Plant-soil feedbacks,and density dependence driven by soil pathogens influence seedling diversity in subtropical forest

Background/Question/Methods

It is believed that soil pathogens play an important role on maintenance of plant community diversity and species abundance. However, most studies inferred community-level species coexistence from a single or a few species and direct evidence that soil pathogens influence species diversity were lacking. On the other hand, studies on different effects of soil pathogens on common and rare species also received conflicting results. Community compensatory trend (CCT) suggested that in the community rare species have an advantage. On the contrary, recently studies about tropical forests demonstrated that rare species suffered stronger negative plant-soil feedbacks (PSF) and negative density dependence (NDD). To test whether soil pathogens differently influence common and rare species, and eventually influence species diversity, three separate and complementary experiments were set up. Specifically, we compared direction and magnitude of density-dependence of different species seedlings in the first experiment, and pairwise PSF in the second experiment, both in shade-house and in field when soil pathogens were present or absent. Finally, field pairwise-quadrat experiment was conducted to compare diversity and evenness on the condition that soil pathogens were present or not.

Results/Conclusions

In shade-house experiment, significant negative density-dependent effects based on mortality rate were found except for the most abundant specie. Moreover, strength of density-dependence was correlated with the relative abundance of corresponding trees (DBH≥1 cm) in our 6ha plot. Common species seedlings showed lower density-dependence than rare species. However, when seedlings were grown in sterilized inoculum, all species did not show significant density-dependent effect, which confirmed that differences of seedling mortality were not due to abiotic factors. Consistent with shade-house experiment, rare species showed stronger negative density dependence in field, although adding fungicide did not totally eliminate density-dependent effect. Plant-Soil Feedbacks based on mortality rate also suggested that rare species suffer stronger negative feedbacks, while common species showed no negative and even positive feedbacks. When soil pathogens were experimentally removed, average Shannon-Wiener and Pielou indices of quadrates were significantly higher than control ones respectively. Shannon-Wiener index increased by15.93% when soil pathogens were absent, compared to 6.97% when they were present; Pielou index increased by 10.78% when soil pathogens were absent, compared to 4.23% when they were present. Our results suggest that as rare species suffer stronger negative density dependence and soil feedbacks mainly driven by soil pathogens than common species, soil pathogens erode seedling diversity.