Parts of China’s Great Wall Are Protected by a ‘Living Cover’ of Biocrusts

The layer of lichen, moss and cyanobacteria helps shield the historic structure from erosion, a new study finds

Biocrust on the Great Wall of China
The biocrust is made up of lichen, mosses and cyanobacteria. Bo Xiao

China’s Great Wall is an iconic cultural landmark that has stood for millennia. But the 13,000-mile-long structure is also at risk of disappearing because of natural erosion.

Now, scientists have discovered that some parts of the wall are better protected from the elements than others.

Moss, lichen and cyanobacteria have created a layer of “biocrusts” on some sections of the wall, according to a study published this month in the journal Science Advances. In addition to strengthening the wall, the biocrusts shield it from wind and water. The “living cover” also insulates the wall from temperature fluctuations, the researchers report.

Yellowish splotches on a wall
The researchers found biocrusts covering 67 percent of the area they sampled. Bo Xiao

The wall was constructed and connected over thousands of years, starting around 220 B.C.E., to help create a “united defense system” for China’s northern border, according to UNESCO, which has listed the Great Wall as a World Heritage site since 1987.

Some sections were built using rammed earth, a process that involves compacting natural materials like gravel and soil to create sturdy structures. Those materials also provided an ideal habitat for organisms to grow—and over time, they created a protective biocrust layer atop the rammed earth.

Scientists in China wanted to better understand the wall’s biocrusts, so they took samples from a roughly 300-mile-long section. When they analyzed the samples, they found that 67 percent of the area they studied was covered in biocrusts.

Man taking samples from the Great Wall of China
Researcher Yousong Cao helped gather samples from the Great Wall for testing. Bo Xiao

They also ran a series of experiments to compare the bare and biocrust-covered samples. The biocrust-covered sections were found to be less porous, and they held up better in tests measuring their strength and stability. Some biocrust samples were three times stronger than the bare rammed earth samples.

These findings make sense, given what scientists know about how cyanobacteria and other organisms operate. They secrete substances that act like natural binders for the rammed earth particles, which function like cement. This helps form a “cohesive network with strong mechanical strength and stability against external erosion,” says study co-author Bo Xiao, a soil scientist at China Agricultural University in Beijing, to Live Science’s Jennifer Nalewicki.

The new research could help inform conservation strategies for the Great Wall and other heritage structures created from rammed earth. Some conservationists have speculated that biocrusts may cause historic structures to deteriorate more quickly, but this study suggests otherwise.

Instead, biocrusts helpfully combine the “protective functions of several conventional measures into one nature-based, cost-effective, eco-friendly and long-lasting strategy,” Xiao tells New Scientist’s James Woodford.

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