Patterns in plant-fungi relationships
A random forest analysis was performed to evaluate the major environmental factors to the topological features of plant-fungi network in the Tibetan Plateau. Plant species richness was the primary factor affecting the number of fungal OTUs links to each plant species and other topological features, such as the number of edges and modularity (Fig. 1a). These observations were also supported by heatmap based on spearman correlation (Fig. 1b) and the multiple regression on distance matrices analysis (Fig. 1c), confirming the leading effect of plant richness on the individual plant species to plant-fungi networks.
Further, there was an overall significant and positive linear relationship between plant species richness per site, and the number of fungal OTUs associated with each plant species in plant-fungi network (Fig 2a).
As a test of the possible explanation that there are only more fungal OTUs linked to each plant species because there is greater fungal richness in the whole soil community in the more plant species-rich samples, we calculated the ratio of fungal OTU richness to plant species richness and estimated its pattern along the plant species richness gradient. In fact, contrasting with the trend seen in Fig 2a for plant-linked OTUs only, the ratio of fungal OTUs in the total soil community to plant species decreased towards higher plant species richness levels (Fig 2b). Thus it appears that potential for association of fungi with plants enhances along the plant richness gradient unbiased by overall fungal richness in the soil, indicating that the tendency of fungi to form links with plants does actually increase with greater plant richness (Fig S3a).
The raising in plant richness increased the connection between plant and fungi at an alarming rate (Fig S3b), with the increasing number of plants (Fig S3c) and fungi (Fig S3d) in IDEN. The modularity and nestedness of the plant-fungi subnetwork in 60 sites also increased with increasing plant richness (Fig 2). While the porportions of functional guilds respectively belong to low, middle and high level of plant richness were similar in the plant-fungi network. (Fig S7a)