Pollution Found to Hinder Fungi Nourishing Europe’s Trees

This is the fungus Boletus subtomentosus, or Suede Bolete. (Imperial College) London

(CN) – Pollution hinders fungi that provide mineral nutrients to European tree roots, according to a new study that suggests this dynamic is sparking malnutrition trends in the continent’s trees.

The report, which analyzed more than 13,000 soil samples across 20 European nations, shows that many tree fungi communities are strained by pollution – evidence that existing pollution limits may not be sufficient.

Trees get nutrients from the soil by hosting fungi, known as mycorrhizal fungi, in their roots. Trees send fungi carbon in exchange for essential nutrients, including potassium, phosphorus and nitrogen, which they acquire from the soil. Some of these fungi can be identified above-ground by the truffles and mushrooms they form.

This symbiotic relationship is vital to the health of trees. Recent studies have recorded signs of tree malnutrition throughout Europe, such as discolored leaves or leaves lost from the crown. However, the mechanisms causing these symptoms are unclear.

In the new report, published Wednesday in the journal Nature, the team presents their results from a 10-year review of European trees, which revealed that local soil and air quality and tree characteristics have a significant impact on mycorrhizae.

The team analyzed 40,000 roots at 137 sites across Europe. This enabled them to identify large-scale trends in mycorrhizae communities, such as their tolerance to pollution.

“There is an alarming trend of tree malnutrition across Europe, which leaves forests vulnerable to pests, disease and climate change,” said lead author Martin Bidartondo, a researcher at Imperial College London and the Royal Botanic Gardens, Kew, in the United Kingdom. “To see if changes in mycorrhizae might be behind this trend, we opened the ‘black box’ of soil. Processes happening in soil and roots are often ignored, assumed or modeled, because studying them directly is difficult, but it is crucial for assessing tree functioning.

A forest in Bavaria, Germany. (William Dotinga/CNS)

“A major finding of the study is that European pollution limits may be set far too high. In North America, the limits are set much lower, and we now have good evidence they should be similar in Europe. For example, current European nitrogen limits may need to be cut by half. Our trees in Europe are not more tolerant than those in North America – their fungi are just suffering more.”

The researchers discovered that the characteristics of a tree, including nutrient status and species, and local environmental conditions – soil variables and atmospheric pollution – were the best predictors of which mycorrhizal fungi species would be present, and to what extent.

While minerals like phosphorus and nitrogen are essential for life, they can be hazardous at high enough concentrations, acting as pollutants rather than nutrients. The new report establishes “thresholds” of these elements – concentrations above which change the community of mycorrhizae.

Some mycorrhizal fungi are replaced by those that are more tolerant of pollution, including species that can capitalize on excess nitrogen from air pollution.

These ecosystem variations can negatively impact tree health.

For example, the researchers propose that some community changes lead to more “parasitic” mycorrhizae which take carbon without providing much in the way of nutrients.

The team says these initial results should be used to craft new in-depth studies into the known connection between mycorrhizae, pollution, soil and tree growth and health.

“The study throws up many new questions in tree health and mycorrhizal diversity,” said first author Sietse van der Linde, who worked as a research associate at Imperial and Kew Gardens during the study. “For example, we found that while mycorrhizae are more specialized than expected – i.e. the majority of species will only associate with certain types of trees – specialist fungi were also less adaptable to changing conditions.”

Laura M. Suz, mycology research leader at Kew Gardens, said the thresholds revealed by the team’s research should inform forest management.

“From now on, with this wealth of new information we can take a broader view of fungi and forests across the continent, and also design new fungal monitoring systems, using this study as the first ever underground baseline to test directly for large-scale drivers of change,” Suz said.

Bidartondo added the study opens the door to investigating more in-depth questions.

“For example, we did not expect that the levels of potassium would affect mycorrhizae as strongly as they did, but we can now try to determine why that is, and what impact potassium levels are having on our forests,” he said.

This is a composite image of some of the hundreds of sample sites in European forests. (Sietse van der Linde)

 

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