What’s eating the Pando Clone?

Monitoring project shows two weeks of cattle is decimating the understory

FISH LAKE, Utah – Conservationists today released a new report showing that cattle grazing in the world-famous Pando aspen clone and neighboring aspen groves exert more than four times as much grazing pressure over less than two weeks as do mule deer grazing over the course of the entire growing season. These findings call into question an earlier study blaming mule deer for the Pando Clone’s waning health and failure to regenerate.

“A recent study determined that there was no relationship between browsing on aspens and the failure of new growth in the Pando Clone, yet blamed mule deer for the Pando’s regeneration failure, so we wanted to find out what was really going on here,” said Erik Molvar of Western Watersheds Project.  “Our hidden cameras showed that mule deer are eating a very modest amount of the understory vegetation, but when cattle arrive on the scene in October, they are wiping out the understory vegetation in a matter of days.”

Our report found 70-90% understory plant consumption by cattle, while plant consumption by mule deer was too light to be estimated. A time-lapse video captured by the motion-sensing cameras illustrates the steady growth of understory vegetation throughout the summer, accompanied by light grazing by mule deer, and the decimation of understory vegetation by cattle once they arrive for a period that lasts for 13 days in the autumn.

“The cattle grazing we documented with our cameras is consuming 70 to 90 percent of the understory vegetation in the Pando Clone, an intensity of livestock grazing far heavier than the 25 percent forage removal that should be allowed based on range science,” said Jonathan Ratner of Western Watersheds Project. “The fact that the Forest Service is allowing this extreme level of overgrazing in the world-famous Pando Clone is symptomatic of the absence of ecologically sustainable livestock management across our western public lands.”

The Pando Clone earned worldwide recognition when it was declared the world’s largest single organism in 1992, an entire grove of aspen trees connected by a single living rootstock and reproducing by sending up sprouts from the root network. But large herbivores appear to be suppressing the Pando Clone’s ability to regenerate, and as a result, this aspen clone appears to be dying back.

“Aspens and mule deer co-evolved over the course of tens of thousands of years in south-central Utah, and under these conditions the Pando Clone grew to be the world’s largest living organism,” said John Carter, a PhD ecologist who runs a nature preserve in southeastern Idaho. “It is counterintuitive that mule deer would be causing the die-back that we’re seeing at the Pando Clone, and in many other parts of the West. Indeed, the information from our cameras indicate that livestock grazing plays a major, and perhaps decisive, role in contributing to aspen decline. You can’t have this severe level of utilization by livestock and not be having a cascade of ecosystem-degrading results.”

The new report calls for excluding grazing from the entire Pando Clone plus a quarter-mile buffer to allow for expansion, and highlights the need for additional scientific inquiry into the role of trampling by cattle in damaging aspen root systems. The role of heavy livestock grazing in damaging aspen groves throughout the West also warrants investigation.

“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 Ratner. “We hope the Forest Service is willing to do its job.”

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This project was funded in part by a grant from the Foundation for Sustainability and Innovation.

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