Migrating flora

French flora is not the same as it was 10 years ago. Researchers have just shown that the identity and abundance of plant species in France changed between 2009 and 2017. The cause? Global warming.

Jean-Michel Dreuillaux / CNRS Photothèque

How are plants affected by global warming? While researchers have long shown that rising temperatures are altering the distribution range of many animals, such as birds and fish, the consequences for flora remained poorly understood... until now. A recent study explored the evolution of plant species between 2009 and 2017, and the results are clear: French flora is changing.

Participatory science

"Crisp, clear and solid results", says Vincent Devictor. The researcher fromMontpellier's Institut des sciences de l'évolution took part in this vast study, carried out thanks in particular to the Vigie-Flore participatory science program. Amateur or professional botanists sample wild flora in the four corners of France and share their observations with the Museum national d'Histoire naturelle. An avalanche of precious data for researchers. " In all, 320 volunteer observers have monitored over 3,000 sites every year for 9 years, using a standardized observation protocol," explains Vincent Devictor. A total of 2,500 plant species were monitored in different environments - urban, agricultural, forest or grassland - out of the 6,000 plant species found in mainland France.

To assess the evolution of the flora, the researchers took into account the thermal preference of each plant. " This is the optimum temperature required for their development, their preferred temperature", explains the researcher. They considered this index for an assemblage of 550 species representative of the common flora of France. " We then calculated the average preferred temperature of this assemblage and observed its evolution over time". The researchers then realized that this famous preferred average temperature had increased between 2009 and 2017.

Preferred temperature

What does this mean in practice? " This means that French vegetation is increasingly composed of species that tolerate high temperatures well, to the detriment of species that prefer cooler climates". Plants with a strong thermal preference are increasingly present and are colonizing new territories. This is notably the case for bearded oats and Madrid bromegrass, which seem to benefit from the situation. Conversely, species with a low thermal preference have seen their abundance decline and have lost ground, such as wild buckwheat or wild chervil, which are disappearing from areas where they were present only a short time ago. A real change in the face of French flora in just 8 years.

Could such a rapid evolution be caused by global warming? To verify this, the researchers studied temperature records at sites monitored by Vigie-Flore observers, and found a significant rise in average temperatures between 2009 and 2017. " This increase seemed to be linked to changes in plant communities," explains the ecologist. Indeed, sites with significant temperature variations are also those that have undergone strong species replacement over time, with less heat-tolerant species giving way to more tolerant ones.

Short time

It's only a short step from there to saying that global warming is responsible for this change in flora... " But before taking that step, we had to make sure that other factors, such as plant preference for nitrates or CO2, were not responsible for this change", adds Vincent Devictor. But before taking this step, we had to make sure that other factors, such as plant preference for nitrates or CO2, were not responsible for this change", adds Vincent Devictor. These parameters did not change significantly over time at the sites monitored. "We can therefore clearly affirm that global warming is directly responsible for these rearrangements of wild flora. This is the first time that a flora response to rising temperatures has been demonstrated on a national scale over such a short period of time", insists the ecologist.

What are the consequences? We could take it as good news that flora is rapidly adjusting to rising temperatures, but it's highly unlikely that this thermal adjustment will be perfect," moderates the researcher. There will be a delay, which could have repercussions for ecosystems as a whole, since plants are the basis for the survival of all other groups," Vincent Devictor points out. Something to ponder during a future stroll through the garrigue... in northern France.