Displacement in the undergrowth

Forest floor

Plants on the forest floor. (Image: Martin Adámek / Czech Academy of Sciences)

A gradual exchange is taking place in the forests of Europe: Many only locally distributed plant species of the undergrowth disappear and are replaced by more common, more widespread species or even non-native species, as a Europe-wide forest study now reveals. The driving force behind this exchange, according to the researchers, is above all the increased nitrogen input into the forest soils – with it, rarer, more specialized plant species are particularly difficult to cope with.

On a global scale, humans have long had a profound influence on biodiversity – it is disappearing more and more. However, it is less clear how biodiversity develops at the local level. “Long-term studies have found that some locations lose species while others gain species without there appearing to be a uniform global trend,” explains Ingmar Staude from the German Center for Integrative Biodiversity Research (iDiv) and his colleagues.

Number of species remains the same, but composition does not

Staude and his team have now investigated how these contradicting trends can be explained and how it looks in Europe’s forests. To do this, they evaluated the data from a total of 68 different locations in temperate forests in Europe, including near-natural forest areas in Thuringia, Brandenburg and Bavaria. Specifically, the researchers wanted to know how the composition of the herb layer in the undergrowth of these forests has changed over the past 40 years. In total, they recorded the stock numbers of 1162 different herbaceous plant species.

The evaluation showed that the number of species at the individual locations had hardly changed, but the species composition did. Many only locally occurring species had disappeared over time, but other, also supraregional plants had taken their place. “On average, the plants that died out at one location had a smaller distribution area than the species that were preserved or were added,” report Staude and his colleagues. “On average, a plant species with average stocking had a ten percent chance of disappearing from a location, whereas it was 60 percent for a plant species with little distribution.”

Nitrogen input promotes gradual unification

However, this means that over the course of time there will be a creeping fight in the undergrowth of our forests: herbs that only occur locally and on a small scale will be replaced by more common, more widespread or introduced species. As a result, the biodiversity in the individual forests does not decrease, but the biodiversity in the entirety of European forests does, because their plant composition is becoming more and more similar. According to the researchers, the overall diversity of species in Europe’s estimated and near-natural forests has already decreased by four percent in the past decades. If one also included forests used for forestry, the decline could be even greater.

But what is the reason for this insidious exchange? “This is not so much due to the smaller population size of such plants, but rather to their ecological niche,” explains Staude. Because less widespread species are often specialists with a smaller tolerance range for certain environmental conditions. In the case of forest herbs, the study found a connection with the increased nitrogen input into the forest soils: “Our results show that the likelihood of extinction of these species increases with nitrogen availability,” the researchers said. According to this, plant species that prefer nutrient-rich soils, such as nettle or blackberry, benefit from the higher nitrogen values. This gives them a competitive advantage over local species that are adapted to poorer soils and is spreading more widely.

“We now have to find out whether the processes that we observe in the forest are similar in other biomes,” says Staude. He and his colleague therefore want to evaluate data on a large scale again – this time, however, for several biomes, for example for the European lean grass and the alpine tundra.

Source: German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig; Specialist article: Nature Ecology & Evolution, doi: 10.1038 / s41559-020-1176-8

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