World’s largest living organism
Is the largest living organism on Earth a tree?
It might be!
In the Fishlake National Park, Utah, USA, there is a forest of approximately 47,000 quaking aspen (Populus tremuloides). These trees are not, however, separate organisms; they are the ramets of a single organism, clones growing from a vast root system that spreads across 43 hectares and has a biomass of perhaps 5897 tonnes. Scientists have nicknamed it Pando, meaning ‘I spread’.
Originally grown from a single seed and aged perhaps up to 14,000 years old (although there are other less reliable claims of up to 80,000 years), Pando is a rhizome that reproduces asexually – vegetatively – by root sucker. Its individual clonal trees live, age and die, but the organism and its single genetic code continues. There are many clonal aspen stands across North America, and the species’ capacity to reproduce asexually as well as sexually – by seed – is shared with many other organisms, so the ability to replicate clonally is a fascinating but common attribute. It is Pando’s vast mass that is distinctive.
The quaking aspen forest at Fishlake was first identified as a giant clone system in the 1960s and claimed as the ‘world’s largest organism’ in the 1970s. A few years later, after millennia of robust expansion, signs of its decline became apparent. Although older trees in the forest remain in good conditions, self-replacement is declining. In consequence, for the first time in its long life the organism as a whole is starting to age. Although clonal aspens can, in principle, persist forever, the Fishlake rhizome could die.
Human intrusion into the forest is having significant direct and indirect negative impacts. Private homes and a campsite have been built among the trees and Interstate 25, constructed in 1918 in ignorance of the ancient rhizome beneath, runs through the middle of the forest. Fire is a key part of the clone’s regeneration strategy, clearing away competitor undergrowth and promoting new sucker growth in burned areas, but fire suppression policies are inhibiting this process. There is also increased herbivore grazing of new growth, and a recent succession of severe droughts is depleting the forest’s energy reserves.
Efforts are being made to protect new growth in the aspen forest by excluding browsing herbivores, and to stimulate regeneration with clearing, cutting and burning. Researchers are conducting comprehensive investigations of the whole clone to determine the effectiveness of different management regimes (Rogers and McAvoy, 2018). This work may prevent further degeneration and encourage sufficient growth to maintain persistence.
So is Pando the largest living organism? There are other huge organisms on Earth – blue whales, giant sequoias, a fungus that extends across 620 hectares, and perhaps other organisms yet to be discovered – but Pando’s supporters use biomass as the significant criterion, and claim the title for their tree. Mass, as a competitive qualification for recognition and preservation, is an effective engagement of public attention, but is size everything? Like every other tree, Pando should qualify for protection not because it is a huge tree, but because it is a tree.
Michael C Grant, ‘The Trembling Giant’, Discover, 14:10 (1993) 82-90. https://www.discovermagazine.com/planet-earth/the-trembling-giant
Jordan Kisner, ‘No Wonder It Quakes’, The American Scholar; (2015): 16-17.
Paul C. Rogers 1 and Jan Šebesta, ‘Past Management Spurs Differential Plant Communities within a Giant Single-Clone Aspen Forest’, Forests 10:12 (2019), 1118
Paul C. Rogers, Darren J. McAvoy, ‘Mule deer impede Pando’s recovery: Implications for aspen resilience from a single-genotype forest’, PLos ONE 13:10 (OCT 17 2018), 1-19.