Compared with a mite or a virus, we humans are enormous. But we share this planet with other organisms that, in turn, dwarf us. At 100 feet, a blue whale is about 18 times longer than the average person; a giant sequoia, three times that. There are even larger giants on Earth, and you don’t have to travel to some far-flung corner of the world to see them. In 1992 two Michigan biologists startled the public by announcing their discovery of a fungus covering an area of 40 acres. Their announcement was soon followed by one from another group of researchers who claimed to have found a 1,500-acre fungus in Washington.
When I and two of my colleagues at the University of Colorado, Jeffry Mitton and Yan Linhart, first read about the fungi, we decided that the record had to be set straight. While the Washington fungus may in fact be the world’s largest organism in area, it is not the largest in mass. Its discoverers have yet to calculate its weight, but they do know that it probably weighs under 825,000 pounds--about double the weight of a blue whale but nowhere near that of a giant sequoia, which can tip the scales at 4.5 million pounds. Yet even the majestic giant sequoia is not the record holder. That honor goes to a tree that my co-workers and I have studied for years: the quaking aspen, a common tree that dapples many mountains of North America. Unlike giant sequoias, each of which is a genetically separate individual, a group of thousands of aspens can actually be a single organism, sharing a root system and a unique set of genes. We therefore recently nominated one particular aspen individual growing just south of the Wasatch Mountains of Utah as the most massive living organism in the world. We nicknamed it Pando, a Latin word meaning I spread. Made up of 47,000 tree trunks, each with an ordinary tree’s usual complement of leaves and branches, Pando covers 106 acres and, conservatively, weighs in excess of 13 million pounds, making it 15 times heavier than the Washington fungus and nearly 3 times heavier than the largest giant sequoia.
Pando reached such vast dimensions by a kind of growth, common to plants, known as vegetative reproduction. A plant sends out horizontal stems or roots, either above ground or below depending on the species, that travel some distance before taking root themselves and growing into new, connected plants. For us humans, who tend to view sexual reproduction as the only means of generating offspring, the method may seem a bit strange. Yet vegetative reproduction happens all around us. Every gardener witnesses it in one form or another. Strawberry plants, for example, send out stringy aboveground stems that can take root and form additional leafy clusters. Vegetative reproduction allows grass to produce lovely lawns (as well as foul language when it spreads into the garden plot). People who raise houseplants routinely take advantage of vegetative reproduction when they make cuttings of their favorite ivy or spider plant and root those pieces in new pots.
In the wild, vegetative reproduction commonly happens on a much grander scale. If you fly across the Southwest, you may see striking geometric patterns of desert shrubs, such as the creosote bush, which usually grows in circles. These circles don’t provide evidence of geometrically savvy visitors from outer space. They’re evidence of new creosote bushes forming at the periphery of a spreading individual while older stems in the center are dying.
Most trees stick to sexual reproduction. In some species, male trees produce pollen in their flowers, which is then used to fertilize the female flowers and produce seeds. In others, a single tree will have the equipment of both sexes. Aspens do indeed have flowers and sexes (Pando is male), but they almost always reproduce vegetatively. They send out roots horizontally underground, from which new shoots called stems (or, more formally, ramets) grow vertically. The new shoots eventually develop into new tree trunks as tall as 100 feet, with branches, leaves, bark--in short, everything you’d associate with an individual tree. Because a root may travel 100 feet underground before sprouting up, and each new trunk can send out its own army of underground roots to form still more new shoots, an aspen individual can attain quite impressive dimensions.
The sum of all the stems, roots, and leaves of one such individual is called a clone. Quaking aspen clones may spread far across a landscape as they continue to reproduce vegetatively. How far one clone can migrate depends on how long it can live.
And how long might that be? The short answer is that we don’t know. It might seem as if all one has to do is count the annual growth rings in the individual stems. Aspen stems that I’ve studied in the Colorado Front Range rarely exceed 75 years. Elsewhere individual stems occasionally reach 200 years. But the age of individual stems tells us almost nothing about the age of the clone they belong to, since its living stems may only be the latest to sprout. The oldest clone with a firm age is an 11,700-year-old creosote bush (researchers were able to date it by measuring the rate at which its circle expands). But aspens may actually be far older. Based on evidence such as the resemblance of some aspen clone leaves to fossilized ones, Burton Barnes of the University of Michigan has suggested that aspen clones in the western United States may reach the age of a million years or more. In principle, clones may even be essentially immortal, dying only from disease or the deterioration of the environment rather than from some internal clock.
As a true organism, a clone is made up of genetically uniform parts. Barring rare mutations, the aspen trunk on the north edge of a particular clone will be genetically identical to the aspen trunk on the south edge and to all in between. We biologists can use molecular techniques to compare the genetic makeup, but an observant hiker can also recognize clones and even distinguish among them. The angle between individual branches and the main trunk tends to be a genetically determined trait that is different from clone to clone. Thus branches on the trunks of one clone may angle off at about 45 degrees, while another clone’s stems show angles nearer 80 degrees.
The time at which clones come out of their winter dormancy also has a strong genetic basis. In spring you can commonly observe that one stand of aspen trees will be bare of leaves while a nearby stand will be fully leafed out. But the most spectacular (though not infallible) indicator of clone identity unfolds with the onset of fall. Some clones turn a brilliant, shining yellow that almost seems to generate sunlight. Others manifest a deep, rich gold, vibrating with many overtones. The leaves of still other aspens turn red; some show a barely perceptible tinge, others a rich scarlet. With experience, one can use these colors as clues to deduce the boundaries of clones. A warning: they can also mislead. Just as a single red maple tree may have dramatic differences in fall coloration between its sunny side and its shady side, aspen clones can vary, too, but the differences may be spread across thousands of different trunks.
