A ‘living skin’ is protecting the Great Wall of China, scientists say

By Ayurella Horn-Muller, CNN

(CNN) — Rammed earth portions of the Great Wall of China — built by compressing natural materials with soils — have been regarded as a weak point in its structure. But these swaths of the iconic landmark developed a natural line of defense against the looming risk of deterioration, a new study has found.

These soil surfaces on the Great Wall are covered by a “living skin” of tiny, rootless plants and microorganisms known as biocrusts that are a source of the heritage site’s staying power, according to soil ecologist Matthew Bowker, a coauthor of the study published December 8 in the journal Science Advances.

“(Biocrusts) are common throughout the world on soils of dry regions, but we don’t typically look for them on human-built structures,” said Bowker, an associate professor at Northern Arizona University, in an email.

Past studies have found lichen and moss biocrusts to be a destructive threat to modern heritage stone structures due to the microbial communities’ long-term impacts on aesthetic value, production of acid and other metabolites, and alteration of microenvironments, which may cause erosion and rock weathering. Those findings have led to the removal of plants growing on the top of parts of the Great Wall. But the effects of biocrusts look different for earthen landmarks, and communities of cyanobacteria and moss actually increase the Great Wall’s stability and improve its resistance to erosion, according to the new paper.

Examining samples taken from over 300 miles (483 kilometers) across eight rammed earth sections of the site built during the Ming Dynasty between 1368 and 1644, the study authors found that more than two-thirds of the area is covered in biocrusts. When the researchers compared the stability and strength of samples layered in biocrust with samples sans “Earth’s living skin,” they discovered that samples with biocrusts were as much as three times stronger than those without.

“They thought this kind of vegetation was destroying the Great Wall. Our results show the contrary,” said study coauthor Bo Xiao, a professor of soil science at China Agricultural University. “Biocrusts are very widespread on the Great Wall and their existence is very beneficial to the protection of it.”

‘Like a blanket’

Made up of components such as cyanobacteria, algae, moss, fungi and lichen, biocrusts dwell on the topsoil of drylands. Covering an estimated 12% of the planet’s surface, the communities of tiny plants and microorganisms can take decades, or longer, to develop. Forming miniature ecosystems, biocrusts stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.

They are able to do so partly thanks to a dense biomass, which acts as an “anti-infiltration layer” for soil pores under the right conditions, as well as a natural absorption of nutrients that promote salt damage. The secretions and structural layers of biocrusts also intertwine to form a “sticky network” of aggregating soil particles that promote strength and stability against corrosive forces threatening the Great Wall, according to the new study.

Climatic conditions, the type of structure and type of biocrust all play a role in a biocrust’s protective function, with its reduction of erodibility “much greater” than its risk of weathering, the researchers found.

Compared with bare rammed earth, the cyanobacteria, moss and lichen biocrust-covered sections of the Great Wall exhibited reduced porosity, water-holding capacity, erodibility and salinity by up to 48%, while increasing compressive strength, penetration resistance, shear strength and aggregate stability by up to 321%. Of the bunch, the moss biocrusts were found to be the most stable.

“(Biocrusts) cover the Great Wall like a blanket that separates the Great Wall from air, from water, from wind,” Xiao said.

Working to keep water out and prevent salt buildup, the biocrusts resist chemical weathering, he noted, producing substances that act as a “glue” for soil particles to bind together against dispersion, making soil properties stronger.

Biocrusts’ role in an uncertain future

Most of the communities that make up a biocrust start from a single organism that grows and makes the environments it grows within suitable for others. Although they are still vulnerable to the impacts of climate change, these constantly evolving organisms are expected to deploy internal mechanisms to adapt to future extremes, said Emmanuel Salifu, an assistant professor at Arizona State University who studies nature-based solutions for sustainable engineering.

That inherent adaptability makes biocrusts great contenders for nature-based interventions to address structural conservation in our warming world, said Salifu, who was not involved with the new study.

“Even if we have warmer temperatures, they are already suited to performing in those conditions,” he said. “We hypothesize that they will be better able to survive if we engineer their growth at scale.”

Wind erosion, rainfall scouring, salinization and freeze-thaw cycles have led to cracking and disintegration across the thousands of miles of structures that link together the Great Wall, which is at risk of severe deterioration and vulnerable to collapse. Rising temperatures and increasing rainfall could also result in a reduction of the wall’s biocrust cover.

Still, the wider construction industry remains divided over the historic conservation potential of biocrusts, according to Salifu.

“The conventional idea is that biological growth is not great for structures. It affects the aesthetics, it leads to degradation, affects the overall structural integrity,” he said. However, there is a lack of concrete research that supports those conclusions, Salifu added, noting that “the jury is still out on that.”

Salifu sees the new study as evidence of the potential advantages to engineering biocrusts for the conservation of earthen heritage sites — though that is still an emerging field. The research establishes that the natural communities of plants and microorganisms “have the capacity to improve the structural integrity, longevity and durability of earthen structures like the Great Wall of China,” Salifu said.

The paper “goes a long way in further pushing the hand on the clock in bringing the industry closer to where we might be able to start thinking about (engineering biocrusts),” he noted.

The study’s authors also say their work makes a case for exploring the possibility of cultivating biocrusts for preservation of other rammed earth heritage sites worldwide.

Beyond its status as a tourist destination that draws millions of visitors each year, the Great Wall has great cultural relevance, which is why the biocrusts preserving it are so significant, Xiao said.

“The Great Wall is the cultural center of Chinese civilization,” he told CNN. “We should do our best to protect it for our next generations. For our children, for our grandchildren.”

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