Effects of a fungal pathogen on recruitment of Ormosia glaberrima seedlings in a subtropical forest

Background/Question/Methods

The Janzen-Connell hypothesis provides explanations for species coexistence and predicts that recruitment of juveniles from adult trees at high density is reduced by natural enemies, particularly soil-borne pathogens. Soil fungi possess relatively low dispersal ability, multiply rapidly in response to changes in host density, and may possess strong host specialization. The transmission rates of fungal pathogens likely depend on the density of host plants, suggesting that host-specific fungi affect seedling survival in a distance- and density-dependent way. According to the Janzen-Connell hypothesis, a single adult tree (locally rare species) located at sufficient distance from aggregated conspecific adult trees (locally common species) should display a rare species advantage by ‘escaping’ from pathogens. However, most ecological studies investigating negative soil feedbacks failed to identify host-specific soil pathogens. In a recent paper (Liu et al., J. Ecol. 2012, published online: 18 Jan), we compared self-replacement of two subpopulations of the subtropical tree Ormosia glaberrima in the Heishiding Nature Reserve (southern China) and tested effects of fungicide treatments in field and growth-room simulation experiments.

Results/Conclusions

The results showed that offspring survival of the subpopulation with high density of adult O. glaberrima trees was negatively affected by a soil fungus, whereas the other subpopulation with a single adult tree was not attacked by a pathogen. The fungus was isolated and identified as Fusarium oxysporum. Inoculation experiments revealed that the fungus was pathogenic on seeds and seedlings of O. glaberrima, but non-pathogenic on other tree species co-occurring with O. glaberrima. Interestingly, the examined O. glaberrima subpopulations differed in their susceptibility to F. oxysporum, suggesting genotypic differences. In conclusion, our findings meet the prerequisites of the Janzen-Connell hypothesis and suggest that soil pathogens may promote species coexistence by keeping self-replacement of locally common species in check. We currently perform experiments to exactly localize F. oxysporum in the field by PCR-based techniques in order to uncover its spatially unequal distribution in O. glaberrima subpopulations.