ABSTRACT
The Great Wall, as a World Heritage Site, is constructed with rammed earth and is currently facing the threat of erosion from wind and rain. Vascular plants and biocrusts are the main coverings of the Great Wall, and their role in mitigating soil erosion has attracted increased amounts of attention; however, the understanding of their underlying mechanisms is limited. Here, we conducted an extensive survey of vascular plants, biocrusts, soil properties (soil organic and inorganic binding materials, aggregates, and texture), soil aggregate stability, and soil erodibility at the top of the Great Wall in four different defensive zones in Northwest China. Vascular plants covered 13.6 % to 63.9 % of the tops of the Great Wall, and their rich diversity was mainly derived from perennial herbs. Moss, lichen, and cyanobacterial crusts collectively covered 36.3 % to 67.8 % of the top of the Great Wall. Redundancy analysis and structural equation modeling revealed that the synergistic effects of vascular plants and biocrusts enhanced soil aggregation stability (including geometric mean diameter, GMD; water-stable macroaggregate content, R) by increasing the accumulation of soil organic carbon (SOC), amorphous iron oxide (Feo), and amorphous alumina (Alo) and promoting the formation of macroaggregates (ASD>0.25 mm) and microaggregates (ASD0.053-0.25 mm). Furthermore, soil erodibility was primarily influenced negatively by the synergistic promotion of SOC accumulation by vascular plants and biocrusts and positively by the reduction in soil sand (PSD>0.05 mm) content by biocrusts. Our work highlights the mechanisms and importance of vascular plants and biocrusts as natural covers for altering the intrinsic properties of soil for the protection of the Great Wall. These findings provide reliable theoretical support for the protection of the Great Wall from erosion by vascular plants and biocrusts and offer new insights for the conservation of global earthen sites and similar wall habitats.
ABSTRACT
The Great Wall, a World Heritage Site and a vertical wall habitat, is under threat of soil erosion. The role of vascular plants and biocrust in controlling soil erosion has attracted attention, yet our knowledge of the underlying mechanism is limited, and there is a lack of systematic strategies for erosion prevention and control. In this study, we quantified the vascular plant community functional composition (including species diversity, functional diversity, and community-weighted mean), biocrust coverage, and soil erosion levels associated with seven different zones (lower, middle, and upper zones on East and West faces, plus wall crest) of the Great Wall. We then employed a combination of linear regression analysis, random forest model, and structural equation model to evaluate the individual and combined effects, as well as the direction and relative importance of these factors in reducing soil erosion. The results indicated that the vascular plant species richness, species diversity, functional richness, community-weighted mean, and moss crust coverage decreased significantly from the crest to the lower zone of the Great Wall (P < 0.05), and were negatively correlated with the soil erosion area and depth on both sides of the Great Wall (P < 0.05). This suggests that higher zones on the wall favored the colonization and growth of biocrusts and vascular plants and that biocrusts and vascular plants reduced soil erosion on the wall. Based on these findings, we propose a "restoration framework" for managing soil erosion on walls, based on biocrust and vascular plant communities (namely target species selection, plant community construction, biocrust inoculation, and maintenance of community stability), which aims to address the urgent need for more effective soil erosion prevention and control strategies on the Great Wall and provide practical methods that practitioners can utilize.
