The Great Wall of China is being protected from erosion by a “biocrust” of moss, lichen and cyanobacteria, much as the wall once shielded the country from northern invasions.
The wall, built and rebuilt many times between about 200 BC and the Ming dynasty, which lasted from 1368 until 1644, once stretched for more than 8800 kilometres. Today, however, less than 6 per cent of its total length remains well-preserved, and over half has either vanished or become severely deteriorated.
Many sections of the wall were built with rammed earth, which is when natural materials including soil and gravel are compacted to create structures.
Bo Xiao at the China Agricultural University in Beijing and his colleagues have sampled a 600-kilometre-long section of the wall and found that more than two-thirds of it is covered in biocrust.
The team found that the layer of lichens, mosses and cyanobacteria contributes to strengthening the wall, keeping it dry and protected from wind and water erosion. The biocrust also acts as an insulator, reducing temperature extremes and lowering the effects of salinity.
Biocrust-covered sections were found to be less porous, with reduced water-holding capacity, erodiblity and salinity, says the team. These sections also showed increased resistance to various forms of mechanical assault.
The findings could change the way managers of heritage sites around the world regard vegetation on ancient structures, especially those that had large sections built with rammed earth. Xiao believes that biocrusts represent a promising and innovative strategy for heritage conservation.
“They offer superior advantages over conventional protective measures and serve as stabilisers, consolidators, sacrificial layers and drainage roofs, combining the protective functions of several conventional measures into one nature-based, cost-effective, eco-friendly and long-lasting strategy,” he says.
The biocrusts on the Great Wall of China could help mitigate the extremes it faces in terms of hot and cold, says Brett Summerell at the Botanic Gardens of Sydney. “They would provide an environment that helps buffer and protect the stability of the structure of the walls.” He says rocks covered in biocrust that he has seen in Australia have less cracking and splitting.