Even biologists can be fooled by aspen stands. One group of researchers, examining the strings of flowers (known as catkins) that quaking aspens produce before leafing out, concluded that the flowers produced one year were of a different sex from those produced the previous year by the same small stand of trees. Knowing that other vegetative reproducers, such as some desert junipers, can be male one year and female the next, the researchers speculated that perhaps aspens could switch sex also.
My colleagues and I were so intrigued by this suggestion that we decided to follow it up more thoroughly. First we identified a number of clones by isolating their unique patterns of enzymes in the lab and then marking the shoots in the field. For several years we then followed their flowering pattern each spring. We found no switching of sexual identity; instead, we discovered that even a small stand of aspen trees may contain more than one clone. We mapped and marked some 160 stems in one such stand. It turned out that there were two clones intertwined in the stand, one male, one female. The previous researchers, we realized, had been tricked into seeing sex switching when in fact they had seen a female clone in their stand flower one year, and a male clone in the same stand flower the next.
Aspen stands are just as complex below ground as above. Their intricate network of roots can ferry nutrients from one part of the clone to another. Roots near an abundant water supply, for example, may provide water to other roots and shoots in a much drier area. These parts of the clone can return the favor if their roots have access to crucial nutrients missing from the wet area. By distributing its water and nutrients over its entire expanse, a quaking aspen clone can survive in a patchy environment where other trees might die off.
It shouldn’t be surprising, therefore, that the quaking aspen is the most widespread tree in North America, forming an almost continuous band between Newfoundland and Maryland in the East and another between Alaska and Washington in the West. Aspens also follow the Appalachian Mountains south to Georgia, and the Rocky Mountains all the way into northern Mexico. In total, this species covers tens of millions of acres in North America.
Wherever they grow, quaking aspens like unstable habitats. In mountainous areas avalanches and mud slides leave barren paths that soon support extensive stands. In fact, it’s possible to date mud slides and avalanches by measuring the age of aspen stems that shoot up immediately following a slide in the scoured area. The distinctive light green of aspen leaves in summer, set off from the deep greens of conifers such as lodgepole pines, frequently marks the zones where winter snow is unstable and tends to avalanche.
Even more than slides of mud or snow, however, it is man’s old friend and nemesis, fire, that ensures aspen survival. At first this might not seem logical, because an aspen stem is particularly vulnerable to fires. Most trees are covered in a bark of dead cells, but the smooth, cream-colored bark of quaking aspens usually remains a living, functioning tissue; it even carries out photosynthesis. The bark succumbs quickly to forest fires, and the entire stem in turn dies.
When a single stem dies, however, the entire clone feels the effect. Normally each stem sends hormones into the root system that suppress the formation of new ramets. But when a stem dies, its hormone signal dies as well. If a large number of the shoots in a stand are wiped out, the hormonal imbalance triggers a huge increase in new, rapidly growing stems. The regeneration of stems can dwarf the original destruction: researchers have counted densities of up to 400,000 aspen stems per acre (Pando has a rather low figure of just over 400 stems per acre).
If an aspen grove does not regularly experience fire or some other disturbance, its days are numbered. Conifers will invade its borders and begin to shade out the stems. Aspens can’t tolerate low levels of light, and they will eventually start to die as the conifers dominate the grove. One consequence of fire suppression by humans in North America has been a drastic reduction in the extent of aspen forests. Pando probably reached such a huge size because until recently he experienced a regular sequence of fires that let him regenerate, spread, and maintain himself. The fires didn’t happen so quickly that they eradicated him, nor were they so infrequent that conifers had time to replace him.
The quaking aspen gained its name because of the way the tree’s leaves tremble in even the slightest breeze. French Canadian woodsmen in the 1600s believed that the trees quaked in fear because the cross on which Jesus was crucified was made of aspen. Now giant aspen clones like Pando have a new reason to tremble: human incursions. Several private homes have recently been built within one section of Pando, and another section has been turned into a campground, complete with parking spaces, picnic tables, and toilets. Paved roads, driveways, and power and water lines built to serve these developments dissect this spectacularly beautiful aspen stand. The presence of people has led the U.S. Forest Service to suppress wildfires, and yet Pando’s remarkable size and longevity are largely a consequence of the cleansing, rejuvenating power of wildfires. Ironically, ending wildfires could well mean the end of Pando.
Realizing that it was affecting Pando’s vitality, the Forest Service recently decided to try to boost its growth by clear-cutting part of the stand. It chain-sawed three clear cuts, totaling about 15 acres, right out of the middle of this magnificent old clone and offered the timber for free to any who wanted firewood. The results have been mixed: because of heavy deer browsing, the first two clear cuts showed minimal regeneration; the third was fenced to keep out the deer. New shoot growth, now one foot tall in the fenced area, appears abundant and healthy. And yet the clear cuts carved from the heart of this individual, clashing as they do with Pando’s surrounding pristine parts, come as a dispiriting shock to me.
Since my colleagues and I nominated Pando as the world’s largest organism, he has captured the attention of dozens of newspapers and radio stations across North America, and some of the reactions have been quite funny. Some see Pando as a threat: I received a call from someone asking, Does this giant clone, spreading vegetatively, pose a threat to the people living in southern Utah? Another person wondered if this recognition of the interconnectedness of nature was the real beginning of New Age philosophy. For us, the real significance of Pando lies in the interest about things botanical he has stimulated. The more we examine the special properties of the quaking aspen, the greater our fascination with the beauty, complexity, and continuing mystery of this tree. If others agree, perhaps we can save clones like Pando from a destiny as firewood.
