The similarity between arbuscular mycorrhizal fungi communities of trees and nearby herbs in a planted forest exhibited within-site spatial variation patterns explained by local soil conditions.

The similarity between the arbuscular mycorrhizal fungi (AMF) communities of trees and neighboring understory herbs in forests remains unclear, which we aimed to clarify. We traced and collected basal roots of 20 randomly chosen Cryptomeria japonica (Cupresaceae) trees and the surrounding soil at four microsites in 1 km² of a Cr. japonica forest. One Chloranthus serratus (Chlorantaceae) herb immediately at the base of each sampled tree was excavated to collect an intact root system. We amplified a partial small subunit of fungal ribosomal DNA (18S) using Illumina MiSeq amplicon sequencing. Soil physicochemical properties were also measured. We detected 670 and 679 AMF operational taxonomic units (OTUs) in Cr. japonica and Ch. serratus, respectively, belonging to Acaulospora, Dominikia, Glomus, Microkamienskia, Rhizophagus, Septoglomus, and Sclerocystis. Seventeen OTUs were detected in the roots of both host species at average relative abundances > 1%. Among them, four dominant OTUs with an average relative abundance > 10% were concurrently detected in the roots of 17 tree-herb sets. The composition and similarity of their AMF communities were spatially varied, significantly driven by spatially varying soil pH, total C, N, C/N, and elevation, but not electroconductivity, supported by the microsite-dependent distributions of their dominant OTUs. We concluded that the similarity of AMF communities between trees and neighboring understory herbs depends on the soil physicochemical conditions that influence the distribution of their dominant AMF.