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)