Fig. 7

A structural equation model was constructed to depict the hypothesized causal relationships among 7 variables, including the evolutionary divergence times (ETs) of different plant species, 2 soil nutrient factors, and 4 AMF parameters. The 2 soil nutrient factors correspond to the first two principal components derived from a PCA of seven soil properties, namely, barren soil factors (BSFs) and fertile soil factors (FSFs), as depicted in Fig. 1. The AMF parameters encompass mycorrhizal colonization, spore density, α diversity, and community structure. In the structural equation model, the mycorrhizal colonization indices employed included colonization frequency (F), colonization density (M), and arbuscular abundance (A). Additionally, 3 α diversity indices, namely, richness, Shannon index, and Pielou's evenness, were included in the current SEM. Similar to the approach used for soil nutrient factors, the first two axes of the principle component analysis (PCA) at the order level (as shown in Fig. 4B) were utilized to quantify data on community structure. In the diagrams, the red solid arrows signify significant relationships between two indices in a given pathway, while the black dashed arrows denote nonsignificant relationships within that pathway. The numbers above the arrows represent path coefficients, with positive and negative values indicating positive and negative relationships between the two indicators, respectively. An asterisk (*) denotes a path with a P value less than 0.05, ** signifies P < 0.01, and *** indicates P < 0.001