Scientists Discover a Living Skin Safeguarding the Great Wall of China
The erosion-threatened Great Wall of China finds protection in an extraordinary living skin of organisms known as biocrust A crucial shield for its uncertain future
Register for CNN's Wonder Theory science newsletter to delve into the latest news on captivating discoveries, scientific progress, and more. A new study has uncovered that the rammed earth sections of China's Great Wall, constructed by compacting natural materials with soil, have been traditionally viewed as a structural vulnerability. However, these segments have actually evolved a natural defense mechanism against the threat of deterioration.
The Great Wall's soil surfaces are covered by a "living skin" of minuscule, rootless plants and microorganisms, referred to as biocrusts, which contribute to the long-lasting nature of the heritage sites, according to soil ecologist Matthew Bowker, who coauthored the study published December 8 in the journal Science Advances.
"Biocrusts are widespread on dry region soils globally, but we generally do not expect to find them on man-made structures," stated Bowker, an associate professor at Northern Arizona University, in an email.
Past research has shown that lichen and moss biocrusts pose a significant threat to contemporary heritage stone structures due to the lasting impact of microbial communities on aesthetic appeal, acid and metabolite production, and microenvironment alteration, leading to erosion and rock weathering. As a result, plants growing on certain parts of the Great Wall have been removed. However, the impact of biocrusts differs for earthen landmarks, and the communities of cyanobacteria and moss actually enhance the stability of the Great Wall and improve its resistance to erosion, as stated in the recent publication.
"Biocrusts are very widespread on the Great Wall," study coauthor Bo Xiao says, "and their existence is very beneficial to the protection of it."
Courtesy Bo Xiao
Samples taken from eight sections of the Ming Dynasty site, spanning over 300 miles, were examined by the study authors. They found that more than two-thirds of the area is covered in biocrusts. Comparing the stability and strength of samples with and without biocrust, it was discovered that those with biocrusts were up to three times stronger. Coauthor of the study, Bo Xiao, emphasized the widespread presence of biocrusts on the Great Wall and highlighted their beneficial role in its protection.
Resembling a protective blanket, biocrusts are composed of cyanobacteria, algae, moss, fungi, and lichen, and can be found on the topsoil of drylands. Occupying around 12% of the Earth's surface, these intricate communities of small plants and microorganisms can take many years to form. Serving as miniature ecosystems, biocrusts play a crucial role in stabilizing soil, enhancing water retention, and controlling nitrogen and carbon fixation.
They can achieve this, in part, thanks to a dense biomass that serves as an "anti-infiltration layer" for soil pores under the right conditions, as well as the natural absorption of nutrients that help prevent salt damage. The secretions and structural layers of biocrusts also combine to create a "sticky network" of aggregating soil particles that enhance strength and stability against corrosive forces threatening the Great Wall, as stated in the new study.
Forming miniature ecosystems, biocrusts stabilize the soil, increase water retention, and regulate nitrogen and carbon fixation.
Courtesy Bo Xiao
The protective function of biocrusts is influenced by climatic conditions, the type of structure, and the type of biocrust. Researchers found that its reduction of erodibility is "much greater" than its risk of weathering.
"Two individuals in China have been arrested for using an excavator to cause damage to a portion of the Great Wall in Shanxi Province, as reported by state broadcaster CCTV."
The authorities in Youyu County received a report on August 24 from CCTV stating that a gap had been made in the wall in Yangqianhe Township.
Upon investigation, the police discovered that a 38-year-old man and a 55-year-old woman had used an excavator to breach the wall, creating a shortcut. This act caused "irreversible" damage to the integrity and safety of that section of the wall, as reported by the broadcaster.
Rewritten
The 32nd Great Wall, a well-preserved section with watch towers dating back to the Ming dynasty (1368-1644), is designated as a provincial cultural relic site.
The police have stated that the case is still under investigation.
Youyu Public Security Bureau
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The Great Wall of China has been damaged in an attempt to create a shortcut. The sections of the Wall that were covered in cyanobacteria, moss, and lichen biocrust showed significant improvements in various properties such as reduced porosity, water-holding capacity, erodibility, and salinity by up to 48%, while also increasing compressive strength, penetration resistance, shear strength, and aggregate stability by up to 321%. Among these biocrusts, moss was the most stable.
Xiao stated that the biocrusts act as a blanket covering the Great Wall, creating a barrier between the structure and the air, water, and wind. Additionally, the biocrusts work to repel water and prevent salt accumulation, resisting chemical weathering. These biocrusts also produce substances that act as a "glue" for soil particles, strengthening the soil's properties by binding them together and preventing dispersion.
The crucial role of biocrusts in an unpredictable future
Biocrust communities typically originate from a single organism that flourishes and creates a conducive environment for other organisms to thrive. Despite being susceptible to climate change, these adaptive organisms are projected to develop internal mechanisms to withstand future environmental extremes, according to Emmanuel Salifu, an assistant professor at Arizona State University specializing in nature-based solutions for sustainable engineering.
A group of Chinese tourists, all wearing protective face masks, are seen visiting the Badaling Great Wall in Beijing, China. The popular landmark had been closed since January 25, 2020, due to the Covid-19 outbreak, but has now reopened with limited access to tourists who meet specific health criteria. (Photo by Lintao Zhang/Getty Images)
Lintao Zhang/Getty Images AsiaPac/Getty Images
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Salifu, who was not associated with the new study, expressed that the natural flexibility of biocrusts makes them excellent candidates for nature-based solutions to address environmental conservation in our increasingly warmer world.
"Despite the warmer temperatures, they are well-adapted to thrive in such conditions," he stated. "We believe that by controlling their growth on a larger scale, they will have a better chance of survival."
The Great Wall, spanning thousands of miles, has suffered from cracking and disintegration due to wind erosion, rainfall, salinization, and freeze-thaw cycles. This has put the wall at risk of serious decay and collapse. Additionally, higher temperatures and more frequent rainfall could lead to a decrease in the wall's biocrust cover.
The studys authors contend their work makes a case for exploring the possibility of cultivating biocrusts to help preserve other rammed earth heritage sites.
Courtesy Bo Xiao
Still, the wider construction industry remains divided over the historic conservation potential of biocrusts, according to Salifu.
Traditional thinking suggests that biological growth is detrimental to structures, impacting aesthetics, leading to degradation, and affecting overall structural integrity," he explained. However, Salifu noted a lack of substantial research supporting these conclusions, stating that "the jury is still out on that."
Salifu interprets the new study as proof of the potential benefits of engineering biocrusts for conserving earthen heritage sites, even though it is still an emerging field. The research demonstrates that natural plant and microorganism communities "have the ability to enhance the structural integrity, longevity, and durability of earthen structures such as the Great Wall of China," according to Salifu.
Excavation of the Xiongnu Elite Tomb 64 containing a high status aristocratic woman at the site of Takhiltiin Khotgor, Mongolian Altai.
J. Bayarsaikhan
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The paper significantly advances the progress towards closer consideration of engineering biocrusts. The authors of the study also argue that their research provides justification for investigating the potential cultivation of biocrusts to preserve other rammed earth heritage sites globally.
Xiao emphasized the Great Wall's cultural significance beyond its appeal to tourists, highlighting the importance of preserving the biocrusts that protect it. "The Great Wall is the cultural center of Chinese civilization," he told CNN, stressing the need to protect it for future generations.
Ayurella Horn-Muller has covered climate change for Axios. Her debut book, "Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South," is due out in the spring.Â
