What's Making Europe's Trees So Sick?

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©. Sietse van der Linde

Pollution appears to be causing a distressing trend of malnutrition for Europe's arboreal citizens.

There has been a worrisome trend of tree malnutrition spreading across Europe, leaving once stalwart forests vulnerable to threats. And we only have ourselves to blame.

A new and comprehensive study, spanning 10 years of research, looked at 13,000 soil samples across 20 European countries. The researchers conclude that many tree fungi communities are being stressed by pollution, indicating what some may call obvious: Current pollution limits may not be strict enough.

“There is an alarming trend of tree malnutrition across Europe, which leaves forests vulnerable to pests, disease and climate change," says lead researcher Dr. Martin Bidartondo, from the Department of Life Sciences at Imperial and Kew Gardens. "To see if changes in mycorrhizae [fungi] might be behind this trend, we opened the ‘black box’ of soil. Processes happening in soil and roots are often ignored, assumed or modelled, because studying them directly is difficult, but it is crucial for assessing tree functioning."

Most simply explained, pollution is changing the fungi that provide mineral nutrients to tree roots. The study, led by Imperial College London and the Royal Botanic Gardens, Kew, found that local air and soil quality have a large impact on mycorrhizal fungi , which they say could explain these sad malnutrition trends in Europe's trees.

Plants and fungi love each other and have an important symbiotic relationship. While we know some of these mycorrhizal fungi from their above-ground forms in the shape of mushrooms and truffles, trees host these fungi in their roots underground to get nutrients from the soil. In return for their gifts of essential nutrients, like nitrogen, phosphorus and potassium, the fungi receive carbon from the tree. Without these offerings, the trees go hungry. Which would explain the signs of tree malnutrition across Europe, like discolored leaves and thinning crowns.

The researchers, found that the characteristics of the tree (species and nutrient status) and the local environmental conditions (atmospheric pollution and soil variables) were the most important predictors of which species of mycorrhizal fungi would be present and their abundances notes a press statement from Imperial College.

While essential for life, an overabundance of minerals like nitrogen and phosphorus – from pollution – can be damaging. The study discovered thresholds of these elements – the points at which the community of mycorrhizae changes. And the species of fungi that are more tolerant to pollution – like ones that can use excess nitrogen from air pollution to their advantage – are outcompeting the ones who suffer. The press statement notes:

These ecosystem changes can negatively affect tree health. For example, the team proposes that some community changes result in more ‘parasitic’ mycorrhizae: those that take carbon but give little back in the way of nutrients.

As bad as it is, at least now there is solid research that can be employed to design new in-depth studies into the link between pollution, soil, mycorrhizae, tree growth and tree health.

First author Dr Sietse van der Linde, who worked at Imperial and Kew Gardens during the time of the research, says that, “The study throws up many new questions in tree health and mycorrhizal diversity."

“The thresholds uncovered in this study should impact how we manage our forests," adds Dr Laura M Suz, mycology research leader at Kew Gardens. "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.”

Another point that came as very surprising (to me, at least) was the comparison of European trees to those in the United States. I always think of Europe as being more advanced in environmental regulation. But Dr. Bidartondo says:

“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.”

'Environment and host as large-scale controls of ectomycorrhizal fungi’ by Sietse van der Linde et al. is published in Nature.