Mistletoe is a semi-parasite that lives on trees, which most people know mainly as a valuable magic potion ingredient from Asterix books, or as a door decoration in Advent: green twigs with white-glassy berries on them. At the moment, however, it is enough to simply look up at the sky, or more precisely at the treetops, to see mistletoe. Because the plants are spreading rapidly almost everywhere in Germany. In agriculture, but also in nature conservation, people are already talking about a plague.
Comprehensive data is not yet available, but in some regions there has been an increase in leaf or pine mistletoe of up to 30 percent in recent years. In addition, modeling studies show how the range could expand further north and east and up into the mountains. “Things are going well for the mistletoe,” says Hanno Schaefer, Professor of Biodiversity at the Technical University of Munich and also privately connected to mistletoe: As a child, he tried to sow mistletoe seeds in the local orchards. Today he’s cutting them out of those trees, a fight he doesn’t think he’s going to win. Because mistletoe, which can settle on more than 400 host tree species, particularly likes to attack old trees or trees that have been weakened by drought stress – and then withdraw water and nutrients from them until the trees eventually die.
There are at least two reasons for the rapid spread: On the one hand, climate change favors the growth of mistletoe in our latitudes, because the evergreen plant, which constantly carries out photosynthesis, is relatively sensitive to temperature, i.e. it does not like deep frost and also rather comfortable ones for germinating in early summer temperatures needed. After birds, mostly mistle thrushes, eat the sticky berries with the seeds in them in winter and stick these seeds to the nearest tree via the droppings, these seeds wait there for spring. If the temperature is right, it germinates and tries to overcome the tree’s immune system and grow with it – a process that can take several years.
Which brings us to the second reason for the spread: it used to be a matter of honor for people, especially in agriculture, to immediately cut mistletoe out of the trees to stop the spread. If you didn’t do that, you had to deal with the neighbors. Some of the branches on meadow orchards were even combed off with coarse brushes in winter to get rid of the sticky germs. But meadow orchards are no longer worth anything in agriculture. Taking care of the trees is no longer economically worthwhile, and a new generation of farmers has largely given up the arduous mistletoe control over the past one to two decades. A realistic means against the spread is currently not known.
“As a scientist you can say: That’s exciting,” says Hanno Schaefer, “but it’s also scary.” After all, it is unlikely that mistletoe will eventually overgrow the world unchecked. Because they need trees. When the trees die, the mistletoe dies too.