Arbuscular mycorrhizal fungi reduce the role of the Janzen-Connell effect in maintaining diversity

Background/Question/Methods: Soil-borne pathogens can contribute to diversity maintenance in tree communities through the Janzen-Connell effect, whereby the pathogenic reduction of seedling performance attenuates with distance from conspecifics. Arbuscular mycorrhizal fungi (AMF) have been reported to promote seedling performance, but it is unknown whether this is also distance-dependent. A complete understanding of the maintenance of plant diversity and community structure requires integrating microbial perspectives and accounting for both positive and negative plant-soil interactions.

In this study, we investigated whether the “negative” distance-dependence driven by soil pathogens exists commonly in the wild, but might sometimes be neutralized by AMF that cause “positive” distance-dependence. We first conducted a field census with two gravity-dispersed tree species and two wind-dispersed species in a subtropical forest, to determine whether newly germinated seedlings suffered distance-dependent mortality (i.e. decrease in mortality with distance from the conspecific tree). We then conducted two shade-house experiments with seedlings of six tree species, by manipulating soil microbes with only AMF, only pathogens, field soil (AMF + pathogens) and sterile soil treatments, to test the following hypotheses: (1) Pathogens reduce seedling performance and this detrimental effect decreases with increasing conspecific distance; (2) AMF promote seedling performance, and this beneficial effect decreases with increasing conspecific distance; (3) The microbes cultured by adult trees have higher colonization or infection rates on roots in soil at closer conspecific distance and (4) Both pathogens and AMF are host-specific and have no effect on seedlings of other species.

Results/Conclusions: In the field survey, the survival of seedlings that geminated during 2009 to 2012 were not significantly related to the distance to the closest reproductive conspecific for all of the four species, indicating that the distant-dependent prediction of the Janzen-Connell hypothesis was not supported in the natural communities. By experimentally separating the effects of AMF and pathogens on seedling performance of six subtropical tree species in a shade-house, we found that soil pathogens significantly inhibited seedling survival and growth and AMF largely promoted seedling growth, and these effects were host-specific and declined with increasing conspecific distance. Together, our field and experimental results suggest that the Janzen-Connell effect plays a minor role in explaining diversity, at least of species occurring in low densities.