Nature’s defenses against erosion

The Great Wall of China still stands 2700 years after its first sections were built. But wind, rain, salt, and freeze–thaw cycles have severely deteriorated the World Heritage site, so much so that only about 6% of the nearly 9000 km structure remains well preserved. To better study the erosion, a research team surveyed numerous areas for new observations. Here Yousong Cao samples one of the sites. For many years conservators thought that the roots of mosses, lichens, and other vegetation exacerbated weathering and erosion of the wall. That thinking, however, has now been overturned.

The Great Wall was built predominantly with rammed earth: damp soil, sand, gravel, and other natural materials that are mixed with a stabilizer and compacted in a frame. Rammed earth is also a habitat for vegetation, and numerous sections of the wall are covered by biocrust—a community of microorganisms and plants. Cao and colleagues found that the sections with biocrust better protect the wall than the sections with none. They note that the biocrust sections have less porosity, water-holding capacity, and salinity than the bare sections. The biocrust also increases the wall’s compressive strength and shear strength. (Y. Cao et al., Sci. Adv. 9, eadk5892, 2023 ; photo courtesy of Bo Xiao.)