Microtopography-induced hydrological heterogeneity promotes the co-assembly of vascular plant and biocrust communities, providing synergistic protective functions for the Great Wall

The Great Wall in China, constructed from rammed earth, faces threats from natural erosion. Vascular plants and biocrusts have enhanced the stability of the Great Wall through various mechanisms; however, understanding of the colonization processes of vascular plants and biocrusts on the wall, as well as their protective mechanisms, remains limited. This study investigated the vascular plant communities, biocrusts, soil moisture content, soil properties, aggregate mechanical stability, aggregate water stability, and soil erodibility factors across seven fine-scale microtopographies of the Great Wall (lower, middle, and upper zones on the east and west faces, as well as the wall crest). After rainfall events and under no-rainfall conditions, soil moisture content was higher at the crest compared to the lower zones on both sides; this demonstrates that microtopography significantly influences soil moisture distribution through rainfall distribution. Soil moisture content was positively correlated with variables associated with vascular plant communities (except evenness) and moss crust (p < 0.05). This suggests that the microtopography of the wall and its induced water heterogeneity drive the distribution pattern of vascular plant communities and moss crusts. Vascular plant communities and moss crusts significantly enhanced the mechanical and water stability of soil aggregates, reducing soil erosion susceptibility by providing physical protective cover, increasing the content of mechanically stable and water-stable macroaggregates, and enhancing soil silt and clay content, soil porosity, and water-holding capacity. Based on the regulation of vascular plant communities and biocrusts by microtopography, a restoration framework was proposed, offering a theoretical foundation and practical methods for the protection of the wall and similar earthen heritage sites.