The Great Wall of China Is Coated in a Living Substance That Shields It From Harm : ScienceAlert

For thousands of years, long stretches of earthworks and stone fortifications known collectively as the Great Wall have stood as testament to the ingenuity and authority of China’s ruling dynasties.

The structure’s astonishing state of preservation is no accident, with conservation and restoration efforts battling to ensure researchers and tourists alike can continue to appreciate the archaeological wonder for generations to come.

A recent study conducted by researchers from China, the US, and Spain could help resolve a debate in the conservationist community over the risks and benefits posed by living material growing into such precious pieces of our history.

Known as a biocrust, lichen, bacteria, fungi, moss, and other small plants can be found growing on just about any mineral surface exposed to the elements, forming thin layers anywhere from a few millimeters to several centimeters deep.

Some fear the physical and chemical processes involved in the growth of such organisms act as a form of weathering, compromising the integrity of underlying structures. As such, they ought to be removed to prolong the integrity of monuments like the Great Wall.

On the other hand, biocrusts play a vital role in protecting soil from being worn away by wind and rain, acting as both a shield and a scaffold for the regolith below. If they protect natural surfaces from being worn away, they may serve as a kind of living armor for unnatural structures.

To gain insights into the specific conditions under which biocrusts might help protect archeology, the small team of researchers conducted an extensive survey on roughly 600 kilometers (just under 400 miles) of the Great Wall, focusing on segments in drier climates.

What is generally referred to as a single wall is in fact a series of fortifications, towers, and battlements built by a succession of rulers from around the 7th century BCE to define their state’s northern-most border.

The more famous sections meander along mountainous ridges like stone serpents, yet many of the older pieces of wall were constructed from clay-rich soil tampered between wooden frames until it was virtually rock hard. In more arid areas, such as the Gobi desert, layers of sand, pebble, and branches of a small shrub called tamarisk were alternated to form a sturdy earthwork.

Time hasn’t been kind to many of these more ancient segments of wall, with wind and occasional rains scouring much of it to its foundations. Of the remaining rammed earth structures studied by the research team, more than two thirds was covered in a biocrust of cyanobacteria and moss, with occasional species of lichen colonizing the wall here and there.

lichen on the great wall of china
Close-up view of biocrusts on the Great Wall. (Bo Xiao)

Samples of the wall beneath the communities of plants and microbes were less porous and didn’t crumble as easily as material collected from walls lacking a biocrust, the team found.

While it’s possible the roots and filaments of the mosses and lichens were contributing a small amount of damage, evidence suggested its ability to bind the rammed earth’s particles outweighed any risk they posed to the overall structure.

“Thus, the protective function of biocrusts produced by their reduction of erodibility is much greater than the potential biodeterioration caused by their biological weathering, making the former a more noteworthy and important aspect in the protection of earthen heritages,” the researchers suggest in their report.

Venturing out to look at a wall covered in a rash of lichen and moss might not give tourists their expected view of Chinese history. And even the researchers concede, there is likely to be a balance of protective vegetation and aesthetic, one that’s much harder to draw a line on using science.

Still, with changes in global climate threatening to rob us of thousands of years of history, future generations might be happy to peer through a patina of green if it means holding onto one of the world’s great architectural treasures a little longer.

This research was published in Science Advances.