Warming as the dominant driver of floristic change in the French Alps

Warming as the dominant driver of floristic change in the French Alps
Over the past half-century, climate change has triggered profound ecological shifts, with the redistribution of species being one of the most consistent responses. In the European Alps, for example, temperatures have risen at roughly twice the hemispheric average since the late 19th century. This has led to earlier snowmelt, leaving plants vulnerable to frost. These combined environmental changes are having a significant impact on mountain ecosystems, resulting in consistent increases in thermophilous species at high elevations. However, the relative importance of climatic drivers remains unclear. Using a 30-year dataset documenting the spatial and temporal shifts of 4,250 plant species across the French Alps, we employed a causal inference framework to quantify the dominant factors shaping floristic change. Mean annual temperature, growing-degree days and water deficit were identified as the three most influential factors, each having an overall positive effect on the occurrence probabilities of species, whereas nitrogen deposition, precipitation and drought had weaker and slightly negative effects. A 1 °C rise in temperature increased species occurrence probabilities by 0.9%, yet responses differed sharply among biogeographic groups. Mediterranean generalists and alpine specialists showed the strongest gains, whereas broadly distributed generalists displayed no significant trends. Functional traits shaped these outcomes, with graminoids benefiting from warming while woody species showed weaker responses. The combined influence of warming and rising water deficits revealed additional trade-offs, as drought-tolerant groups such as Mediterranean generalists and alpine specialists responded positively to both drivers, in contrast with the weak or neutral responses of generalist taxa. By applying a causal inference framework, we identified the relative importance of multiple drivers and showed that temperature is the dominant force shaping shifts in the French Alpine flora, while interactions among climatic drivers are equally crucial for understanding how species respond.