Forests are vital for climate regulation and biodiversity, yet they face unprecedented threats from human activities and climate change. Understanding what drives tree species diversity is essential for conservation. While factors like temperature, rainfall, and soil fertility have been widely studied, the role of mutualistic interactions, particularly mycorrhizal symbioses, remains underexplored. A recent paper by Feng Jiang et al. (with input from WILDCARD’s partner Wageningen University & Research), published in Science Advances (2025) delves more deep into the topic.
Mycorrhizal fungi form partnerships with trees, exchanging nutrients for carbon and offering protection against pathogens. Two major types dominate: ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi. EcM fungi tend to provide stronger protection and alleviate negative plant-soil feedbacks more effectively than AM fungi. This has led to the EcM dominance hypothesis, which predicts that forests dominated by EcM trees will have lower species richness. Conversely, the mycorrhizal mixture hypothesis suggests that diversity peaks when both EcM and AM trees coexist, due to complementary nutrient acquisition strategies. A third view, the integrated hypothesis, proposes that both mechanisms operate together, with diversity peaking in mixed communities but declining in EcM-dominated forests.
Until now, these hypotheses lacked global-scale testing. Jiang and colleagues addressed this gap by analyzing over 400,000 forest plots worldwide, examining how EcM tree proportion relates to species richness across latitude and aridity gradients.
Key Conclusions
The study revealed that the relationship between EcM tree proportion and species richness varies with environmental conditions:
- Low latitudes & moist conditions: A strong negative relationship supports the EcM dominance hypothesis. Here, EcM fungi reduce negative plant-soil feedbacks, favoring conspecific survival and lowering overall diversity.
- High latitudes & arid conditions: A unimodal relationship supports the mycorrhizal mixture hypothesis. Stressful environments weaken negative biotic interactions, making niche partitioning between EcM and AM trees more important for coexistence.
- Intermediate conditions: A negatively unimodal pattern aligns with the integrated hypothesis, suggesting both mechanisms operate simultaneously.
Overall, the study concludes that mycorrhizal symbioses are a critical, yet overlooked, driver of global forest diversity, interacting with climate and soil factors. This insight expands our understanding of biodiversity maintenance from an aboveground perspective to include belowground mutualisms.