Nitrogen pollution causes shifts in distribution of European forest plants

Biodiversity under threat

Nitrogen deposition caused by air pollution, rather than climate change, turns out to be the primary driver behind shifts in the distribution of European forest plants. Accordingly, these plants are found to be moving westward more frequently than northward. An international team of researchers, including Utrecht University ecologist Jonas Lembrechts, demonstrates this in a publication in Science. The results indicate that biodiversity shifts are influenced not only by global warming, but by a combination of human-induced ecosystem disturbances.

Climate change can cause the distribution of species to change. For instance, it is often assumed that global warming causes plant species in Europe to shift from the warmer south to the colder north. But is this truly the case?

Jonas Lembrechts

To be able to answer this question, Lembrechts and his colleagues analysed the movement of 266 forest plant species across Europe over recent decades. They used data from European forests, where surveys were conducted at various points in time to determine exactly which plant species were present. Some of these locations were first surveyed as early as 1933. The study also included data from the Speulderbos forest in the Veluwe region near Garderen.

Nitrogen generalists

The researchers found that European forest plants were 2.6 times more likely to migrate westward than northward. When they examined the environmental factors that best explained this movement, they discovered that nitrogen deposition from atmospheric pollution, rather than climate change, was the primary driver. Additionally, nitrogen generalists—plants that are not significantly impacted by high nitrogen levels—were particularly likely to have moved westward. While sulphur pollution from historic acid rain may also have contributed to these shifts, it does not appear to be the main factor.

It is like playing a lottery; you do not know with which species things will really turn bad in the end.

Less biodiversity

The rapid spread of nitrogen generalists is occurring at the expense of plant species that thrive in nutrient-poor soils. These plants are often more specialized, meaning they need specific conditions to grow. As nitrogen levels in the soil increase, these conditions disappear and areas become more similar.

Lembrechts: “It is clear that if nitrogen emissions in the Netherlands do not decrease, plant diversity will become increasingly impoverished. The forests will then be dominated by brambles, nettles, and other nitrogen generalists, such as spruce, cow parsley, and Norway maples.”

If we want to preserve biodiversity in the future, it is essential to understand the complex interactions between these various factors.

Airplane analogy

With these shifts in plant species, the animals and other creatures that rely on them are also at risk.

Lembrechts: “When people ask me why the disappearance of species is a concern, I like to use the more commonly made analogy of an airplane. An airplane is made up of many different parts, each serving a purpose. While a plane might not crash immediately if it loses a few components - though an observant passenger might feel uneasy seeing a screw fly past the window - there comes a point when the plane is no longer safe. You just cannot predict which missing part will be the last straw. Nature operates in a similar way. Every species has a role, and while we can afford to lose some species, it is like playing a lottery; you do not know with which species things will really turn bad in the end. And we do need healthy nature, as it provides essential services, such as clean air, clean water, healthy soil, and crop pollination.”

Climate change

Lembrechts emphasizes that the study, which was led by a team from Ghent University, does not imply that climate change has no impact on species displacement. Instead, the results indicate that the movement of forest plants is affected by a combination of human influences, not solely global warming. Lembrechts: “If we want to preserve biodiversity in the future, it is essential to understand the complex interactions between these various factors.”

Publication

Unexpected westward range shifts in European forest plants links to nitrogen deposition
Science, 10 October 2024. DOI: 10.1126/science.ado0878

Pieter Sanczuk, Kris Verheyen, Jonathan Lenoir, Florian Zellweger, Jonas J. Lembrechts*, Francisco Rodriguez-Sanchez, Lander Baeten, Markus Bernhardt-Römermann, Karen De Pauw, Pieter Vangansbeke, Michael P. Perring, Imre Berki, Anne Bjorkman, Jörg Brunet, Markéta Chudomelová, Emiel De Lombaerde, Guillaume Decocq, Thomas Dirnböck, Tomasz Durak, Caroline Greiser, Radim Hédl, Thilo Heinken, Ute Jandt, Bogdan Jaroszewicz, Martin Kopecký, Dries Landuyt, Martin Macek, František Máliš, Tobias Naaf, Thomas A. Nagel, Petr Petřík, Kamila Reczynska, Wolfgang Schmidt, Tibor Standovár, Ingmar Staude, Krzysztof Świerkosz, Balázs Teleki, Thomas Vanneste, Ondrej Vild, Donald Waller, Pieter De Frenne

*Affiliated with Utrecht University