Allelochemical and soil fungi co‐determine conspecific density dependence in a temperate forest

Abstract Ecological theory predicts that the high local diversity observed in plant communities can be maintained by soilborne pathogens and allelopathic autotoxicity, both of which trigger negative conspecific density dependence (CDD). In contrast, mutualistic fungi and allelopathic promotion may counteract these biotic processes. Although negative, positive, and neutral CDDs have been documented, the interactive roles of soil fungi and allelopathy in regulating CDD remain unclear. We conducted an experiment in which phenolic acids were added to pot‐grown tree seedlings associated with different mycorrhizal fungi to quantify the effects of allelochemicals on seedling growth, soil fungi, and plant–fungus interactions. We then combined these results with long‐term inventory data (2005–2021) to test how these mechanisms relate to CDD in a natural temperate forest. We found both positive (i.e., promotion) and negative (i.e., autotoxicity) responses of tree species to allelochemicals. Moreover, allelopathic effects on seedling growth were generally stronger than fungal effects, and allelochemicals altered plant–fungus interactions in a dose‐dependent manner. Contrary to expectations, ectomycorrhizal trees exhibited stronger negative CDD than arbuscular mycorrhizal trees, driven largely by allelopathy, although ectomycorrhizal fungi mitigated some allelopathic autotoxicity in certain species. Overall, our results highlight the ecological role of allelopathy in regulating species coexistence, showing that allelopathic autotoxicity was more important than fungal pathogens or mycorrhizas in driving CDD in this temperate forest, through effects on both plants and plant–microbe interactions.