The Pando aspen grove, located in central Utah, is the largest organism on the planet by weight. From the surface, it may look like a forest that spans more than 100 U.S. football fields, but each tree shares the exact same DNA and is connected to its clonal brethren through an elaborate underground root system. Although not quite as large in terms of area as the massive Armillaria gallica fungus in Michigan, Pando is much heavier, weighing in at more than 6 million kilograms. Now, researchers say, the grove is in danger, being slowly eaten away by mule deer and other herbivores—and putting the fate of its ecosystem in jeopardy.

“This is a really unusual habitat type,” says Luke Painter, an ecologist at Oregon State University in Corvallis who was not involved with the research. “A lot of animals depend on it.”

Aspen forests such as the Pando grove and many others reproduce in two ways. The first is the familiar system in which mature trees drop seeds that grow into new trees. But more commonly, aspen and some other tree species reproduce by sending out sprouts from their roots, which grow up through the soil into entire new trees. The exact amount of time it took the Pando grove to reach its modern extent is unknown, says Paul Rogers, an ecologist at Utah State University in Logan. “However, it’s very likely that it’s centuries old, and it’s just as likely that it’s millennia old.”

Scientists first noticed the Pando shrinking in the late ’90s. They suspected elk, cattle, and most prominently deer were eating the new shoots, so in the new study Rogers and colleagues divided the forest into three experimental groups. One section was completely unfenced, allowing animals to forage freely on the baby aspen. A second section was fenced and left alone. And a third section was fenced and then treated in some places with strategies to spur aspen growth, such as shrub removal and controlled burning; in other places it was left untreated.

Aerial photos of the Pando grove spanning 1939 to 2011, which show the grove thinning over time

USDA Aerial Photography Field Office, Salt Lake City, Utah

The results were surprising: Simply keeping the deer out was enough to allow the grove to successfully recover, the team reports today in PLOS ONE. Even in the fenced-off plots where there was no burning or shrub removal, young trees were thriving.

The good news, at least for Pando, is that it appears that keeping out the deer is enough to solve the problem. But fencing the entirety of the grove is neither practical nor palatable, says Rogers, who partners with the U.S. Forest Service’s Rocky Mountain Research Station in Fort Collins, Colorado, as part of the Western Aspen Alliance, a group committed to improving aspen management and restoring their ecosystems. “Everybody, including myself, doesn’t want fences around this iconic grove. We don’t want to go to nature to see a bunch of fences.”

The alternative, he says, is to do something about the mule deer population. The thinning of the forest has only started to occur in the past century or so. This time frame roughly coincides with when humans entered the area, building cabins, banning hunting, and removing carnivores like wolves that would ordinarily prey on the deer. These human activities, Rogers says, has turned Pando into a safe haven for the deer, artificially inflating their numbers in the area.

With the new data in hand, he’s planning to advocate for a culling of the deer population in the area. Although that may seem extreme, it may be the only chance to give Pando a chance a long-term survival. “The real problem,” Rogers says, “is that there are too many mouths to feed in this area.”

Source: One of the world’s largest organisms is shrinking | Science | AAAS