May 26, 2024

Pando: The World’s Largest Organism

Pando is the world’s largest organism: a clonal colony of quaking aspen trees, that has been growing in Utah for the last 14 000 years.

Extent of ice during last 'Glacial Maximum'
Extent of ice during last ‘Glacial Maximum’

The entire history of human civilisation has occurred since the end of the ‘Last Glacial Maximum’.

Sometimes called ‘the last ice age’ (most scientists dislike this term), this period of global cold saw much lower average temperatures than today; around 9 degrees centigrade, compared to 15 currently. For 35 000 years, enormous glaciers covered much of the earth’s surface.

In the northern hemisphere, the present-day United Kingdom, Scandinavia, Canada and the northern United States, were covered by ice sheets.

Homo sapiens were already present during this period, but the spread of our species was much reduced. Large parts of the earth were simply too cold to be habitable.

North America during the last Glacial Maximum
North America during the last Glacial Maximum

Around 20 000 years ago, temperatures began to rise.

Exactly why, is uncertain. Analysis of ice cores from Antarctica, and soil samples from the sea floor, show rising levels of carbon dioxide, which could have supplied the required warming. But what may have caused this CO2 increase, is not known.

Other theories for the big thaw include an increase in the sun’s temperature, possibly caused by fluctuations in Earth’s orbit, or that the giant glaciers covering half the planet simply became unstable due to their size. Their collapse could have triggered a chain reaction, including changes to ocean currents, that lead to higher temperatures.

Whatever the mechanism, as temperatures climbed the glaciers melted, and the planet was reshaped. The sea rose an estimated 10 metres, drowning large swathes of land and creating a new coastline, replaced by new regions that the retreating ice uncovered.

By about 14 000 years ago, temperatures stabilised again, now at a higher level. The modern era, known as the ‘Holocene’, had begun.

Quaking Aspen
Quaking Aspen

The ‘Quaking Aspen’ is a deciduous tree, native to North America. The tree grows tall, up to 25 metres, and thin, and its trunk has pale bark covered in dark spots. It is one of the commonest tree species on the continent.

Its unusual name is derived from its leaves.

These are lighter than those of most trees, and shake, even in the lightest breeze. There is a folk story that goes along with this: the traditional explanation is that Jesus Christ was crucified on a cross fashioned from an Aspen, and the trees still quake to register their sorrow.

While this feature is unusual, it likely gives the tree an evolutionary advantage:

‘Their leaves hang from flexible stems and flutter in the breeze, exposing first their upper and then their lower surfaces to the sun.

This means that both sides of the leaf can photosynthesise. This is in contrast to other species, where the underside is reserved for breathing.

Thus, quaking aspens can generate more energy, and grow faster.’

– Peter Wohlleben, ‘The Hidden Life of Trees’

As well as their appearance, quaking aspen also have another unusual feature.

The trees can be male or female and grow from seeds.

As adults the trees establish a large root structure, that becomes the centre of the organism. This then pushes up new shoots, that develop into new Aspens. The trees grow, mature, die, and are replaced, while the root system continues. Each new tree is a clone of the original.

And in the right conditions, some of these Aspen clone clusters, can grow to enormous sizes.

Location of Fishlake National Forest
Location of Fishlake National Forest

In 1976 two scientific researchers, Jerry Kemperman and Burton Barnes, were travelling through the western United States looking for large Aspen clones for a study.

In the Fishlake National Forest in Utah, a large old growth forest near Richfield, they found what they were looking for.

Their techniques were rudimentary. On foot, exploring the forest, Kemperman and Barnes collected stem, leaf and bark samples, that they then compared, looking for like characteristics.

In a paper published later that year, they used this approach to identify two huge Aspen clone groups, one measuring an estimated 24 acres, the other an astonishing 106 acres.

But their research was not confirmed for nearly 20 years. In 1992, Michael Grant, Jeffrey Mitton, and Yan Linhart of the University of Colorado returned to the location, and were able to conduct genetic testing of the Aspens described by Kemperman and Barnes.

These tests provided definitive proof.

Aerial view of Pando
Aerial view of Pando

In Fishlake National Forest is the world’s largest, and heaviest, living organism: a giant quaking aspen clone group, so large that a national highway passes through the middle of it, with trees growing on either side.

