Mycorrhizal fungi are essential to the establishment of the vast majority of plant species but are often conceptualized with contradictory roles in plant community assembly. On the one hand, host-specific mycorrhizal fungi may allow a plant to be competitively dominant by enhancing growth. On the other hand, host-specific mycorrhizal fungi with different functional capabilities may increase nutrient niche partitioning, allowing plant species to coexist. Here, to resolve the balance of these two contradictory forces, we used a controlled greenhouse study to manipulate the presence of two main types of mycorrhizal fungus, ectomycorrhizal fungi and arbuscular mycorrhizal fungi, and used a range of conspecific and heterospecific competitor densities to investigate the role of mycorrhizal fungi in plant competition and coexistence. We find that the presence of arbuscular mycorrhizal fungi equalizes fitness differences between plants and stabilizes competition to create conditions for host species coexistence. Our results show how below-ground mutualisms can shift outcomes of plant competition and that a holistic view of plant communities that incorporates their mycorrhizal partners is important in predicting plant community dynamics.

菌根真菌对于绝大多数植物物种的建立至关重要，但在植物群落组装中通常被概念化为相互矛盾的角色。一方面，宿主特异性菌根真菌可能通过促进生长使植物在竞争中占据主导地位。另一方面，具有不同功能能力的宿主特异性菌根真菌可能会增加营养生态位分配，使植物物种共存。在这里，为了解决这两种矛盾力量的平衡，我们使用受控温室研究来操纵两种主要类型的菌根真菌的存在，即外生菌根真菌和丛枝菌根真菌，并使用一系列同种和异种竞争密度来研究菌根真菌在植物竞争和共存中的作用。我们发现，丛枝菌根真菌的存在平衡了植物之间的适应性差异并稳定了竞争，为宿主物种共存创造了条件。我们的结果表明，地下共生关系如何改变植物竞争的结果，并且包括其菌根伙伴在内的植物群落的整体观点对于预测植物群落动态非常重要。