Quantitative study of the multilayer complex structure of shrub-herbage-biocrusts based on structural equation model

Shrub-herbage-biocrust is a common structure of vegetation in arid and semi-arid regions, characterized by distinct horizontal and vertical structural features. Quantifying the three-dimensional vegetation structure is fundamental to understanding its ecological functions. Focusing on shrub-herbage-biocrust communities with varying three-dimensional structures on the typical Loess Plateau, we employed unmanned aerial vehicle and quadrat surveys to investigate community morphological traits, screen representative indicators of horizontal and vertical structures, and construct a three-dimensional structure index based on structural equation modeling (SEM). The results showed that the projective coverage of shrub, herbage, and biocrusts were representative indicators for the horizontal features of the three-dimensional structure, whereas the product of shrub height and canopy width, alongside herbaceous plant height, were representative indicators for the vertical structural features. The weights of each vegetation layer in the three-dimensional structure were in order of herbaceous layer (0.39) > biocrust layer (0.36) > shrub layer (0.25), and the weights of horizontal and vertical indicators for each layer were quite similar (0.49-0.51). Vegetation total coverage ranged from 30.9% to 80.2%, and vegetation interlacing coverage ranged from 11.5% to 113.0%, indicating strong structural heterogeneity within the community. The constructed three-dimensional structure index based on SEM ranged from 0 to 1, with increasing values indicating an increased degree of interleaving among vegetation layers and a tendency toward more complex three-dimensional structures. There was a significant positive linear correlation between the three-dimensional structure index and interlaced vegetation coverage, indicating that SEM could effectively quantify the three-dimensional structural characteristics of shrub-herbage-biocrust communities. This study would provide methodological support and a theoretical foundation for in-depth research on the structure-function relationships of shrub-herbage-biocrust communities, as well as for practical ecolo-gical restoration efforts.