If you’ve ever walked through an aspen grove, you’ve seen hundreds or thousands of white trunks propping up a sky of silvery green leaves trembling in the wind.

In the fall, all the leaves in one grove will go from green to gold at the same time.

This is because the entire grove is really just one organism, a massive root system from which many trunks sprout, grow, die, and are replaced by new trunks.

The largest aspen grove in the world, in Utah, is named Pando: Latin for “I spread out.”

It sprouted 80,000 years ago from a seed the size of a pepper grain and now supports almost 50,000 trunks, making it the heaviest living thing in the world—and one of the oldest.

But Pando is slowly declining. Most of its trunks are now more than 100 years old; new ones aren’t growing to take the places of those that die.

Researchers think it has to do with elk and mule deer. A century ago, ranchers and trappers removed their natural predators from the area. Local populations of elk have grown to more than 77,000 and mule deer to 300,000.

And the grazing elk and deer are eating the aspen saplings.

Studies that fenced off sections of the grove have seen young trees return, growing 10 feet in just a few years.

Ranchers don’t want to reintroduce non-human predators, who might endanger their livestock.

So, the solution may be to fence Pando to protect it from deer. Or to change hunting practices to thin the herds.