Clonal colonies in the forest
The aspen tree—known variously as the quaking aspen, trembling aspen, American aspen, and golden aspen, but most importantly as Populus—inhabits cool weather regions of the Northern Hemisphere from the United States across to Italy, Russia, China, and Japan—and even a small cone of mountains in the highlands of northwest Algeria.
Aspen tend to thrive in recently disturbed landscapes and are typically the first large plant species to sprout in places impacted by wildfires, avalanches, logging, or other major events. They grow quickly, create ethereal groves with white trunks backed by shimmering green, and in fall their yellow color seems to infuse the air with a dream-like glow.
But there’s one novelty which makes aspen stand out from almost every other tree species: The trees grow roots near the surface, and the roots sprout new trees. The new trees are genetically identical to the parent tree, and a grove of aspen—often covering more than 100 acres—is literally a single living organism.
A single clonal colony of individual male quaking aspens in Fishlake National Forest in the high country of south-central Utah has been identified as the heaviest known organism and among the oldest known living organisms. The grove, known as Pando, is thought be to be dying due to a variety of factors, including grazing, development, drought, and fire suppression.
“Pando is the biggest aspen ‘clone’ ever identified, the single most massive living organism known on Earth,” The Salt Lake Tribune reported.
“A clonal colony or genet is a group of genetically identical individuals, such as plants, fungi, or bacteria, that have grown in a given location, all originating vegetatively, not sexually, from a single ancestor,” explains Wikipedia. In the case of aspen, above ground they appear to be separate trees, but below ground they are interconnected and clones of the same plant. Pando is estimated to be 80,000 years old, even though individual aspen trees usually don’t live for more than 150 years.
Aspen are likely the largest clonal colonies on Earth, but they are not the only ones. Other examples include trees and shrubs such as the willow, blackberry, fig, and banyan; vines such as wisteria; ferns such as goldenrod; herbaceous flower plants like the strawberry, and non-woody plants like Narcissus and Crocus.
Quaking aspen, P. tremuloides, according to a 2013 study in the Journal of Biogeography, have the largest natural distribution of any tree native to North America. A sampling of quaking aspen DNA found two major genetic clusters—one in southwestern North America and another to the north. The one in the southwest included two subclusters, while the northern cluster had no subclusters; the boundary line between the two was roughly the southern extent of the ice sheet during the Last Glacial Maximum—roughly speaking, the U.S.-Canada border.
A 2016 study discovered that quaking aspens in the northern region of North America have had a high level of gene flow between populations, while those in the Rocky Mountain foothills of Alberta had increased allelic richness, likely thanks to favorable environmental conditions for reproduction.
Beautiful though they may be, scientists have found that aspens in the Rocky Mountains have been in decline for decades, with stands found dying next to neighboring trees which are unaffected. This decline has happened in two ways—stand death, likely due to ungulate browsing, fire suppression, and the resulting crowding by conifers—and “sudden aspen decline,” a more widespread, severe, and rapid dieback occurring on a landscape scale likely due to a combination of infestations and warm drought conditions.
“The best rule of thumb to know for figuring out whether an aspen grove is healthy is whether you can see through it,” explains the U.S. Forest Service. “If you can see through an aspen grove…it may be just a matter of time before this aspen grove dies off and disappears from the landscape.”
Because of their genetic clonal nature, aspen tree biology is particularly complex, notes one exploration of sudden aspen death. When stressed by drought, aspen trees have to pull water from the soil more strongly, which creates bubbles in the water that can block water and nutrient transport.
“If the clone dies,” notes phys.org, “thousands of trees can be lost.”