Plant-Soil Feedback Contributes To Rare Tree Species Maintenance In A Subtropical Forest

There are a large number of rare species in forest ecosystems,which play important roles in maintaining the forest biodiversity.Why rare species are rare and how they can persist in forest communities are hot issues in biodiversity study.According to the modern coexistence theory,a species should have a positive population growth at the initial stage when it invades a stable community established by its abundant competitors,namely gaining advantage when rare.This has been framed as the “rare species advantage” hypothesis.With the evidence of negative plant-soil feedback(PSF)caused by soil fungal pathogens growing,rare tree species should be able to escape from soil specific pathogens due to their low densities,thereby gaining advantages in PSF and persisting in forest communities.However,many previous studies showed that rare species suffered more from soil pathogens than common ones,being contrary to the prediction of “rare species advantage” hypothesis.So far,there have been debates about how species abundance is related with the strength of PSF,and there still is a lack of experimental evidence to reconcile debates.Therefore,clarifying the relationship between abundance of tree species and the strength of negative PSF is important for fully understanding the persistence of rare species.Through in situ sampling of fine roots on Baishanzu 25 ha forest dynamic monitoring plot(in Zhejiang province)and an inoculation experiment in the greenhouse of Baishanzu,this study explored the relationships between fine root traits and pathogen defence,the relationships between abundance of tree species and rhizosphere pathogen accumulation,and the relationships between abundance of tree species and strength of PSF,as well as synthetically discussed the relationship between abundance of tree species and PSF in the end.The main results of this thesis are as follows:(1)The relationships between fine root traits and the richness of pathogens in roots depended on the mycorrhizal type of each focal tree species.For arbuscular mycorrhizal(AM)tree species,the fine root diameter was negatively correlated with the richness of pathogens in fine roots,while the specific root length was positively correlated with the richness of pathogens in fine roots.For ectomycorrhizal(ECM)tree species,the richness of pathogens in fine roots was only negatively correlated with the relative abundance of ECM fungi.These results revealed how fine root traits affected the defence of fine roots against pathogens,and provided explanation for the relationships between fine root traits and strength of PSF.(2)The accumulation rates and amount of rhizosphere pathogens had important relationships with the abundance of AM tree species.In AM species,the abundance of tree species was negatively correlated with the richness and relative abundance of rhizosphere pathogens.Moreover,the richness of rhizosphere pathogens significantly increased with the increasing DBH of rare AM tree species,while it had a decreased trend with the increasing DBH of common AM tree species(but being nonsignificant).However,the abundance of ECM tree species had no significant relationship with the richness or relative abundance of rhizosphere pathogens.(3)Results of the PSF experiment showed that the relationship between abundance of tree species and the PSF strength depended on the local densities of conspecific focal tree species where the inoculation soil was sampled.When 10 species with contrasting local densities(high versus low)were inoculated with soil sampled from the sites with high local densities of conspecific tree species,significant negative correlations between species abundance and PSF strength were detected;if these 10 tree species were inoculated with soil sampled from the sites with low local densities of conspecific tree species,there was no significant relationship between species abundance and PSF strength.Moreover,there were the same results after adding the remaining five species with no contrasting local densities to the models.The fine root diameter was significantly positively correlated with the PSF strength,while the specific root length was significantly negatively correlated with the PSF strength.The path analysis further showed that the species abundance could be explained by fine root traits and the diversity of rhizosphere pathogens,while fine root traits,the variation of diversity of rhizosphere and root pathogens caused by the local densities of conspecific tree species(where the inoculation soil was sampled)jointly explained the PSF strength.In conclusion,this thesis systematically explored the differences in the accumulation rates of pathogens,PSF strength,and the roles of fine root traits in pathogen defense among multiple tree species with contrasting abundance.Based on the estimation of PSF strength through a greenhouse inoculation experiment,it also provided a potential explanation for driving the formation of species abundance and rare species advantage.By incorporating local densities of conspecific tree species into uncovering the relationships between species abundance and PSF strength,this study verified the hypothesis of “rare species advantage” with multiple tree species for the first time,and explained for the failure of previous studies in detecting the advantage of rare species through PSF experiments possibly due to not taking local densities of focal species into consideration as well as using the conditioning stages.The accumulation rate of soil pathogen in rhizosphere decides the abundance of each species.The advantage of rare tree species in PSF is that they have lower local density and are easier to escape from their specific soil pathogens.These findings of this study have profound implications for accurately understanding the maintenance of rare tree species in forest ecosystems.