Conspecific and heterospecific plant-soil feedbacks influence survivorship and growth of temperate tree seedlings

1. The Janzen-Connell (J-C) Model proposes that host-specific enemies could maintain high tree species diversity by reducing seedling performance near conspecific adults. An implicit, but untested assumption of the J-C Model is that negative conspecific feedbacks would promote replacement by heterospecific seedlings. 2. In a glasshouse experiment, we tested plant-soil feedbacks as a J-C mechanism in four temperate tree species. We assessed effects of conspecific- relative to heterospecific-cultured soil extracts on seedling survival, total mass and performance for each focal species. To test the implicit assumption of replacement by heterospecific seedlings, we also compared relative performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. We also tested whether soil microbes caused these plant-soil feedbacks and whether low irradiance increased seedling vulnerability to pathogens. 3. When grown with conspecific versus heterospecific soil extract, Acer rubrum mass decreased, Quercus rubra mass increased and Fraxinus americana increased survival. Conspecific extract reduced Acer saccharum mass in low light but increased it in high light. To integrate survival and growth, we examined seedling performance [(mean total mass x mean survival time)/(days of experiment)] at low and high light. In conspecific versus heterospecific soil extract, seedling performance was lower in two, higher in four and neutral in 18 of 24 cases, suggesting no advantage to dispersing away from conspecifics. Based on relative seedling performance within a soil extract, conspecific seedlings were disadvantaged in two, favoured in three and neutral in 19 of 24 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance were chemically mediated, occurring regardless of sterilization. Microbes lacked host specificity and reduced performance regardless of extract source. Additionally, microbial factors reduced seedling performance for Q. rubra regardless of light availability, and for A. rubrum and F. americana only in high light. 5. Synthesis. These chemical-mediated plant-soil feedbacks probably influence community dynamics, but are inconsistent with the J-C Model. Even when a species' seedlings responded more negatively to conspecific than heterospecific soil, heterospecific seedlings were not necessarily favoured in that species' soil, precluding heterospecific replacement as an explanation for coexistence.