this thing in a zoo," Dr Rogers said.

Instead, Dr Rogers is arguing for a temporary fence to give Pando "breathing space", and then allowing professional hunters limited access to cull the deer.

Pando decline could make way for 'Humongous Fungus'

 

The total organism weighs nearly 6,000 tonnes.

Pando was first recognised as a single organism in the 1970s. More recently, geneticists have confirmed that there are around 46,000 clone aspen trees coming from the one root system.

Although its age hasn't been confirmed, it's believed to have established itself as the last ice age receded, putting it in the order of 14,000 years old.

And according to the United States Department of Agriculture, its total weight is just under 6,000 tonnes, making it the biggest single organism by mass on the planet.

Also vying for the title is the "Humongous Fungus" — a single mushroom covering around 8 square kilometres, mostly underground, in Oregon.

Pando's survival strategy up until this point has been very successful. It puts most of its energy into fast growth, and little into producing defences like thorns or poison.

But that strategy has been breaking down over the last 50 years in the absence of predators to control the deer, according to Dr Rogers.

He said the uniformity of Pando provided a unique opportunity for researchers to analyse that strategy in isolation.

"We can hopefully use the lessons learned here on this forest of one tree, to export to more complex landscapes where there's many genotypes and different levels of defence and growth," he said.

The impact that removing predators has on ecosystems has gained more prominence recently, according to Dr Crowther.

"When you remove a large predator, numbers do go out of control," he said.

"You do get that in Australia. Where there are no dingoes, you get a lot of kangaroos."

Research published earlier this year found that dunes on either side of the dingo fence, which runs from New South Wales to Western Australia, were forming differently as a result of a "trophic cascade" triggered by the exclusion of dingoes.

Although returning predators to Utah is likely to be the most sustainable long-term solution, Dr Rogers fears what will happen if parties can't agree on short-term action.

"It will no longer be the largest organism on earth, it could collapse significantly ... which is a lose-lose for everybody."