Ecological theory predicts that high local diversity observed in plant communities could be maintained by soilborne pathogens and allelopathic autotoxicity that trigger negative conspecific density dependence (CDD), but mutualistic fungi and allelopathic promotion could simultaneously counteract these biotic processes. Here, we combined a phenolic-acid addition experiment of tree seedlings associated with different mycorrhizal fungi with an extensive field survey to test the allelopathy-fungi mechanisms relate to CDD in a natural temperate forest. Overall, allelopathic effects on seedling growth were generally stronger than fungal effects, and allelochemicals altered how plants interacted with soil fungi in a dose-dependent manner. Contrary to expectation, ectomycorrhizal trees suffered stronger negative CDD than arbuscular mycorrhizal trees driven largely by allelopathy, although ectomycorrhizal fungi could offset some of this allelopathic autotoxicity. Together, allelopathy may thus be an important driver of CDD via affecting both plants and plant-microbe interactions, although the precise effects should be species specific.