Heat and drought threaten the truffle harvest

Since 2011, WSL has been running a monitoring programme for the Burgundy truffle, a common and popular edible mushroom in Europe. At the study sites in Switzerland and southern Germany, volunteers and researchers who own truffle dogs harvest the fruiting bodies at "their" site every three weeks. They weigh, measure and describe them, and send a piece to WSL for genetic testing. Tree growth and climate data are also measured at the site.

Using computer models, ecologist Brian Steidinger from the University of Konstanz and WSL evaluated this information in conjunction with climate data. He found a clear decrease in truffle abundance in dry and hot years. "Our results show that the Burgundy truffle is endangered by an alarming trend towards increasing summer drought in Europe," says Steidinger. The results have now been published in the journal Global Change Biology.

One-quarter less truffles per degree of heat

A one-degree higher average summer temperature lowers the yield by almost a quarter (22%), in some sites even up to 70% less. With three degrees more warmth, the truffle hunters found no truffles at all. "However, the downward trend over the years was interrupted in the exceptionally cool and wet summer of 2014, when many truffle fruiting bodies were formed," says Martina Peter, head of the WSL Ecological Genetics research group. According to this, the underground fungal filaments, the so-called fungal mycelium, should still be present, but the fruiting body formation is affected.

The result took the researchers unaware: Burgundy truffles are also common in very dry areas, for example in Spain, and are thus considered undemanding. Apparently, however, the Central European varieties do not tolerate the same high temperatures as the southern ones. "This suggests that the truffles may form many small, locally adapted populations and that they may not be able to tolerate the whole range of temperatures in their area of distribution," says Peter. The reason for this is probably that truffles tend to genetically exchange within a small area, as Peter has found out in corresponding studies. "So the more southern genetic variants that are now advantageous with climate change probably don't arrive here fast enough."

Because extreme weather conditions are likely to increase with climate change, Swiss and German truffle hunters could increasingly go empty-handed: "If no fruiting bodies are produced, some truffle occurrences could die out," says Peter. One possible solution would be to identify drought-resistant truffle varieties and seed them in local trees. "You have to carefully check whether this makes financial and ecological sense or does more harm than good," says Peter.

In the longer term, the decrease in fruiting bodies is a problem for the fungus, because these produce spores and ensure its sexual reproduction. During this process, the genetic material is mixed anew. Spores that bring new genetic variants increase genetic diversity, which might facilitate the adaptation to new environmental conditions. "Our results suggest that these truffles will confront the challenges of climate change with less flexibility than expected," says Steidinger. Truffles are symbiotic fungi that supply their host trees with vital nutrients and water. The new findings are thus of importance for forest ecosystems under climate change.