Vital mycorrhizae in tree soils "vulnerable to air pollution"

Symptoms of malnutrition in trees across Europe could be down to stresses on the mycorrhizal fungi communities around their roots, according to an extensive ten-year study.

Some of the forests sampled - image: Sietse van der Linde
Some of the forests sampled - image: Sietse van der Linde

Led by Imperial College London and the Royal Botanic Gardens, Kew, the study examined 13,000 soil samples from 137 forest sites in 20 European countries, in order to identify large-scale trends in mycorrhizal communities, including their tolerance to pollution.

The team found that tree species and nutrient status, and the local environmental conditions of atmospheric pollution and soil variables, were the most important predictors of which species of mycorrhizal fungi would be present and their abundances.

Other recent studies have noted signs of tree malnutrition across Europe, such as discoloured leaves or leaves lost from the crown, without identifying the mechanisms behind these.

Lead researcher Dr Martin Bidartondo, from the Department of Life Sciences at Imperial and Kew Gardens, said: "There is an alarming trend of tree malnutrition across Europe, which leaves forests vulnerable to pests, disease and climate change.

"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. Our trees are not more tolerant than those in North America - their fungi are just suffering more."

Minerals like nitrogen and phosphorus can damage organisms in high concentrations, and the study discovered thresholds of these elements' concentrations above which the mycorrhizal community changes, with some fungi outcompeted by those more tolerant of pollution.

The reserachers propose that some of these community changes favour more "parasitic" mycorrhizae that take carbon but give little back in the way of nutrients.

First author Dr Sietse van der Linde, who worked at Imperial and Kew Gardens during the research, added: "We found that while mycorrhizae are more specialised 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."

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