Immense and Ancient Quaking Aspen
Jeff Mitton
Natural Selections (appeared September 10, 2000)
Quaking aspen is a handsome species in any season, but its fall colors draw thousands of people into the mountains every autumn.
Aspen’s bright yellow and warm orange fall foliage is apparent to everyone, but most people do not appreciate how truly extraordinary aspen is. Quaking aspen is he most genetically diverse plant species, it is the most widely distributed tree in North America, it may live longer than any other species, and an aspen is the world’s largest (by weight) living organism.
Quaking aspen, Populus tremuloides, can be found from the mountains of Mexico to northern Alaska, from the Atlantic to the Pacific, and from sea level to more than 11,400 feet. At the University of Colorado’s Mountain Research Station, aspen grow at tree line, where they are twisted and pruned by winter winds, and are less than three feet tall.
Aspen produce seeds that are wafted by the wind, just as they are in cottonwoods, which are close relatives. But the success of these tiny seeds is poor in arid regions. In Colorado, aspen spread more commonly by a form of asexual reproduction called suckering. A solitary tree can send out lateral roots that will sprout another tree. This process will produce a stand or even a small forest. But what appears to be a stand of trees is a group of vertical stems, called ramets, connected by a common root system; it is a single individual, a clone.
Careful observation will allow you to distinguish clones in the forest. Clones can be distinguished by the time that their buds flush in the spring, their fall colors, bark color, the angle of the branches, flecking on the bark, leaf shape and gender. Aspen is dioecious, so a clone is either male or female; gender is identified by examining their flowers, or catkins, in the spring. Female clones are slightly more abundant at low elevation, but 90% of the clones above 11,000 feet are male.
Persistent root systems allow aspen to colonize and persist in disturbed sites, such as avalanche runs and frequently burned areas. A mature lodgepole pine or spruce-fir forest may contain just a few, widely spaced, suppressed stems of aspen beneath a dense canopy. But in the spring after a fire or an avalanche clears out the conifers, numerous ramets spring from the expansive root system, and grow several feet tall in a few months.
The most massive, or heaviest, single individual living today is a clone of aspen on the shore of Fish Lake in southern Utah. The giant is a male, nicknamed Pando (meaning “I spread”), that covers 122 acres. Some clones of fungus cover more area than this clone of aspen, but they are diaphanous or invisible, so their weights are trivial in comparison. We estimated that Pando has 47,000 ramets, and weighs more than 13 million pounds, more than triple the weight of its nearest rival, the giant sequoia named General Sherman.
Most ramets in the Front Range are relatively young; our sampling of the largest ramets revealed ages of 60 to 120 years. But the age of a ramet tells you nothing about the age of a clone. Most biologists agree that clones of aspen could be 10,000 years old, and some biologists speculate about ages in excess of a million years. But this is all conjecture¾the ramets and roots continually wither and are replaced by new growth, so no piece of wood reveals the full age of a clone.
As you hike through aspen forests, watch carefully. In some cases, you will not be walking through a forest, but through a clone, a single individual.
The fall colors of aspen illuminate western mountains in autumn
Photo by Jeff Mitton
Pando, a male clone of quaking aspen, Populus tremuloides, is the largest (by weight) living individual. It covers 122 acres of hillside and lake shore at Fish Lake, in southern Utah. Note the two humans, near the center of the photo, for scale. This is just one margin of the enormous, wandering clone.
Photo by Jeff Mitton