Jeremy Borderieux , Jean-Claude Gégout, Josep M Serra-Diaz

The ongoing climate change is triggering plant community thermophilization. This selection
process is ought to shift community composition toward species adapted to warmer climates
but may also lead to biotic homogenization. The link between thermophilization and
homogenization, and the community dynamics that drive them (colonization and extinction)
remain unknow, but is critical for understanding community responses under rapid
environmental change.
We used 14,167 pairs of plots to study shifts in plant community during 10 years of rising
temperature, in 80 forest ecoregions of France. We computed community mean thermal
optimum (thermophilization) and Δβ-diversity (homogenization) for each ecoregion and
partitioned these changes into extinction and colonization dynamics of cold- and warmadapted species.
Forest understory communities thermophilized on average by 0.12 °C decade-1 and up to
0.20 °C decade-1 in warm ecoregions. This rate was entirely driven by extinction dynamics.
Extinction of cold-adapted species was a driver of homogenization, but it was compensated
for by the colonization of rare species and the extinction of common species, resulting in
the absence of an apparent homogenization trend.
Here we show a dieback of present cold-adapted species rather that an adaptation of
communities via the arrivals of warm-adapted species, with mutually cancelling effect on
β-diversity. These results suggest that a future loss of biodiversity and delayed biotic
homogenization should be considered.