A team of researchers, including U of G integrative biology professor Hafiz Maherali, found that fungi — not trees — are what control forest diversity. More specifically, whether a tree and its fungi have an “internal” or “external” relationship affects growth patterns.

Tree species diversity is critical for maintaining forest biodiversity and ecosystem function, including everything from carbon storage to nutrient cycling. But the factors regulating tree diversity have remained unclear. Scientists have long known that plants and soil fungi form symbiotic relationships called mycorrhizas, with plants providing carbon in exchange for nutrients.

Most tree species, including maple and ash, have internal mycorrhizas with fungi that colonize in tree roots. But other species, such as pine and oak, have external mycorrhizas that produce a protective sheath around the root.

In the study, published in Science, the researchers examined 55 tree species from 550 forest locations in North America. They found the type of relationship determines where trees grow.

In both greenhouses and forests, they found that trees with external mycorrhizas might grow together more densely because their roots are protected from pathogens in the soil.

Trees with internal mycorrhizas growing in the same soil as their parent tree are exposed to the parent’s pathogens. “Pathogen attack causes these trees to grow further away from their parent, a process that increases forest diversity,” says Maherali.

The results may help in forest management and restoration and and in better understanding the effect of invasive species.

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