How Trees Become More Resilient Through Symbiotic Relationships
When trees and soil fungi enter into a close relationship, both partners benefit. Many tree species have further improved this cooperation: a simultaneous symbiosis with two different groups of mycorrhizal fungi. These trees cope better with water and nutrient deficiencies — important characteristics for forestry in the face of climate warming.
Despite their vast root systems, trees are often unable to absorb sufficient water and nutrients from the soil to develop in a healthy manner.
This is why most land plants have, over the course of evolution, entered into symbiotic relationships with fungi. These so-called mycorrhizal fungi, which either envelop the roots or penetrate into the cells of the root system, receive from the trees a portion of the sugar produced through plant photosynthesis as an energy source. In return, the trees receive nutrients such as phosphate and nitrate as well as water from the fungi — a kind of unromantic marriage of convenience.
Dual Symbioses Expand Habitat
Researchers at the University of Zurich (UZH) and Agroscope demonstrate, based on more than 400 tree species distributed worldwide, that many have further improved this system of cooperation. While most tree species only form relationships with certain representatives of different groups of mycorrhizal fungi, “a proportion of all woody plants simultaneously enter into an alliance with two fungal variants,” says Ido Rog from the Institute of Plant and Microbiology at UZH.
The researchers now show that such a “dual alliance” makes trees more fit: they are less susceptible to drought and cope better with nutrient deficiencies. “This allows them to colonise a much larger area than tree species that enter into symbiosis with only one type of fungus,” says lead author Rog. The greater geographical range and expanded ecological niche space of these trees are independent of phylogenetic descent and evolutionary history.
More Resistant to Heat and Drought
The simultaneous relationship with two different mycorrhizal fungi enables trees to access a more diverse nutrient supply, as their roots cover a more extensive soil profile and can cope with a wider range of soil properties. «Our results suggest that polygamy with two fungal variants served trees as a strategy to adapt to harsher environmental conditions — that is, to colonize nutrient-poorer niches and withstand more severe climates,» says UZH Professor Marcel van der Heijden. For instance, the distribution of trees with dual symbioses is significantly more pronounced in drier areas than in locations with higher rainfall.
«In forestry, this knowledge could help to select, in the future, those tree species that are specialized in polygamous fungal relationships. These trees are likely to cope better with advancing global climate change and could be used to colonize drier climate zones,» says van der Heijden.
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