Mycorrhizal fungi promote a greater diversity of tree species

Fungi, especially those that are “mycorrhizal”, are natural allies of the forest because they improve the acquisition of nutrients from trees. But which of the mycorrhizal feeding strategies gives the greatest diversity of trees in a forest: strategy A (ectomycorrhiza) or strategy B (arbuscular mycorrhiza)?

Biologists from the University of Montreal and the Institute for Research in Plant Biology asked themselves the question and found that the answer was neither, but rather a combination of the two, proving that there is strength in numbers, or rather diversity.

The powerful nutritional capacity of mycorrhizae

“A mycorrhiza, from the Greek myco-, ‘mushroom’ and rhiza, ‘root’, is a type of symbiotic relationship between a plant and a fungus that has existed since the colonization of land by plants, several million years ago. “, explained Alexis Carteron, lead author of the study.

“This positive association for both partners is undoubtedly the most widespread and important form of ‘mutualistic symbiosis’ in terrestrial ecosystems.”

Carteron holds a doctorate. in biology from the University of Montreal and is currently working as a postdoctoral researcher in the Department of Environmental Science and Policy at the University of Milan, Italy.

In botany, mycorrhizal fungi have long been known to provide plants with significant nutritional benefits by extending their root system up to 10 times the original root area and allowing them to better absorb water and minerals from the soil. For example, mycorrhizal fungi are able to dissolve phosphorus in the soil, making it available to plants. In return, the plant provides the fungi with sugar from photosynthesis.

“For some time, there has been growing interest in the important role of mycorrhizal fungi in plant biodiversity,” said study director Etienne Laliberté, holder of the Canada Research Chair in Functional Biodiversity of Plants. plants at the University of Montreal.

Two Strategies for Mushrooms

The two main types of mycorrhizae, ectomycorrhizae and arbuscular mycorrhizae, appear to influence tree species diversity in forests in different ways.

Ectomycorrhiza affects about 2% of plant species, mainly conifers in northern hemisphere forests. Arbuscular mycorrhiza, the oldest and most widespread form of symbiotic association between fungi and plants, affects 80% of terrestrial plants. The two differ in how the fungus attaches to the roots of the plant.

Scientists have observed that forests whose soils are colonized by arbuscular mycorrhizae are more diverse. Species-rich tropical rainforests, for example, are composed primarily of arbuscular mycorrhizal trees, while species-poor boreal forests are dominated by ectomycorrhizal trees.

For this reason, researchers believed that arbuscular mycorrhiza promotes the coexistence and diversity of plant species, while ectomycorrhiza promotes the dominance of only one or a few species.

Experimental studies of young trees and large-scale observations in different terrestrial biomes (geographical areas sharing similar climate, fauna and flora) also seem to support this hypothesis. It is, however, questioned by the new study.

More than 80,000 forest plots analyzed

Forest trees interact with each other and with mycorrhizal fungi at a very localized level (within a few meters) over periods of several decades. Carteron and Laliberté therefore had to test the hypothesis on forest patches (i.e. an area of ​​a few hundred square meters) in a multitude of forests to determine if the results could be generalized.

“We analyzed approximately 82,000 forest plots across the United States and concluded that plots very heavily dominated by ectomycorrhiza or arbuscular mycorrhiza had lower tree diversity,” Carteron explained. “Surprisingly, it was forests with a mixture of both mycorrhizal strategies that had a greater number of tree species. Our results therefore indicate that the dominance of any mycorrhiza, regardless of type, appears to decrease the diversity of forest trees.”

Can mycorrhizae help fight climate change?

While mycorrhizal dominance can be determined at multiple scales, such as root system, forest patch, and biome, this study highlights the importance of considering the impact of mycorrhiza on ecological processes at the forest patch level. At this scale, the study shows that the coexistence of mycorrhizal strategies can promote plant diversity.

“Sometimes forests with a mixture of mycorrhizal strategies are overlooked by biologists because they are considered less abundant,” the researchers noted. “However, our study showed that this is not always the case and that these mixed types could, in fact, constitute a large part of the world’s forests.”

Such forests can represent a crucial avenue for research and forest management targeting greater ecosystem services: “Combating and adapting to climate change is a good example of the services that an ecosystem can provide, because a forest thrives with a great diversity of trees constitutes a favorable reservoir for climate change. balance,” said Carteron.

The study was funded by the Natural Sciences and Engineering Research Council of Canada, the Fonds de recherche du Québec — Nature et technologies and the Bourse d’excellence Hydro-Québec — Université de Montréal.

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