Scientists have warned that an ancient forest widely considered the largest single living thing in the world is dying, despite efforts to preserve it.

The Pando aspen is an enormous expanse of 40,000 trees, all of which are clones with identical genetic compositions, meaning they are classified together as one individual.

Thought to be up to 80,000 years old, the colony known as the “trembling giant” is a contender for the oldest organism as well as the heaviest and largest.

In total the trees, which originate from a single underground parent clone, cover 43 hectares of Utah’s Fishlake National Forest.

But in recent years a tragedy has been quietly unfurling. Despite their best efforts, scientists think this natural wonder that has lasted millennia may not survive a few decades of human interference.

“While Pando has likely existed for thousands of years – we have no method of firmly fixing its age – it is now collapsing on our watch,” said Professor Paul Rogers, an ecologist at Utah State University.

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After analysing Pando’s condition comprehensively, Professor Rogers and his colleague Professor Darren McAvoy examined a 72-year aerial photo sequence that revealed its steady decline.

The forest has gradually thinned as humans expanded into it, cutting down areas that have never properly recovered.

Additional pressure over the years has come from drought, and the intrusion of hungry deer into the forest that have hampered efforts to restore it.

“After significant investment in protecting the iconic Pando clone, we were disappointed in this result,” said Professor Rogers.

Early efforts to protect the forest from deer using fencing and allowing young shoots to grow showed some promise.

However, when these methods were expanded across a larger area they failed to prevent animals from getting in. Professor Rogers said deer appeared to be finding ways to either enter through weak points or somehow hopping the eight-foot barrier.

The results of the study were published in the journal PLOS ONE.

In his role as director of the of the Western Aspen Alliance, Professor Rogers has seen the trends recorded in Pando playing out across the aspen forests of the western states.

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“In addition to ecological values, Pando serves as a symbol of nature-human connectedness and a harbinger of broader species losses. Here, regionally, and indeed internationally, aspen forests support great biodiversity,” he said.

“This work further argues for ‘mega-conservation’ as a departure from traditional individual species-habitat approaches.

“It would be a shame to witness the significant reduction of this iconic forest when reversing this decline is realisable, should we demonstrate the will to do so.”