Dassel. Gnarled dry branches instead of large, round crowns: This is what many ash trees in Germany have looked like for years. An introduced fungus is threatening the tree species. Bitter: The ash is actually an ideal tree for global warming. Researchers therefore want to save the tree species - and have a plan.
Floors, stairs, kitchens or hockey sticks - ash wood can be used in many different ways. More than 900 animal and plant species are also related to the tree species, explains Christopher Sander. The scientist is conducting research at the Northwest German Forestry Research Institute on so-called ash shoot dieback - the disease that has been troubling the tree species for years.
Ash trees feel particularly comfortable in very dry and hot areas, as forester Hendrik Büning explains. In his forest in Solling, southern Lower Saxony, a few dozen of them stand along an avenue on a forest path. Büning doesn't know how long they can stand there. The only thing that is certain is that at some point he has to cut them down or they will break off on their own. Because the trees are infected with false white stem cup, a fungus from East Asia. It is the reason for the dieback of ash shoots, which was first discovered in Germany around the turn of the millennium.
The fungal species is deadly for native ash trees. Research leader Gitta Langer from the Northwest German Forest Research Institute explains: The fungus attacks the trees, among other things, on the leaves, falls to the ground with them in autumn and forms spores in the spring that attack the tree again - until at some point only individual branches and later the roots and the entire tree die. What is crucial is that, unlike East Asian ash trees, the fungus in European ash trees also grows from the leaf into the wood. The local white stem cup doesn't do that. The tens of thousands of infections with the foreign fungus become a problem for the tree, which also becomes susceptible to other diseases.
How far the infection has progressed is difficult to tell from the outside. Mushrooms at the root and a thin crown are indications. “As long as the trunk is still alive, new shoots can still be formed,” explains Langer. The time until death varies by several years. And then there are ash trees that are hardly susceptible to the fungus.
But why? Gitta Langer and her team also ask themselves this question. The researchers have now found some of these hopeful specimens. They are used to grow new trees in the laboratory. Because: “There is currently no cure,” emphasizes Langer.
However, the less vulnerable trees will probably not be enough to carry the ash tree into the future. There are simply too few of them and their gene pool is therefore too small, explains Langer. They would therefore also have to be crossed with genetic material from the more susceptible trees.
This is where the ash trees on the forest path in Solling come into play again. In many regions, such as Hardegsen a few kilometers further, infected ash trees are felled using harvesting machines. It would be too dangerous to leave them in the forest, for example when forest workers are on duty there, explains forester Nils Flechtner.
In Solling, however, the infected trees are allowed to continue to grow. Among other things, because of a rare lichen that lives on them. To be on the safe side, dead branches are regularly cut off - actually “too time-consuming and expensive,” says forester Büning.
In addition to crossing with less susceptible trees, the researchers also have another hope for the ash tree's survival: adaptation. Other tree species have also repeatedly shown that they can adapt to changing conditions, explains Michael Rudolph from the Lower Saxony State Forests, which is responsible for the state forest. But the old ash trees could no longer do that. This requires long-lived ash trees that reproduce naturally.
For the researchers and foresters in the forests, it's a game of time. “We have to bridge the time until the fungus and tree have adapted,” says forester Büning. But that shouldn't take too long: the next threat, the Asian ash beetle, is already on the way. The weakened ash trees are a hit. In Europe, the beetle has already been detected in Slovakia and Hungary, among others.
Climate change could help: According to Langer, the hot temperatures have recently slowed the growth of the dangerous fungus somewhat. However, planting East Asian ash trees would be of little help. Although they are adapted to the fungus, they could bring new pests that pose a threat to other European trees.
If the ash tree could be saved, it would be good news for the forestry industry. Of course, the ash is not the one miracle tree for climate change, as Michael Rudolph emphasizes. It is too uncertain what threats may arise in the future. In addition, the species currently only accounts for a few percent of the forest area; will not be planted at all because of the shoot death.
But the ash could be one of many tree species for the future. It grows quickly and is ready to harvest after 60 to 70 years, even if it is usually left for longer. For comparison: Spruce is harvested between 80 and 100 years ago.
