Nestled in the spruce trees in the Harz mountains of northern Germany is a bark-eating pest not much bigger than a sesame seed.
Known as “book printers” for the lines they eat into the bark that fan out from a single spine resembling words on a page, these eight-toothed beetles have always been part of the local forest. Officials expect the bugs to typically kill a few spruces each summer as they find suitable trees to lay their eggs — they burrow into the tree’s cambium, or growing layer, hampering it from getting the nutrients it needs to survive.
But the tiny insects have been causing outsized devastation to the forests in recent years, with officials grappling to get the pests under control before the spruce population is entirely decimated. Two-thirds of the spruce in the region have already been destroyed, said Alexander Ahrenhold from the Lower Saxony state forestry office, and as human-caused climate change makes the region drier and the trees more favorable homes for the beetles’ larvae, forest conservationists are preparing for the worst.
“Since 2018, we’ve had extremely dry summers and high temperatures, so almost all trees have had problems,” said Ahrenhold. Spruce trees in particular need a lot of water so having less of it weakens their defenses, and they’re not able to produce their natural tree resin repellent, he said.
As the planet warms, longer droughts are becoming more common around the world, with hotter temperatures also drying up moisture in soil and plants.
And even though the beetles tend to target weakened trees, in dry years the population can reproduce so much “that the beetles were even able to attack healthy spruce in large numbers,” he said. “In some regions there are now no more spruces.”
Experts say there’s no easy solution, but forest managers work to remove trees that might be susceptible to beetles as early as possible and use pesticides where they’re needed.
Michael Müller, the Chair of Forest Protection at the Technical University in Dresden, said there are “very strict requirements for the use of pesticides” which can be very effective in getting rid of the bugs, although the chemicals are sometimes frowned upon for their potentially harmful environmental side effects.
“It’s of course preferable to take the raw wood out of the forest and send it for recycling or to store it in non-endangered areas outside the forest,” he said, but noted that requires a separate logistical operation. On trees that are still standing, he said, it’s not really possible to remove the beetles.
Müller added that forest conservation measures can “sometimes take decades from being implemented to taking effect” and other factors, like storms and drought, and other species, such as game and mice that can also hamper plant growth, are potentially more damaging to the forest in the long run than the bark beetle.
But he said that conservation efforts are limited by external factors, like the changing climate. “After all, we can’t irrigate the forests,” he said.
In the longer term, mixing other tree species into the forest could be a solution, Ahrenhold said. “It makes sense to plant other conifers that can cope better with these conditions, especially on south-facing slopes and on very dry soil,” he said.
Having too many spruce trees in the Harz is a result of centuries of planting big concentrations of the tree, according to Richard Hölzl, an environmental historian at the Five Continents Museum in Munich.
“Clausthal-Zellerfeld is one prime example of a mining area in the Harz where they very early on tried to establish artificial reproduction for spruce to have it for mining construction works,” said Hölzl.
Officials realized by the 19th century that planting just one type of tree over and over again wasn’t a good idea ecologically, but “the economy countered that realization because spruce was such an attractive species,” he said. Spruce was the preferred tree for industrial forestry, paper mills and pulp.
Still, without the warmer and drier weather from climate change, the bark beetles wouldn’t be flourishing in all that spruce.
“There is a long, long prehistory, but there’s also the (climatic) change now,” said Hölzl. “We can’t really blame our forefathers for that.”
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