Spruces are being eaten by bark beetles, and jewelled oak beetles are attacking oak trees. Beech and maple trees are also dying. Germans have to say goodbye to the German forest.
Ossified oaks, sparse beech groves and deep, dark forests of spruce: the forests that people like to roam through during their vacation weeks right now will not be around for much longer, according to forestry experts. “People like to imagine the forest as it was in their childhood, but the forests are changing now,” says Henrik Hartmann, head of the Institute for Forest Protection at the Julius Kühn Institute in Quedlinburg.
The trees are struggling with the effects of climate change. Heat waves, long dry periods and storms are weakening them. While the forest has suffered from the extreme weather of recent years, many pests such as insects and fungi have benefited from the rising temperatures. They are thus driving forward forest conversion, says Ralf Petercord, silviculture expert at the Forestry Ministry in North Rhine-Westphalia.
Spruce: Bark beetle larvae eat the bast layer
This is most clearly seen in the spruce trees. For centuries, forestry relied on the fast-growing trees, and pure stands emerged everywhere – which in recent years have been a feast for the larvae of the spruce bark beetle. In some regions, such as the Harz Mountains, the insects have appeared in such large numbers since 2018 that there are hardly any old spruce forests left there. The spruce has the highest mortality rate of all tree species.
At least the high altitudes of the German low mountain ranges were still considered safe by many, but even where it was once cool and moist enough for spruce trees, the bark beetle is attacking the stressed conifers en masse. “Even at altitudes of 1000 to 1500 meters, the temperatures are sufficient for the bark beetle to swarm, attack spruce trees and reproduce,” says Markus Kautz from the Baden-Württemberg Forest Research Institute. Pure spruce forests may ultimately only remain in the rough high altitudes of the Alps.
Oaks: Jewel beetles
Almost half of the oaks examined in the most recent forest condition survey also showed a significant thinning of the crown. Such weakened trees are sought out by the two-spotted oak beetle, whose larvae live under the bark in a similar way to those of the bark beetle. In some regions, the beetle has already caused damage to pedunculate and sessile oaks that threatens the survival of the species.
“If oak forests have already been damaged, for example by the oak-feeding community, by flooding or something else, then the beetle can become a problem,” says Dominik Wonsack, also from the Baden-Württemberg Research and Testing Institute. “Then the trees can no longer fend off the beetle.” Then entire oak stands could die.
Book: Complex disease causes problems
Without human influence, the forests in Germany would be dominated by beech trees. Beech species currently make up 16 percent – but they are often not doing well either. “By the end of the century, beech trees will no longer be able to grow to a height of 30 to 40 meters, and the forests will become sparser and lower,” predicts Hartmann, director of the Quedlinburg institute.
In addition, the so-called beech complex disease, which often occurs after a heat or drought event, is causing problems. “It starts with cracks in the trunk and a slime flow.” Then the bark comes off, the wood rots and various fungi and wood-breeding insects such as the beech bark beetle appear. “We don’t know 100 percent what is going on,” says Hartmann. It may be related to bacterial infestation.
Ash and maple trees attacked by fungi
With maples, however, the enemy is known: the fungus Cryptostroma corticale. If it attacks a tree, the bark peels off in large areas, and sooty black dust appears underneath: spores of the fungus. Ten years ago, the sooty bark disease was not even an issue among foresters, says Hartmann, but now entire stands are infected. “This is usually fatal for the tree.”
The native ash populations have been massively decimated by ash dieback. The reason is yet another fungus: false white stem cup (Hymenoscyphus pseudoalbidus). If it attacks trees, shoots and branches die, leaves wither and dry up, the trunk changes color – ultimately the ash tree dies. Recently, however, researchers at the Leibniz Centre for Agricultural Landscape Research (ZALF) described a new idea in a study: they found a bacterium on healthy ash trees that can inhibit the growth of the fungus. Research is to be carried out to see whether this helps in practice.
Balance no longer works
“Basically, the trees defend themselves against insects and fungal infestation, and they do so very effectively,” says forestry expert Petercord from North Rhine-Westphalia. “But the balance between trees and the other organisms often no longer works.” If it is too dry, for example, rot pathogens could enter through the roots, or the tree does not have enough strength to produce defenses against nibbling butterflies.
Petercord has actually already given up on the ash, and he is also very critical of the sycamore maple and the red beech. The development of the oak is also not good. “The main tree species will all lose area,” he is certain. In return, other species have the chance to grow much more frequently in Germany: the hornbeam, for example, the European white elm or the alder.
What was native does not have to remain native
One thing is clear to the experts: pure stands have no future. Spruce forests have been converted into mixed forests for more than 30 years, as the German Forest Protection Association explains. And elsewhere? Which trees should foresters plant now? “We need a variety of approaches because we don’t know what the climate will be like,” says Institute Director Hartmann. All experts are talking about a permanent mixed forest in which young trees stand next to trees that are as old as possible.
Hartmann believes that we need to try out which species should be included. “We can keep native tree species, but not everything that was native up to now will still be native at the end of the century,” says the scientist. That is why species from other regions of the world are also coming into focus. “We should look at these species differently, not as alien species, but perhaps as native species in the future.”
A new European study involving the Thünen Institute of Forest Ecosystems also comes to the conclusion that forests should be supplemented with trees from other regions in the future. The report speaks of “assisted migration”, in which tree species are selected from distant regions because they are best adapted to the future climate. After all, trees planted today would have to be able to cope with the climate in 100 years.
Forest condition survey 2023 Bavarian Forest National Park: Spruce bark beetle situation Article on the bacterium against ash dieback Forest knowledge: Beech complex disease Forest knowledge: Sooty bark disease LWF Bavaria: Ash dieback Waldwissen.net on ash dieback