Acid rain, over-fertilization, scorching summers - the living conditions for plants in Europe have dramatically changed over the past decades. Until now, the big picture of how this affects biodiversity was missing.
"If you only pick some data from the last twenty years, it can quickly lead to incorrect conclusions," says ecologist Jürgen Dengler, funded by the SNSF under the National Research Programme "Biodiversity and Ecosystem Services" (NRP 82). He and his team at the Zurich University of Applied Sciences (ZHAW) participated in an international study that fills this gap.
The analysis was based on a vast database, the European Vegetation Archive (EVA). This contains over two million surveys conducted for research or conservation projects. Such investigations record plant species and their frequency within defined sample plots, typically ranging from one to 400 square meters. Everything is counted, from tiny forget-me-nots and daisies to thirty-meter-high beeches.
The European team utilized 650,000 of these surveys, conducted between 1960 and 2020, for a temporal analysis. For most of these areas, there was previously no information on factors like nutrients or light conditions, such as whether the soil contains a lot of nitrogen or if a dense canopy shades the forest floor.
The project team reconstructed this using the presence of so-called indicator plants that prefer specific environmental conditions. One example is dandelion, which thrives particularly well on nitrogen-rich meadow soils. "For the project, we developed a common European consensus system from over thirty different systems for classifying indicator plants," says Dengler.
Fertilizers, traffic, and industry pose challenges.
With the help of AI, the researchers then evaluated the survey records collected over decades. This allowed them to determine how plant communities have been composed over 60 years in four different habitats - forests, meadows, shrublands, and wetlands - and what has changed in that time. The indicator plant system helped link these new developments to a change in environmental conditions.
One trend stood out: there are more and more nitrogen-loving plant species in all habitats, such as large stinging nettles in forests and broad-leaved dock in meadows. The cause is evident. Due to fertilizers, livestock farming, and nitrogen oxides generated by traffic and industry, more and more nitrogen accumulates in the soil. The increase of such species is problematic as they displace plants like orchids that prefer nutrient-poor soils.
The good news for local biodiversity: Current studies show that this trend is now slightly declining in Switzerland. "It seems that regional measures such as the reduction of fertilizers are having an effect here. But across Europe, there is still no sign of this," says Dengler.
Further effects were only noticeable in certain habitats. Indicator values in meadows are shifting towards shade-loving species - possibly because the vegetation is becoming denser due to nutrient inputs or lack of management across Europe. According to Dengler, there are vast fallow areas in Eastern Europe due to socio-economic reasons. This overgrowth leads to less light reaching the ground. Small, sun-loving species like thyme and primroses, therefore, have fewer opportunities to thrive.
Temperature effects surprisingly minor.
One result was rather unexpected: "Vegetation responds much slower to temperature increases than we would predict," says Dengler. Native species are therefore not yet significantly displaced by warmth-loving plants from southern countries or other continents.
One explanation is that these species usually do not live in direct proximity and need to cover great distances for recolonization - either through seed dispersal or as accidental passengers during the transport of goods. This expected effect may thus be lagging behind rising temperatures.
An exception is the Swiss mountains. In recent years, it has been shown that increasing numbers of warmth-loving species are advancing to higher altitudes. These include typical lowland grasses such as perennial ryegrass or meadow foxtail. They do not have to cover long distances but only shift their habitat slightly upwards - hence this development might already be detectable.
In the coming years, Dengler will conduct analyses within the SNSF project VegCHange specifically for Switzerland. He intends to document changes in a fine grid of about 100 square kilometers - while the international research covered around 25,000 square kilometers: "We have just as much data as elsewhere. They are just not yet available in a centralized database."
The results aim to be prepared for practical use. Stakeholders, such as those from politics or conservation, can contribute additional research questions or desires for data processing. This initially happens in Graubünden, where data on diverse landscapes from arable land to moors to larch forests are available. Gradually, other cantons will be included as well.
"This way, we can identify where the greatest species losses in Switzerland are and then counteract with the right strategy," predicts Dengler. And it will likely also become clear where biodiversity is at a good level, and it suffices to maintain the status quo.