Michael Grant subsequently wrote an article about the find in ‘Discover’ magazine, and it was here that he gave the tree its name: Pando, which in Latin means, ‘I spread.’

Individual Pando Aspens, each genetically identical
Individual Pando Aspens, each genetically identical

Pando is located about 1 mile southwest of Fish Lake.

It’s 100 acre size comprises about 47 000 individual Aspen trees, connected to one root system so large that if unwound, it would stretch 20 000 km: approximately half way round the world. The entire organism is estimated to weight approximately 6 million tonnes.

Each Aspen tree – scientists sometimes refer to them as ‘stems’ – is genetically identical to every other, although they take on their own appearance as they age. Each lives between 100 and 150 years, and is replaced by a new shoot emerging from the root; a process known as ‘suckering’.

The overall age of the tree has been disputed.

Aerial view of Pando
Aerial view of Pando

Dating a clonal colony like Pando is not easy.

Conventional methods used to date trees, like counting their rings, will only provide the age for the individual Aspen stem, not the entire organism. This has lead to a wide range of ages being claimed.

The National Parks Service, which overseas the forest Pando lives in, has stated an age estimate of 80 000 years. But most scientists think this unlikely.

The cold temperatures during the previous Glacial Maximum meant that the area in Utah where Pando grows, known as the Colorado Plateau, was likely covered in ice. It would have been a frozen wasteland, unsuitable for any large organism.

Glaciers have appeared, and melted, several more times during the Holocene period, leading some scientists to assign an age to Pando of a few thousand years. This has been questioned as well.

The size of the clonal colony, much larger than any other identified, means a significant amount of time would be required for Pando to grow to its current size. While debate, and investigation, continues, the consensus opinion puts Pando’s age at around 14 000 years.

Quaking Aspen seeds
Quaking Aspen seeds

It is hard to imagine, but this entire, vast, living thing would have grown from one single seed, the size of a small nut.

14 000 years ago, the retreating glaciers left a broad, fertile expanse in what would become Utah. The landscape was likely dramatic; thousands of years of glaciation would have carved up the terrain, which also featured active volcanoes, and regular earthquakes.

Fish Lake sits in a basin between two fault lines. The land is well watered, and the volcanic activity would have left it mineral rich; perfect soil for trees. Into this setting an aspen seed, probably carried by the wind, settled and began to grow.

Marks pressed into clay: the oldest known writing is 9 000 years younger than Pando
Marks pressed into clay: the oldest known writing is 9 000 years younger than Pando

14 000 years is before all of recorded human history. Writing is a relatively recent invention, dating only to about 3 500 BCE.

It is before the Roman Empire, the Mongols, the Egyptian Pharaohs, before Mesopotamia. The oldest known city, Jericho in the West Bank, is about 11 000 years old. Humans had barely even set foot in the Americas: they probably arrived around the same time, either by boat or via the land bridge into Alaska, as Pando first sprouted.

The first human beings to reach Fish Lake, the ‘Fremont’ people, did not arrive until 8 000 years later, around 2000 BCE.

Deer investigate Pando
Deer investigate Pando

While Pando has thrived in its location, and now enjoys National Forest protection, it is not without threat.

The young Aspen shoots are vulnerable to wild deer and elk, that eats the plants when they are young. Cattle is also grazed in the area, although has less impact. Global heating and climate change are also having an influence, gradually changing this fragile ecosystem, as happened at the end of the last Glacial Maximum.

Studies conducted in 2018 and 2019 indicated that Pando has declined in size, and the rate of new shoot growth was not sufficient to replace the older Aspen trees as they died. Monitoring is ongoing.

The Pando stamp
The Pando stamp

Scientists are not sure exactly how long a clonal Aspen colony could live, given stable conditions. Pando is the oldest that has been discovered, but even longer lifespans have been proposed.

‘Quaking aspen is an intriguingly multifaceted species, and may be the most genetically diverse plant studied to date. Individuals may reach ages in excess of 1 million years.’

– Michael Grant and Jeffrey Mitton, ‘Bio Science’ magazine

In 2006, Pando was honoured as one of the ’40 Wonders of America’, and appeared on its own commemorative stamp.


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