A Nominee for Biggest Organism
By WILLIAM K. STEVENS
Published: December 15, 1992
WHAT is the world's largest living organism?
Scientists in Colorado have nominated a new candidate for the title: a 106-acre, 6,000-ton stand of genetically identical quaking aspens, cloned naturally from a single individual and growing from a single root system in Utah.
In April, other scientists reported the discovery of a giant fungus of the species Armillaria bulbosa that they said was among the biggest and oldest organisms on the planet, bigger than the giant sequoia and the blue whale. They did not claim it was the largest organism in the world, but the possibility was widely raised.
The fungus covers more than 30 acres in the soil of a forest in northern Michigan. Its network of rootlike tentacles and above-ground mushrooms was spawned by a single spore 1,500 to 10,000 years ago; and although there were many tiny breaks in the fungus, the scientists wrote in the British journal Nature that it was genetically uniform from one end to the other and therefore could rightly be considered a single individual.
If the Michigan fungus can be so characterized, say the Colorado professors, the Utah aspens surely qualify as well. In fact, they say, there are no breaks in the interconnected filigree of roots from which some 47,000 aspen stems grow.
"Many people think when they're hiking through the mountains that they're hiking through an aspen forest, but in many cases what they're hiking through is aspen clones," says Dr. Jeffry Mitton of the University of Colorado. Along with two colleagues, Dr. Michael Grant and Dr. Yan Linhart, he nominated the system of cloned aspens as the world's biggest organism in a letter in the Nov. 19 issue of Nature. Common Root System
"If you could look into the soil and trace the roots of all these things that look like independent trees," Dr. Mitton said, "you would find that they are in fact connected by a common root system, like the leaves on a silver maple tree are connected by the branches and the main stem." As the organism sends out its roots, he explained, it also sends up new sprouts, called ramets.
The result, he said, is a single organism "that can literally climb over mountains and across meadows."
There are thousands of these clonal systems in the Rocky Mountain region, said Dr. Mitton, and he and his colleagues have studied many of them. By analyzing genetic variation in proteins, they determined that the aspen stems springing from a given root system were genetically identical. As with human twins, said Dr. Mitton, the exact match can be noticed on close inspection: leaves on two ramets cloned by the same system are the same size and shape, for instance, and an entire stand of clones changes color in the fall in exactly the same way at the same time. The result can be a quiltlike pattern over the countryside with some stands orange, some yellow, some still green and some already bare.
The particular clonal system that the Coloradans have nominated as the world's largest organism grows in the Wasatch Mountains south of Salt Lake City. It was first described in the journal Science in 1975 by Dr. Burton Barnes, a forestry professor at the University of Michigan.
The Colorado scientists, working from Dr. Barnes's description and relying on their own work with other aspen clones, estimated that the Utah clonal system weighs about 13.2 million pounds, or more than 60 times the Michigan fungus.
Quaking aspens reproduce both sexually and asexually. A clonal system is formed when an individual tree produced sexually begins sending out its clones asexually. Sexual reproduction is relatively rare and has been "frustrated for thousands of years" because the dry climate inhibits seed development, said Dr. Mitton. Asexual clonal reproduction has taken up the slack. The clonal systems may be thousands of years old; while an individual aspen stem withers after an average of 65 years, the organism as a whole goes on and on.
Photo: Colorado scientists say some stands of quaking aspens consist of trees identical in genetic structure that grow from a single root system. These aspens, turning color simultaneously, are in Gothic Valley, Colo. (Dr. Jeffry Mitton/University of Colorado)