Does the mycorrhizal mediation hypothesis capture biogeographical patterns in plant-soil feedback? The case of conspecific negative density dependencies

Why are there so many plant species in nature? Because there exist mechanisms that reduce plant fitness when a species becomes abundant, opening up opportunities for coexistence. The best known example of this is the accumulation of species-specific pathogens and herbivores in the vicinity of parent plants that prevent the establishment of seedlings belonging to the same species, giving rise to so-called conspecific negative density depedencies. Conspecific negative density dependencies arise when conspecifics receive lower fitness gains by soilborne mutualists, the mycorrhizal fungi, or more severe fitness reductions from pathogens and herbivores compared with heterospecifics in the proximity of parent trees. There is a widespread belief that conspecific negative dependencies in species engaging with arbuscular mycorrhizal fungi are considerably more negative than in plant species engaging with ectomycorrhizal fungi.

Veresoglou and Lambers developed a perspective that the actual differences in conspecific negative density dependencies between plant species belonging to the two types of mycorrhiza are smaller than what most peers believe. The two authors attribute many of the strongly negative estimates of conspecific density dependency that have accrued across databases, in relation to trees associating with arbuscular mycorrhizal fungi, to the prevalence of studying density dependencies in temperate forests. Temperate forests contain very few trees associating with arbuscular mycorrhizal fungi, which, through bringing about a propagule limitation, could prevent the formation of a healthy arbuscular mycorrhizal symbiosis in the younger seedlings. This is an idea that is captured in the mycorrhizal mediation hypothesis. The outcome is that conspecifics of arbuscular mycorrhizal trees receive less help with their nutrition, as well as less protection from pathogens from their mycorrhizal partners than they would have received, at a propagule sufficiency. This puts them at a competitive disadvanatage against ectomycorrhizal trees. Less work has targeted tropical and subtropical systems and in those systems the estimates of density dependencies more likely reflect actual differences between the two mycorrhizal types.

To concolidate this idea, the authors reanalyzed an existing global dataset. They show that the differences in conspecific negative dependencies between the two mycorrhizal types diminished in forests that contain a high relative abundance of trees that associated with arbuscular mycorrhizal fungi. The researchers’ finding appeared March, 2025 in Soil Ecology Letters.

Stavros Veresoglou said, "There are many triggers of conspecific negative density dependencies and we only superficially understand them right now. For example, the degree to which species belonging to different mycorrhizal types engage in different strategies of dispersal and how thse cascade to conspecific negative density dependencies. Propagating oversimplified ideas in the discipline, even if these end up being the case, does a disservice to mycorrhizal ecology.”

DOI: 10.1007/s42832-025-0293-3