Rare tree species host a high diversity of rhizosphere fungi

Plant hosts play a crucial role in structuring rhizosphere fungal symbioses which in turn influence the composition of tree species in forest ecosystems. However, there is little understanding of how changes in host species abundance (e.g. rare vs. common host species) affect the rhizosphere fungal community. By sampling rhizosphere soils from 272 adult individuals representing 68 tree species in a 25-ha subtropical forest plot, we investigated the relationship between tree species abundance and rhizosphere fungal diversity. We also conducted a greenhouse experiment by planting seedlings of five test tree species in pots filled with the same heterospecific soils to assess this relationship under the same soil (fungal) condition. The results of the in situ rhizosphere soil sampling revealed that the diversity of total rhizosphere fungi decreased with tree species abundance. Similarly, the greenhouse experiment showed that the least abundant species, Callicarpa membranacea (CM), exhibited the weakest host selection effect, as indicated by its highest total rhizosphere fungal richness. After controlling for differences in total rhizosphere fungal richness, the greenhouse experiment showed that CM hosted a high proportion of moderate-specialization fungi compared to other more abundant tree species except the most abundant species Schima superba which appeared to host the highest proportion of high-specialization fungi. Our results further showed that both the above-ground biomass and the biomass allocation ratio of CM seedlings were positively correlated with total rhizosphere fungal diversity. These findings suggest a potential role for a high total rhizosphere fungal richness in sustaining this least abundant species in the field. Synthesis. Our study demonstrates that rare tree species harbour higher total rhizosphere fungal diversity than common ones, reflecting their weaker selection on soil fungi. Tree species with varying abundance may exhibit differential selection preferences for fungi with different specialization levels. This variation in host selection in both strength and preferences could offer new insights into understanding rarity and commonness of forest tree species.