fall aspen pando utah.JPG This is all one giant tree. Yes, really. Mark Muir/USDA

Think of a tree and what comes to mind probably has some leaves, some roots, and a trunk.

But not all trees follow that formula. Some form what are called clonal groves: swaths of forest connected underground by a single network of roots, with each trunk genetically identical to the others.

The most famous example of a clonal grove is Pando, a grove of quaking aspen in Utah's Fishlake National Forest.

Its name means "I spread" in Latin, and for good reason. Pando is among the largest and oldest organisms on Earth. Its 47,000 stems cover more than 100 acres.

It's tricky to tell exactly how long Pando has been around, since the individual trunks only live up to 100 or 150 years. But scientists estimate Pando is at least 80,000 years old — and possibly more like a million years old. Those estimates are based on how quickly aspens spread.

Scientists have sequenced the genome of a couple dozen shoots of Pando and confirmed the main swath really is a clone, with very closely related but not quite identical trees surrounding Pando proper.

Clonal aspens like Pando can reproduce the traditional way, but they aren't as good at it. That's because they have three sets of chromosomes instead of the usual two. So when conditions aren't perfect, they stick to sending up new shoots from the mass of roots under the grove.

Pando has survived this way for a long time, but scientists are worried it may not be able to clone itself quickly enough to stay alive in the future. They've noticed that most of the aspen trunks are relatively old and when new shoots do develop, they don't last long enough to become full-fledged trees.

Scientists have a couple of theories why that is, but one of the most common is that deer and other animals nibble away at the shoots before they grow old and thick enough to protect themselves. In response, sections of Pando have been fenced off. Deer still sometimes sneak in, but in places where the fence is well-maintained, young shoots inside the fence are doing better.

See more of Pando in this video: