How big is the "world's largest tree"? The answer is totally beyond your expectation!

As the saying goes, "a drop of water cannot form an ocean, and a single tree cannot form a forest." A drop of water cannot form an ocean, but a single tree can form a forest. In plants, the characteristic of clonal growth allows plants to produce different numbers of ramets through asexual reproduction . Over time, the ramets increase in number, and a single tree becomes a forest.

I'm sure you're all familiar with the following passage:

"Our boat gradually approached the banyan tree. I had the opportunity to see its true appearance: it was a big tree with countless branches, and roots grew on the branches. Many roots hung down to the ground and into the soil. Some branches hung down to the water surface. From a distance, it looked like a big tree lying on the water."

In 1933, Ba Jin went south to Guangdong and saw a large banyan tree forest on the sandbar in the middle of Tianma River in Xinhui County, Jiangmen City. He was inspired and wrote "Bird's Paradise". According to local rumors, the banyan tree forest in Tianma River originated from a water banyan tree planted in the Ming Dynasty. It took more than 300 years to form the current wonder of more than 10 acres.

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In this popular essay, Ba Jin initially believed that the banyan forest was made up of many trees, but was immediately corrected by his friends. Some said the forest was made up of one tree, while others said it was made up of two trees. In fact, to this day, no scientist has conducted an experiment to tell us how many trees there are in the bird paradise.

However, in Utah, USA, scientists actually discovered a tree that has grown into a forest covering an area of ​​43 hectares and has grown itself into a legend .

01

This forest is really just one tree

The tree is called Pando , and its scientific name is Populus tremuloides, which mainly grows in forests in northern temperate regions. Pando consists of more than 40,000 seemingly independent "trees", but in fact they share a root system and are just different clones of the same poplar tree .

Pando, a single tree that forms a forest. Image source: earthdate.org

Pando was first discovered in 1976 when scientists used aerial photographs combined with leaf, bark and stem features to determine that a grove of poplar trees in the Fish Lake Basin in Sevier County, Utah, was likely a clone of a single poplar tree.

You might think that it is a bit hasty to determine that these poplars are clones of each other based only on morphological characteristics such as leaves. Scientists have certainly thought of this. More than 20 years after Pando was first discovered, scientists have confirmed through genetic data that this forest is really just one tree!

The "Pando" forest (green area) in the Fish Lake Basin. Image credit: friendsofpando.org

Specifically, two studies have examined the genetics of Pando:

One group of scientists collected poplar leaves from more than 800 sampling points in Swan Beach and Fish Lake in Utah, USA, including the famous Pando. They extracted the DNA of the samples and used 9 microsatellite loci to determine the relationship between different samples (microsatellite is a neutral molecular marker with a fast mutation rate and is often used in population genetics analysis).

The results were as expected by the scientists. Among the 256 ramet samples collected from Pandolin, only 6 genetically different variants were detected, and the genetic relationship between these variants and other individuals was very close.

Pando under the night sky. Image source: friendsofpando.org

In a separate study, scientists collected 209 rhizome samples and analyzed them using seven microsatellite loci, identifying a genetic entity that corresponded to the previous morphological identification and delimiting the range of Pando to 43.6 hectares .

Seeing this, don’t you want to sigh and say, this tree is really big!

02

Growing in the land of ice and fire

In addition to shocking people, Pan Duo's discovery also provided scientists with a great opportunity to study asexual reproduction of plants. What exactly caused these poplar trees to switch from sexual reproduction to asexual reproduction has become a question of concern to everyone.

Although scientists have not yet really figured out why Pando gave up sexual reproduction and chose to continue its life through asexual reproduction, they believe that the turbulent environment may have played a key role in the emergence of Pandolin .

The area where Pando grows is not as peaceful as the pictures above show. This land has experienced volcanic eruptions that blocked out the sun, and the Ice Age when no vegetation survived. Various catastrophic events in the Fish Lake Basin, such as earthquakes, landslides, volcanoes, and floods, have posed considerable challenges to the survival of plants.

Geological map of the Fish Lake Basin. Image source: friendsofpando.org

It is speculated that on a warm spring day, thousands of years after the end of the last Ice Age, an aspen seed trembled in the air and finally landed on the southeast edge of the Fish Lake Basin.

This land is scattered with huge volcanic rocks and covered with traces of glacial etching. It may be difficult for many people to move forward, but for seeds, it is a rare and perfect home: there are no other plants competing for sunlight, and the erosion of glaciers makes the land more conducive to the extension of roots .

Pando's root system. Image credit: friendsofpando.org

As a male poplar, Pando can only produce pollen but not seeds. If nothing unexpected happens, it may grow smoothly into a big tree on this land, and that's all.

However, during Pando's growth, an unknown force disrupted its growth. Pando began to continuously produce new branches through rhizomes and march across the vast land, and a legendary story unfolded.

03

Will Pando's story end?

Zhuangzi once wrote in "Xiao Yao You": "In ancient times there was a tree called Da Chun, which took eight thousand years as spring and eight thousand years as autumn." For Pan Duo, Spring and Autumn are just the blink of an eye, and it seems that Pan Duo's story will never end.

But the greatest challenge to Pando's survival came with the increase of human activity. The range of Pando began to shrink as cattle from ranchers and wild herbivores (such as deer and elk) ate the young stems .

In response to the predicament facing Pando, scientists have conducted research. Through analysis of aerial photographs taken at different times, they found that the area of ​​Pando is indeed shrinking, and mottled windows have appeared in the originally lush forests.

Range changes in Pando. Image credit: References

In order to better protect Pando, the forestry department built fences to prevent animals such as elk from entering and eating the tender stems of Pando. This move has indeed alleviated the crisis faced by Pando.

Fawns wander outside Pandolin's enclosure. Image credit: friendsofpando.org

People return to the earth after a hundred years, and plants and trees reincarnate year after year. We don’t know how long Pando can exist, but we hope that the story written by this miracle of life will not end hastily due to human influence.

References:

[1].DeWoody J, Rowe CA, Hipkins VD, Mock KE. “Pando” lives: molecular genetic evidence of a giant aspen clone in central Utah. Western North American Naturalist. 2008 Dec;68(4):493-7.

[2].Mock KE, Rowe CA, Hooten MB, Dewoody J, Hipkins VD. Clonal dynamics in western North American aspen (Populus tremuloides). Molecular Ecology. 2008 Nov;17(22):4827-44.

[3].Rogers PC, McAvoy DJ. Mule deer impede Pando's recovery: Implications for aspen resilience from a single-genotype forest. PloS one. 2018 Oct 17;13(10):e0203619.

Produced by | Science Popularization China

Author: EVEE (School of Life Sciences, Peking University)

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

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