Dying forests: How researchers are trying to save the ash – knowledge

In Norse mythology it is said that when the world tree Yggdrasil begins to wither, all life is about to perish. Yggdrasil unites the worlds of humans, elves, dwarves, giants and gods. And Yggdrasil is an ash tree.

You don’t have to believe in elves and dwarves, but the withering of the distinctive feathery ash leaves, the brownish-black discoloration on the shoots, cancerous swellings and dead tissue on ash trunks can no longer be overlooked in the forests. And that with the mass death of the ash trees, a piece of the world as we know it will perish – that sounds pathetic, but it’s not entirely unrealistic.

In the case of Fraxinus excelsior, the common ash tree found in Germany, the number of regrowing trees is currently even higher than that of dying trees. That’s why Gitta Langer, phytopathologist and head of the department for mycology and complex diseases at the Northwest German Forest Research Institute, speaks more of a race between pathogens and trees: “At the moment, the common ash is not threatened with extinction in our country. But whether it will survive as a commercial tree, is the big question. And whether we can rely on the ash tree when it comes to the climate-friendly conversion of the forests is also not certain.”

Planting and sowing the ash is not recommended

As of today: no, forest farmers no longer rely on ash trees. The official specialist journals of the state forest administrations advise against planting and sowing ash trees.

From earliest times, the tree has been worshiped in myths. Its wood was ideal for tool handles and weapons because of its hardness and pliability. According to legend, Achilles killed Hector with a spear made of ash wood, grown on Mount Pelion, carved by the centaur Chiron. To this day, ash is used to make sports equipment, parquet flooring and toys. Leaves and roots were considered a remedy for snake bites, fresh bark was said to heal small wounds. Ash leaves in tea help against rheumatism. The ash feels at home on dry, calcareous soils as well as where it quickly becomes too wet for other tree species, on streams or alluvial forests. It was considered an important support in the ecosystem to cushion global warming.

But then came the mushroom. Since the first confirmed detection twenty years ago in Schleswig-Holstein, the phytopathologists have been powerlessly following how the crowns of the infected trees have thinned out year by year. In Japan, a German mycologist finally came across descriptions of the same fungus, from then on everyone agreed: It is the false white stem cup, Hymenoscyphus fraxineus, original registration address: Asia. The import of just one or two ash plants brought Hymenoscyphus to Poland in the 1990s.

The mortality rate increases for young and old ash trees. Alarmed by the dramatic spread, governments in many European countries have pumped money into research into control strategies. In Switzerland, where Hymenoscyphus has already relegated the ash from second to third place among the most common tree species, the “Resistash” or “Survivash” research initiatives sound like military rearmament. In Germany, people prefer to let hope resonate in the project name and in 2020 the joint project “Frax for Future” was equipped with almost ten million euros from the forest climate fund.

The four main areas with a total of 27 sub-projects deal with the genes of the pathogen and the host as well as the relationships between the organisms, the possibilities of long-term monitoring of the damage and the question of how best to deal with the affected forests in terms of silviculture.

Phytopathologist Langer heads the focus in the research network, which deals with the interaction between fungus and ash. What is already certain: Actually, the infection of stem and root is a dead end for the fungus. “For Hymenoscyphus, it makes absolutely no sense to harm the ash tree for the purpose of reproduction. If the host population dies, it just ends up depriving itself of its own livelihood.” With the import of the fungus, humans have provoked a biological incident that, contrary to all evolutionary logic, causes long-term damage to host and fungus alike.

In Asia, the fungus lives in company with the Manchurian ash without showing any symptoms. The mycelium grows into the petiole. At some point the leaf falls to the ground, the fungus hibernates and forms the fruiting body and thousands of spores the next year. The cycle starts over. In the case of the common ash tree in Europe, on the other hand, the fungus continues to grow into the small shoots, where it damages the pathways for water and nutrients and the growth tissue. The shoots died the next spring. “We don’t know what mechanism it uses to penetrate the leaves,” says Langer. “The spores also penetrate into the base of the trunk, which leads to damage there, so-called necrosis, through which other dangerous pathogens can penetrate. The tree constantly loses leaves and can make less photosynthesis and at the same time is attacked at the base of the trunk by rot fungi and insects.” In the fallen ash leaves, the fungi can produce hundreds of thousands of spores for at least five years – in terms of plant hygiene: a nightmare.

Open detailed view

Combating the fungus with chemicals, such as a broad-spectrum mycotic, is strictly forbidden in the forest: “Without fungi, there can be no forest,” says Gitta Langer. Fungi live in community with trees, provide nutrients and water, break down dead wood, ensure the exchange of information between plants and thus keep the cycle of forest life going.

Langer and the teams she coordinates pursue two strategies: weaken the pathogen and strengthen the host. In one project, the route of infection via the trunk is simulated under artificial conditions in the laboratory and in the greenhouse. If you know how the tree is trying to defend itself, you might be able to support its counter-strategies. Another approach aims to find less aggressive fungal individuals within the stem cup population. In theory, in the spirit of assisted evolution, one could anticipate natural selection and try to give these more benign fungal strains an advantage, pushing them out on their deadly relatives.

Is it possible to weaken the fungus with a virus?

Researchers are digging deeper into the epigenetic bag of tricks with an RNA interference experiment. Gabi Krczal from Agro Science, an institute for plant research in Rhineland-Palatinate, is trying to start where the DNA information of the fungus is read for the construction of proteins. The production of proteins should be specifically prevented by means of double-stranded RNA introduced from the outside. Essential genes are practically muted. “Components for RNA interference and their functions have already been identified in many fungal species, and we have also been able to do this for Hymenoscyphus,” says Krczal. The artificial RNA molecules will be tested on infected ash seedlings.

The search for possible opponents of the fungus also looks promising. In the case of another tree disease, chestnut bark canker, viruses are already being used, which are let loose on the fungal pathogens and inhibit them. Cornelia Heinze from the Institute for Plant Sciences and Microbiology in Hamburg is currently isolating viruses from all kinds of fungi that exist in Germany. After hundreds of samples, some viruses were discovered that could be troubling Hymenoscyphus. Cornelia Heinze sees the main advantage of the strategy that, once the virus has been transferred to Hymenoscyphus, it could spread naturally among the population: “On the other hand, it cannot spread to another type of fungus, which this method also ensures with regard to ecological concerns power.”

In a clone archive, the ash stock of the future would be secured in the long term

The researchers at the Thünen Institute are also hoping that there are ash trees in nature that are more resistant than others and will recover from ash dieback. The aim here would be to genetically decode the resistance and to reproduce the particularly resistant trees by grafting. In a clone archive, the ash stock of the future would be secured in the long term.

In Norse mythology, the dragon Nidhöggr gnaws on the world ash. Hymenoscyphus gnaws on the ash shoots and trunk in the here and now. At least twice a year, the 14 test plots distributed across Germany from north to south are checked to see how the trees are doing: recently, no one was found that was healthy.

There are now approaches on how to counteract the culprit. But there is a lot of basic work that is still a long way from concrete control measures in the forest. In the case of gene regulation and the use of viruses, there will have to be extensive tests to determine whether other dangers are lurking in these interventions. Then the approval process follows. All this takes time, a lot of time. That’s how long Hymenoscyphus goes on. Funding for “Frax for Future” has been secured for another year and a half.