Climate shapes biodiversity–mycorrhiza interactions to control forest carbon

While mycorrhizal dominance is known to influence biodiversity-ecosystem functioning relationships, the joint effects of mycorrhizal dominance and tree diversity on forest productivity and carbon storage remain poorly understood, particularly under varying climatic conditions. To address this gap, we analyzed data from 5,215 forest plots across China and 27,467 plots across the USA to assess how tree diversity and mycorrhizal associations interact to influence tree productivity and carbon storage under contrasting climate regimes. In China, biodiversity-mycorrhizal interactions exhibited strong climate dependency. In temperate forests, increasing ectomycorrhizal (EcM) dominance weakened the positive effects of tree diversity on carbon storage-a negative interaction further intensified by warming, but reversed under higher precipitation. By contrast, subtropical forests showed consistently positive diversity-EcM synergies, largely unaffected by climate. In the USA, these patterns were inverted. In temperate forests, EcM dominance enhanced the diversity–productivity relationship under cool, dry conditions, but this synergy reversed under warmer and wetter climates. In subtropical regions, initial antagonistic interactions became increasingly positive with rising temperature and precipitation. These discontinuities likely arise from climate-driven shifts in nutrient limitation and mycorrhizal functional strategies, whereby EcM-associated species gain a competitive advantage under cooler or nutrient-poor conditions, while arbuscular mycorrhizal (AM)-associated communities dominate in warmer and more productive environments. Together, our results demonstrate that mycorrhizal-diversity interactions are highly context dependent and vary markedly across continents, underscoring the pivotal role of the climate–mycorrhizal–diversity triad in shaping forest ecosystem functioning and providing a critical ecological basis for improving carbon modeling and guiding forest management under climate change. • Mycorrhizal dominance strongly modulates biodiversity–ecosystem functioning relationships. • Climate regulates the strength and direction of diversity–mycorrhizal effects on productivity and carbon storage. • Opposite interaction patterns emerge between China and the USA, revealing continental-scale divergences . • Findings highlight the “climate–mycorrhizal–diversity” triad as a critical driver of forest carbon dynamics under global change.