A paper published in Nature Communications Biology contributes to the growing appreciation for the outsize role that microbes play in everything from human digestion to crop yields: Microbes in the soil—fungi in this case—appear to be influencing forest diversity on a global scale.

Forests on Earth exhibit a marked gradient from the equator toward the poles: Tropical forests near the equator tend to include a large number of different species, whereas forests nearer the poles support less plant diversity.

One explanation for this phenomenon maintains that soil pathogens, including bacteria and fungi, help create this gradient. Species-specific pathogens accumulate near adult trees, and their abundance can diminish the success of juveniles growing near their parents, thus promoting species diversity. This effect is stronger in warm, wet climates, contributing to the greater diversity in forests near the equator.

However, a new study led by Camille Delavaux, a lead scientist at ETH Zurich in Switzerland, adds a twist to this established story. Mycorrhizal fungi—soil fungi that form mutually beneficial relationships with the majority of plant roots globally—appear to be counteracting the effects of harmful soil pathogens in ways that influence global patterns of forest diversity.