Conspecific Negative Density Dependence Of Tree Species With Contrasting Mycorrhizal Types And The Response Of Rhizospheric Fungal Community To Host Plant And Habitat

Conspecific negative density dependence(CNDD)has been well-recognized as one of critical mechanisms to maintain the tree species diversity in forests,in which plant-pathogenic fungi are one of the primary drivers for inducing CNDD.Actually,the rhizospheric soil of tree species also harbors a variety of symbiotic fungi(e.g.,mycorrhizal fungi).This structure caused by the infection of mycorrhizal fungi in plant roots can mitigate the harmful effects of pathogenic fungi and reduce the intensity of CNDD,thereby affecting the composition of tree species community.Host plants,simultaneously,combining with soil physicochemical properties,influence the composition and structure of fungal community,achieving dynamic balance between the above-ground and below-ground parts.In order to explore the effects of mycorrhizal types and rhizospheric fungal communities on the strength of CNDD and the response of fungal communities to host plant and soil properties,better revealing the mechanism of CNDD in maintaining forest tree species diversity,we conducted two studies which were based on Barro Colorado Island(BCI)plot in the tropic and Baishanzu plot in the subtropic,respectively.Firstly,using the 33-year census datasets of the BCI plot,we tested whether there was a significant change in the strength of CNDD at a relatively long-time scale,and then we explored how the CNDD strength of tree species differed among different mycorrhizal types,thereby expecting to improve our understanding of CNDD in maintaining biodiversity.After taking the mycorrhizal types into consideration,we further tested whether the relationship between tree species abundance and the CNDD strength was different from that of analyzing all tree species together.Secondly,through sampling the rhizospheric soils of focal tree species in the Baishanzu plot and operating the ITS r RNA gene-based high-throughput amplicon sequencing,we managed to analyze the composition of their fungal communities,and tried to explore whether rhizospheric fungal community affected the CNDD strength of their host species with different mycorrhizal types.Moreover,we also tested the response of fungal community to host plants and environmental factors.This major finding of this study provided a theoretical basis for the mechanism of CNDD in maintaining tree species diversity in the tropical and subtropical forests.The main conclusions are as follows:(1)The mycorrhizal type of tree species had no significant effect on the change of CNDD strength.There was no significant difference in CNDD intensity between different mycorrhizal tree species,neither in the tropical forest nor the subtropical forest.(2)In the different climate zones,the correlation between abundance and CNDD strength in different mycorrhizal tree species varied.In the BCI plot,the abundance of arbuscular mycorrhizal(AM)tree species had a significant negative correlation with the CNDD intensity,being similar to the results when all the tree species were analyzed together,but this correlation was not detected in ectomycorrhizal(ECM)tree species.In the Baishanzu plot,although rare AM and ECM tree species were detected to experience stronger CNDD,this phenomenon was not found for ericoid mycorrhizal(ERM)tree species.(3)The net pathogenicity of the rhizospheric fungal community varied among tree species associated with different mycorrhizal types.We defined the net pathogenicity of the rhizospheric soil based on the ratio of the number of pathotrophic fungi to symbiotrophic fungi,finding that the number of pathogens in ECM tree species was lower than that of symbiotic fungi and the net pathogenicity of ECM tree species was significantly lower than that of AM and ERM tree species.(4)The mycorrhizal type of tree species has a greater influence on the fungal community.In the exploration of the effect of plant taxonomy,functional traits and soil properties on the composition and structure of rhizospheric fungal community,the tree species identity,the mycorrhizal type,the carbon and nitrogen content of leaves,the fungal infection rate,the soil p H and the nitrate nitrogen concentration were significantly related to the α diversity of fungal community,but only the mycorrhizal type and soil properties significantly affected the β diversity of fungal community.In summary,although the composition of the rhizospheric fungal community seems to play a weak role in the change of CNDD strength,the mycorrhizal type of one focal tree species can significantly affect the composition of the rhizospheric fungal community.This study emphasizes the importance of incorporating the mycorrhizal type of tree species into uncovering the mechanism maintaining forest biodiversity,and thus improves our understanding of the mechanisms that contribute to maintaining forest biodiversity and promoting the coexistence of tree species.