The biodiversity of European forests is undergoing a silent and unexpected transformation. Or maybe not anymore.
A study published in Science, conducted by a team of 31 international scientists led by Pieter Sanczuk of the Forest & Nature Lab at Ghent University, revealed that European forest plants are migrating westwardand not just northward as one would have expected due to the climate change.
The work, entitled “Unexpected westward range shifts in European forest plants link to nitrogen deposition”, analyzed the shifts of forest plants over several decades, using data from 2954 permanent and semi-permanent vegetation areas distributed in five European biogeographical regions: Atlantic, Alpine, Boreal, Continental and Pannonian. The first measurements date back to 1933 and were accompanied by subsequent surveys between 1987 and 2017, with median intervals of approximately 39 years between one survey and the next.
How the study was carried out
To make these findings, the researchers compiled a large database of vegetation patches, located in forests that have not been converted to other uses since the 18th century. These plots, distributed in the main European biogeographical regions, were monitored over time to evaluate the presence and abundance of plants, focusing in particular on those less than 1.3 meters tall. The analyzes did not include intensively managed plantations or areas that had experienced major disturbances such as logging.
The data collected on this large sample of vegetation showed an unexpected result: 39% of monitored plant species are moving westwardwhile only 15% is heading north, as would be expected due to global warming. The main culprit for this westward shift is atmospheric nitrogen.
Nitrogen as a driver of plant migration
The high levels of nitrogen, coming mainly from pollution generated by traffic, industries and agricultureare profoundly changing the composition of European forests. This gas, which arrives in the form of rain and dust, acts like a fertilizerfavoring the growth of plant species more tolerant to these conditions, often to the detriment of more specialized ones. Nitrogen-loving plants such as wood sorrel and silver fir seedlingsare rapidly expanding westward, where pollution is greater, and aggressively competing with local vegetation.
Pieter Sanczuk and his team highlight that «Nitrogen pollution, and not just climate change, is the main factor behind these surprising shifts. Nitrogen deposition allows a rapid spread of highly competitive species, which end up establishing themselves in areas with high levels of atmospheric nitrogen to the detriment of specialized plant species.”
The ecological consequences of westward migration
Nitrogen tolerant species they are supplanting native vegetationwith potentially devastating consequences for local biodiversity. Specialized species, which require habitats with few nutrients, risk being replaced by more generalist and invasive plants.
Ingmar Staudeco-author of the study and researcher at the University of Leipzig, explained: «Climate change is often cited as the main cause of changes in the range of plant species, but in recent decades we have seen that nitrogen deposition is the real driving force. This raises questions about how ecosystems can adapt to higher temperatures if the change in biodiversity is dominated by other environmental factors, particularly air pollution.”
The long-term implications
The analysis of the vegetation areas conducted by the researchers is one of the most complete ever carried out. The plots studied did not suffer major disturbances between initial and subsequent surveys, and the data collected represent an invaluable resource for understanding long-term changes in European forest ecosystems. The results show that plant species they are moving at an average speed of 3.56 kilometers per year.
“Understanding the interactions between climate change and nitrogen pollution is crucial for land managers and policy makers,” say the study authors. “Only through stricter regulation of emissions and intelligent forest management can we hope to protect the biodiversity of our forests.”