Aspen clones take the size prize
At this time of year, aspen are the most obvious trees in the boreal forest. Along with prairie-crocuses and swans, the first shimmer of tender green from aspen leaves is a welcome sign of spring.
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Aspen regenerating in burn areas are often clones growing off of one parent tree.
(photo: Jill Johnstone)
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Aspen -- Populus tremuloides -- are the most widespread trees in North America, growing everywhere from northern Mexico to Alaska. But there are other characteristics that make aspen stand out as well.
Some researchers claim that aspen form the largest living organisms on the planet, and could also be among the oldest.
No, there are no giant single aspen trees hidden away in a remote mountain valley. Aspen most often reproduce by cloning, meaning that numerous trees sprout off of suckers from a single parent.
After measuring one aspen clone growing near Salt Lake City, Utah, researchers estimated that it weighs more than 6,000 tonnes, making it three times heavier than the largest giant sequoia tree and giving it the record as the world's largest living organism.
Nicknamed Pando, Latin for "I spread," this clone covers an area of more than 43 hectares and has more than 47,000 individual stems.
Jim Pojar, now the executive director of the Yukon branch of the Canadian Parks and Wilderness Society, worked as a forest ecologist with the B.C. government for 25 years. He is also a co-author of Plants of the Western Boreal Forest and Aspen Parkland, a field guide regularly used in this area.
Pojar says that there could be bigger aspen clones than Pando around; it's just that no one has tried to measure them. "There are huge clones in Alberta as well. You can tell because they green up in spring and change colour in the fall at the same time."
Aspen that reproduce by cloning can also be extremely long-lived. Individual stems may live less than 200 years, but the clone itself survives much longer as new stems continue to replace the dead ones.
Some clones are estimated to be at least 8,000 years old, making them possibly the oldest organisms on the planet. Some researchers even speculate that aspen could theoretically be immortal if no natural catastrophe kills the clone. Fossilized leaves of aspen trees dating back a million years look almost identical to aspen leaves today.
While Pojar thinks Canadian aspen clones could easily rival Utah's Pando in size, he thinks that American clones are most likely older than those in Canada.
"American aspen clones that are south of the limit of glaciation would have occupied their sites for a longer period of time than those in Canada," he says.
While clones are genetically homogeneous, aspen as a species is genetically diverse, probably because aspen make the most of chances to mix their genes by reproducing sexually.
"There must be a lot of pent-up sexual energy in these clones. After the Yellowstone fires, there was a flurry of sex among the ashes. That burn was actually good for the Yellowstone ecosystem," says Pojar.
After fires, aspen roots can easily survive underground, ready to take advantage of the loads of nutrients released into the soil by the masses of burned vegetation.
"Plus the blackened soil warms up fast and then the flush of nutrients into the system seems to trigger more sex," says Pojar.
As anyone with pollen allergies can confirm, aspen produce huge loads of pollen. As they are dioecious, meaning that trees are either male or female, aspen rely on wind to carry pollen between the sexes.
"It's a long distance relationship," explains Pojar.
One study found that male and female aspen are distributed unevenly, with more males growing at higher altitudes in harsher conditions while females were more common in moist, more protected pockets at lower elevations.
Aspen pollen is produced in the furry looking catkins that hang on the trees at this time of year. Pojar suspects that the catkins appear before the trees leaf out as that makes it easier for the wind to disseminate the pollen.
"The catkins don't have petals or a smell, so they are not trying to attract something like an insect pollinator," he says.
Aspen can at times be confused with birch trees, which also have catkins and oval leaves, but the bark of the two species is different. While birch bark is often covered with lichens, aspen does not support such growth and the bark is usually bare.
Aspen bark also often has a greenish tinge, indicating the presence of chlorophyll. Unlike most trees, aspen can photosynthesize with its bark.
"At this time of year, before the leaves are out, that is probably really important for them. It is probably one reason that moose and beaver like to gnaw on them and not on birch bark," says Pojar.
- Aspen are one of the species tracked by the PlantWatch program, which was the subject of last week's yourYukon column. If you want to keep track of when aspen (and aspen clones) leaf out each spring, contact the Yukon's PlantWatch coordinator at plantwatch@ycs.yk.ca or (867) 668-5678. You can also check the PlantWatch website at www.plantwatch.ca.
