Biocrusts protect the Great Wall of China from erosion

A study has found that a thin layer of biological growth, known as biocrusts, covers 67.1 percent of eight Ming-era sections of the Great Wall of China built with rammed earth. These crusts, made of cyanobacteria, mosses, and lichens, appear to protect the structure rather than weaken it, contradicting previous assumptions.

Rammed earth construction and its vulnerability

The Great Wall is not a uniform stone structure. In many parts, particularly in dry regions, it was built using rammed earth, a technique involving layer by layer compaction of soil, sometimes mixed with rubble and vegetation. These sections, built during the Ming Dynasty (1368–1644), are now exposed to wind, rain, and soil salinization, which can accelerate their disintegration.

Biocrusts' impact on the wall's structure

Biocrusts reduce the wall’s porosity, water retention, erodibility, and salinity by 2 to 48 percent compared to bare rammed earth. They also increase compression, penetration, and shear resistance by 37 to 321 percent. Moss-dominated biocrusts are especially effective, offering up to three times more resistance than sections without such cover.

Variation in biocrust effectiveness

Not all biocrusts are equally protective. Those dominated by mosses, found in slightly more humid semi-arid areas, are more effective at strengthening the wall than those made mostly of cyanobacteria. This conclusion comes from a study published in Science Advances, which challenges the belief that vegetation accelerates the wall’s degradation.

New insights for heritage conservation

A 2025 study in Heritage Science used drones to survey erosion on the Ming Great Wall of Gansu, identifying soil salinization, topography, and rainfall as key factors. These findings, combined with the biocrust study, suggest that Chinese authorities should reconsider their restoration practices, potentially preserving these living layers instead of removing them.