Analysis of a negative plant-soil feedback in a subtropical monsoon forest

1. The Janzen-Connell hypothesis provides explanations for species coexistence and predicts that recruitment of tree juveniles is reduced by host-specific enemies, particularly soil biota. Previous studies, however, have not fully addressed the aspect of host specificity. Using a legume tree (Ormosia glaberrima) in a subtropical monsoon forest as a model, we experimentally investigated the mechanisms underlying a Janzen-Connell effect. 2. A negative plant-soil feedback was identified by a field census and confirmed by planting surface-sterilized seeds at different densities around focal trees. Fungicides were applied to test whether the effects were caused by fungi. In a growth-room inoculation experiment, effects of collected soil samples on seedling survival were examined and compared to soil samples treated with fungicides. Sequencing of the internal transcribed spacer region (ITS) sequence of the 28S ribosomal RNA gene was used for pathogen identification. The fungus causing seedling mortality was isolated and characterized by ITS rDNA sequencing and inoculation experiments. 3. A Janzen-Connell effect was identified at a field site, in which O. glaberrima was a locally common species. In situ treatments with fungicides and a corresponding growth-room simulation experiment showed that seedling survival depended on the distance to focal adult trees and that a fungal pathogen attacked seeds and seedlings. No negative plant-soil feedback was observed at another field site with a single O. glaberrima tree, indicating a locally rare species advantage. 4. The disease-inducing fungus was identified as Fusarium oxysporum. Inoculation experiments showed that the isolated fungus was pathogenic on O. glaberrima seedlings, but non-pathogenic on seedlings from other tree species co-occurring with O. glaberrima. Moreover, susceptibility of O. glaberrima depended on seed provenance (likely genotype). 5. Synthesis. We demonstrate that an observed negative plant-soil feedback on a locally common legume tree is caused by a host-specific pathogen. Our data fully support the criteria of spatially unequal pathogen distribution and host specificity proposed in the Janzen-Connell model. Taken the interaction between O. glaberrima and F. oxysporum as a paradigm, we suggest that host-specific pathogens, locally accumulated around parent trees, are important determinants of tree community structure.