The earthen sections of the Great Wall of China were constructed by compacting natural materials with soil.
This is considered a weakness in the structure of the Great Wall.
However, a new study has found that this iconic area has developed a natural defense against the looming threat of degradation.
Enhancing Erosion Resistance
“This “living skin” consists of small, rootless plant species and microorganisms…
According to ecologist Matthew Bowker, a co-author of the study published in the journal Science Advances, the soil surfaces on the Great Wall are covered by a “living skin” made up of small, rootless plant species and microorganisms known as biological crusts.
These crusts are the enduring strength of the Great Wall. Bowker, an Associate Professor at Northern Arizona University, stated: “Biological crusts are common worldwide in dryland soils, but we usually don’t find them on human-made structures.”
Previous research has found that biological crusts of lichens and mosses pose a destructive threat to modern heritage stone structures due to the long-term effects of microbial communities on aesthetic value, acid production, and other metabolites, as well as changes in the microenvironment that can lead to erosion and weathering.
These findings have led to the removal of plant species growing on top of the Great Wall. However, according to the new study, the effects of biological crusts vary across terrestrial landmarks. Meanwhile, cyanobacterial and moss communities actually enhance the stability of the Great Wall while improving the area’s erosion resistance.
Researchers examined samples collected from over 300 miles (483 km) across eight earthen sections of the site built during the Ming Dynasty from 1368 to 1644. The team found that over two-thirds of the area was covered in biological crust.
The researchers compared the stability and durability of samples layered in biological crust with those lacking “the Earth’s living skin.” They discovered that samples with biological crust were three times more durable than those without.
“Many opinions suggest that this type of vegetation is destroying the Great Wall. However, our results show the opposite. Biological crusts are very common on the Great Wall. Meanwhile, their presence is highly beneficial for protecting this area,” stated Bo Xiao, co-author of the study and a Professor of Soil Science at China Agricultural University.
Biological crusts are very common on the Great Wall.
“Like a Blanket”
Composed of components such as cyanobacteria, algae, moss, fungi, and lichens, the biological crust lives on the surface layer of dryland soils. Covering approximately 12% of the planet’s surface, these communities of tiny plants and microorganisms can take decades or longer to develop. Forming miniature ecosystems, biological crusts stabilize soil, enhance water retention, and regulate nitrogen as well as carbon fixation processes.
They achieve this partly due to their dense biomass, acting as a “waterproof layer” for soil pores under suitable conditions, as well as naturally absorbing nutrients that promote the breakdown of salts.
According to the new study, the secretions and structural layers of the biological crust also interweave. This forms a “sticky network” that aggregates soil particles to enhance strength and stability against erosion threats to the Great Wall.
Researchers found that climatic conditions, the type of structure, and types of biological crust all play a role in protective functions. They significantly reduce erosion risk, “much more than” weather threats.
Compared to bare earth, the biological crust covering the cyanobacteria, moss, and lichens of the Great Wall exhibited reductions in porosity, water retention, erosion potential, and salinity by up to 48%. At the same time, this cover enhanced compaction, penetration resistance, and stability by up to 321%.
Among these, the moss crust is considered the most stable. “The biological crust covering the Great Wall acts like a blanket separating this area from air, water, and wind,” researcher Xiao noted.
He pointed out that by preventing water infiltration and salt accumulation, the biological crust counters chemical weathering. This creates substances that act like a “glue” to bind soil particles together against dispersion, strengthening the soil’s properties.
Most communities that form biological crusts begin from a single organism. This organism also adapts its environment to suit other organisms.
Emmanuel Salifu, an Assistant Professor at Arizona State University who studies nature-based solutions for sustainable engineering, stated that these organisms remain vulnerable to the impacts of climate change. However, they continuously adapt to changes in their environment.
Salifu, who was not involved in the new study, noted that this inherent adaptability makes biological crusts strong candidates for nature-based intervention measures.
This aims to address the issue of preserving structures in our warming world. Even as temperatures rise, they remain suitable for functioning under those conditions. They are likely to survive better if we design the development of these crusts on a large scale.
Salifu considers this new research as evidence of the potential benefits of cultivating biological crusts to preserve heritage sites on Earth, even though it remains a relatively new field.
The study demonstrates that natural communities of plants and microorganisms can enhance structural integrity, longevity, and durability of earthen structures like the Great Wall of China.
The authors of the study also indicated that their work lays the groundwork for exploring the potential of nurturing biological crusts to preserve other earthen heritage sites worldwide.
In addition to being a tourist attraction that draws millions of visitors each year, the Great Wall holds significant cultural meaning. This is why the preservation through biological crusts is so important.
According to Professor Xiao, the Great Wall is the cultural center of Chinese civilization. Therefore, making every effort to protect it for future generations is extremely necessary.
