What’s killing the world’s most massive living thing? New study of Utah’s Pando aspen grove blames cows, not deer.

Each fall, about a thousand cattle pass through southern Utah’s Pando aspen grove, believed to be the world’s most massive living organism, pausing for a week or two on their way from summer to winter pastures.

During the herd’s brief stay, up to 90% of the plant growth beneath the trees in the unfenced portions of the grove is grazed away, according to a new study that fingers livestock, as opposed to mule deer, as the chief culprit in the world-famous Pando’s troubling decline.

Western Watersheds Project, an advocacy group that seeks to reduce livestock grazing on public lands, conducted the study over the course of last year’s growing season using four motion-detecting trail cameras, two rigged inside Pando and two in neighboring stands of quaking aspen, Utah’s state tree.

“At our monitoring sites, we documented 4.5 times the amount of cattle use herbivory in two weeks than the mule deer use over six months,” the report said, urging the U.S. Forest Service to fence off the 106-acre grove, whose trees are genetically identical and share the same root system.

The amount of forage taken far exceeds the 50% threshold set by Forest Service guidelines, according to the report.

“The level of grazing use by these cattle essentially cleans out the understory” with not much left for native animals and insects," said co-author Erik Molvar, Western Watersheds’ executive director. “The cattle are stripping the larder bare, leaving the deer nothing to eat but aspen. They could be driving the deer to eat more aspen.”

Located near the southwest shore Fish Lake in Fishlake National Forest, Pando’s name comes from the Latin “to spread out.” Now, the grove actually is contracting, plagued by the same problems afflicting aspen ecosystems around the West. Young trees aren’t coming up fast enough to replace aging ones. Herbivore ungulates dine on the nutritious shoots, wiping them out before they can reach maturity. Other factors include the suppression of fire, a natural force that reinvigorates aspen groves, and fungal diseases.

Citing past studies, the Forest Service contends it is mostly a resident herd of mule deer munching Pando out of existence.

“The deer in there are basically a year-round population,” said John Zapell, spokesman for the Fishlake National Forest. “The cattle come as they move off the range in fall. They are in there for less than two weeks. ... When I walk in the unfenced areas, there is not a lot of young aspen that have come up. Is that the cattle or deer?”

He contends it is an unsettled question and that Pando’s decline is likely associated with both wild and domestic ungulates.

The issue was studied by Utah State University professor Paul Rogers, head of the Western Aspen Alliance, who concluded deer play a big role in Pando’s failure to regenerate. He had not thoroughly read the latest report, so he was not prepared to offer a full comment Friday, but he did note it was not peer-reviewed science.

“An organization putting out their own report, with their own potential biases, is much different than someone being anonymously vetted by independent reviewers,” Rogers wrote in an email. “Cattle are certainly part of the problem, but there are significant areas within Pando where cattle are excluded and Pando is not successfully regenerating.”

Aspen and mule deer, however, have been sharing the landscape for thousands of years, evolving with each other, according to Molvar’s co-author, John Carter, an ecologist who runs a nature preserve in southern Idaho.

“It is counterintuitive," Carter said, “that mule deer would be causing the dieback that we’re seeing at the Pando clone, and in many other parts of the West."

It is only since the arrival of domestic livestock that the West’s aspens have begun disappearing, replaced by conifers.

“Cattle have a greater propensity to consume aspen sprouts in autumn, when the nutritional quality of other understory vegetation declines," the report states, “and the virtual elimination of understory vegetation by this high intensity livestock use may also cause mule deer to switch to aspen shoots, further amplifying the impacts.”

Ungulates are excluded from two fenced areas covering about half the Pando grove, although deer have been spotted inside one. The woven-wire fences stand eight feet hight and were built in 2013 and 2014. Aspen regeneration is occurring in the one built in 2013, with young trees reaching heights of up to 12 feet.

“Outside the exclosures, you have heavy browsing on aspen to the point where new shoots are being grazed,” Molvar said. “That destroys Pando’s ability to perpetuate itself. The old trees may live only 120 to 130 years before they topple over.”

Once an aspen reaches 6 feet, it is safe from browsing ungulates.

Rogers has proposed fencing off the entire grove’s 12,000-foot perimeter, which would cost $60,000 plus maintenance costs, to keep out all ungulates, as a way to reverse the Pando’s decline. Effectively fencing the grove would be difficult and blight the landscape, at least for several years, with an unsightly structure. Falling trees will create breaches and human visitors would sometimes leave gates open. And the grove straddles State Road 25.

The study authors endorse fencing, although they recommend adding a quarter-mile buffer to allow for the grove’s expansion.

“We can determine, absolutely, the relative contribution of livestock or deer to the killing of Pando by simply removing livestock from those pastures for five years and measuring the results,” said Jonathan Ratner, another co-author who works for Western Watersheds. “We hope the Forest Service is willing to do its job.”