A ‘living skin’ is protecting the Great Wall of China from erosion

Biocrusts, thin layers of moss and bacteria, may prevent wind and rain from damaging the world famous site

The Great Wall of China used to be much greater. What stands today is only a fraction of the expansive fortifications built on the country’s northern borders starting more than 2000 years ago and then eroded by time. But many sections of the remaining walls seem to be getting preservation help from an unlikely source: thin layers of bacteria, moss, lichen, and other organisms known as biocrusts, which grow on the surface of soils.

A study published today in Science Advances finds that these so-called “living skins” have likely protected parts of the Great Wall from wind, rain, and other corrosive forces. And with advances in technology and research, scientists might eventually propagate new biocrusts to spare the wall from further degradation.

The work is “innovative and creative,” says Nichole Barger, an ecologist at the Nature Conservancy who was not involved in the new research. She notes it’s not necessarily surprising, however, given the growing recognition of the protective effects of biocrusts: These webs of growth are known to help stabilize dryland ecosystems and prevent soil erosion.

Many of the Great Wall’s most well-known and visited sections are made of stone or brick, but other parts were built out of soil compacted by workers, often called rammed earth. Over time, this material can break down as rain seeps in, wind blows the soil away, salt crystals form inside, and temperatures fluctuate.

But this compacted soil, much like the natural soils surrounding it, can also become home to biocrusts. These layers of growth have been estimated to cover some 12% of the planet’s land surface, and are concentrated in regions with drier climates, including northern China. They come in a variety of forms, from thin networks of bacteria mere millimeters thick to denser layers of moss and lichen up to a few centimeters in height.

Soil scientist Bo Xiao at the China Agricultural University and his colleagues wanted to know whether biocrusts play a stabilizing role for a humanmade structure such as the Great Wall. Along the sections they studied, biocrusts—mostly made of moss or photosynthetic microbes called cyanobacteria—covered more than two-thirds of the structure. The team took samples from different parts of the wall covered in biocrusts and compared their physical properties with those of bare, biocrust-free rammed earth.

Moss and other organisms thrive on sections of the Great Wall of China made of compacted soil.BO XIAO

Compared with the bare sections, biocrust-covered rammed earth was less porous and had higher shear strength and compressive strength, the team reports today.

The researchers suggest these properties and others linked to biocrusts protect the Great Wall from degradation in a few ways, including by reducing wind erosion, preventing water and salt from seeping in, and increasing the overall stability of the rammed earth. Perhaps unsurprisingly, thicker, moss-dominated biocrusts were generally more protective than thinner ones dominated by cyanobacteria.

Bettina Weber, an ecologist at the University of Graz, praises the group’s effort to examine whether protective effects of biocrusts could apply to the cultural heritage site. She suggests their findings could help introduce biocrust research to new scientific fields.

The Great Wall results run counter to the prevailing notion in heritage conservation that plant growth will damage buildings or archaeological sites, the study points out. But a lot of that fear comes down to the fact that plants can damage buildings with their root growth, says study author Matthew Bowker, an ecologist at Northern Arizona University—and biocrusts don’t have penetrative root systems.

Biocrusts in general are under threat. Some recent studies have cautioned that as the climate changes and intensive land use spreads over the next few decades, many biocrusts may disappear, taking their protective benefits with them. That loss could have consequences for the Great Wall, the new study cautions. Bowker notes that as the climate gets hotter along the Great Wall, the thicker, moss-dominated crusts could give way to thinner, cyanobacterial crusts, which require less water.

Recently, labs around the world have been researching whether they can restore damaged or degraded biocrusts by spurring their regrowth—though Bowker says these efforts remain in the R&D stage. One challenge is that scientists are still trying to understand how long it takes different types of biocrusts to grow in various climates and levels of disturbance, with estimates ranging from years to centuries. 

But Barger pointed out that intentionally growing biocrusts along a relatively small feature like the Great Wall would be easier than trying to restore biocrusts over hundreds of thousands of acres. And because what’s at stake is “a cultural symbol of China [and] Chinese civilization,” Xiao says, it will be important to find effective ways to keep this site standing for generations to come.

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