The Janzen-Connell hypothesis proposes that host-specific natural enemies could maintain high tree species diversity by reducing seedling survivorship near conspecific adults and/or at high conspecific seedling densities. Negative feedback between plant and soil communities could be an important mechanism of such non-competitive distance and density-dependent (NCDD) mortality. In a greenhouse experiment, we assessed: 1) lifespan and growth responses of Acer rubrum, Acer saccharum, Fraxinus americana and Quercus rubra seedlings to extracts taken from soils that had been cultured by adults of each of these species; 2) whether these relationships were influenced by the species and density of seedlings culturing the soil prior to extraction; 3) host-specific soil pathogens as the mechanism creating these plant-soil feedbacks; and 4) whether low light availability increased species vulnerability to these pathogens.

Results/Conclusions

Species-specific feedbacks between adult trees (but not seedlings) and the soil influenced lifespan and/or growth for all species, but contrary to the Janzen-Connell hypothesis, not all species benefited from escaping conspecific cultured sites. Instead seedlings for some tree species were harmed more in soils cultured by heterospecifics than conspecifics. For instance, F. americana seedling lifespan was reduced with soil extract cultured by Q. rubra vs. conspecific adults. Infection by Fusarium morphotypes, isolated from roots of dead seedlings, impacted seedling performance differentially among three of the four species and were neither strictly host-specific nor strictly generalist. In addition, disease reduced seedling performance for some species regardless of light availability and for others only in high light. Seedlings responded to soil origin in both non-sterile and sterile extracts suggesting that chemical mechanisms also were operating. Although a secondary influence in comparison to irradiance, species-specific adult-soil feedbacks impacted seedling performance differentially among species and thus have the potential to influence forest community dynamics.