The World’s Largest Organism

The question of what makes living things unique from each other is a subtle one.  Is a bee hive one colony organism, or is it a thriving city with fifty thousand hard-working, like-minded souls?  Is a siphonophore an individual or a group?  What about a blood cell or a sponge cell (which can form a whole new sponge)?

The Quaking Aspen (Populus tremuloides)

As another example of this question (and in keeping with this site’s recent tree theme) consider the quaking aspen, Populus tremuloides.  This lovely slender tree is native to cooler regions of North America south of the permafrost.  The aspen has glossy bright green leaves and pale smooth bark with occasional black score marks and scars.  Along with the poplar, cottonwood, and other aspens, P. tremuloides is a member of the willow family (Salicaceae).  It is a plant frequently used by landscapers for its winsome beauty.

So what is remarkable about this tree (aside from its hardiness and prettiness)?   Quaking aspens colonies are among the most successful clonal colonies on Earth.  Indeed, the largest living organism currently known is a quaking aspen colony in Utah known as “Pando” (which means “I spread” in Latin).  Spread over more than 100 acres, Pando masses more than 6000 tons and consists of over 47,000 individual trunks (these clone trees are properly called ramets).  Such a colony should be thought of as an immense root network as much as it is thought of as constituent trees.  It spreads and propagates by underground suckering.  Even if forest fire destroys all of the ramets,  the colony can swiftly regenerate from its hidden, protected roots.  This ability to survive forest fires and other cataclysms is one of the reasons quaking aspens out-compete hardier (but slower growing) conifers.

Aspen Colony

As Pando’s ramets wear out and die they are replaced, but the colony itself stays alive.  Most botanists estimate Pando to be 80,000 years old, but it could plausibly be a million years old or older (it could also be younger or could have split into multiple colonies—there are uncertainties dealing with something so huge, strange, and ancient).  I suppose I should say “he” rather than “it” since Pando is a male (although the poor fellow probably hasn’t flowered in 10,000 years).  Many aspens no longer reproduce sexually—possibly because the environment has changed since the ice ages but also because they are—or were—so successful at spreading asexually.  Across the west and in parts of Canada, clonal colonies of Aspen are now beginning to die out.  Scientists are unsure whether this is because of fire restriction policies of the twentieth century (which were slowly giving an edge to pine trees), further climate change, or because of some wholly unknown factor.  Whatever the case, people love quaking aspens, and are planting them everywhere.  There was one in front of the stucco row-house in the suburbs where I lived during junior high.  Perhaps some of these landscaped specimens will grow into immense clonal colonies themselves over the millennia.

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