Photo by Hafiz Maherali

To understand forest diversity, look not just to the trees but also to the fungi, says a new international study involving a University of Guelph professor.

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.

The findings were published in the latest issue of the journal Science and featured in the Globe and Mail.

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,” Maherali said.

The researchers examined 55 tree species from 550 forest locations in North America. “It’s one of the largest studies to date in terms of species examined,” Maherali said.

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.

The researchers found that the type of relationship determines where trees grow.

In both greenhouses and forests, they found that trees with external mycorrhizas may 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,” Maherali said.

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

With so many sample trees in the study, Maherali said, the researchers had to distinguish among specimens that were distantly and closely related.

“The challenge was: Are we seeing differences between the two different mycorrhizas because of the fungi? Or are we seeing differences because we’re comparing maples versus pines or maples versus oaks, which are already quite different, regardless of the fungi they associate with?”

As well, two species might respond in the same way to fungi because they grow under similar conditions or because they are closely related. “This can complicate interpretation,” said Maherali, whose lab interpreted the findings and the effects of shared ancestry. “By using information on the shared ancestry of species, we could factor it out of the results, which helped reveal the fungal-specific effects.”