The Janzen-Connell effect as a driver of grassland diversity?

Background/Question/Methods The negative impact of adult organisms on the recruitment of juveniles was described for coral reefs and tropical forests by Janzen and Connell. They claimed that predator accumulation reduced recruitment success in close proximity to the parent. This process has been proposed as a major driver of diversity in the tropics. However, its magnitude, frequency and actual implication for the maintenance of species richness have since been a matter of dispute. We studied negative plant-soil feedbacks in temperate grasslands as a potential equivalent of the traditional Janzen-Connell effect. We carried out an experiment with 24 plant species belonging to three functional groups (grasses, forbs and legumes). Soils from three-year-old field monocultures of the same 24 species were used to grow plants from seed in the glasshouse, either on soil from their own species or on soils from other species. Plants were grown in small communities with intra- or interspecific competition. Different soil treatments were applied to selectively remove certain groups of suspected feedback agents (such as pathogenic soil organisms or allelochemicals).

Results/Conclusions We found strong negative feedbacks of untreated soils from field monocultures on recruits from the same species. While germination success was not affected, plant biomass was reduced dramatically on soils previously occupied by the same species compared to other soils. The disadvantage was strongest in interspecific competition and resulted in a biomass reduction of 50%. One of our soil treatments, sterilisation, completely removed the negative effects. Separate exclusion of only fungi or allelochemicals did not reduce the negative feedbacks. Fertilisation decreased the feedbacks to some extent, possibly by mitigating other detrimental effects. The results of the soil treatments suggest that the major feedback agents were non-fungal pathogenic soil organisms. Both the strength and prevalence of the observed effects imply that negative soil feedbacks are important processes in temperate grasslands. By operating in a density-dependent way on plant recruitment and competitive ability, pathogen-driven soil feedbacks could act as a type of Janzen-Connell effect and maintain diversity in grasslands.