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It is tiny, but its effect is devastating: the ash dieback pathogen, Hymenoscyphus fraxineus, attacks the Common ash (Fraxinus excelsior), causing its shoots to die. The pest is a fungus that was probably introduced into Europe from East Asia in the 1990s with imported ash plants. Since then it has spread epidemically, also in Switzerland. More than 90 percent of all ash trees in Switzerland are infected, and many have died.
The fungus forms its fruiting bodies and thus its spores on ash leaves on the ground. It is impossible to remove it from the forest. But ten percent of Swiss ash seem to be resistant to the pathogen, or at least to tolerate it. This is where research comes into play. When Swiss Forest Protection, whose office is based at WSL, asked foresters to identify healthy trees, 397 ash trees were reported. Specialists from an environmental engineering firm inspected these trees in 2018 and recorded various data on the sites where they were found, such as regeneration, the forest communities and the density of the ash trees. The aim is to find out why these trees are resistant to the fungus.
“There are certainly more healthy ashes than the 397 trees reported. But it is difficult to identify them in a stand where many of the ash trees are diseased,” explains Valentin Queloz, Head of Swiss Forest Protection. The ash trees found are now being tested further in the laboratory. Genetic analyses could shed light on why some ash trees are susceptible to the fungus and others are not.
While researchers are eagerly looking for solutions to the ash dieback, practitioners are also challenged. “The still healthy ash trees should be spared and the ash regeneration under these trees should be given enough light to have a chance at all,” says Valentin. The future of the ash in the Swiss forests may depend on these trees. (Lisa Bose, Diagonal 1/19)