Turning point in the productivity of western European forests associated with a climate change footprint

• Global changes, such as changing climate, are affecting forest productivity worldwide. Observed trends in productivity are depending on the focal spatial scale and on the considered time window. Stable trends at large spatial scale can mask divergences at smaller scale while short time windows limit the capacity to reveal non-linear trends such as turning points. • Capitalizing on 40 years of the French national forest inventory data from >100 tree species, we explored trends in forest productivity over a 550,000 km 2 area crossing 4 biogeographical regions, and spanning a large climatic gradient from Mediterranean to mountain or semi-continental conditions. • We find a decrease in productivity of living trees between 1978 and 2022 in 95 % of the regions. These declining trends persisted even after accounting for dead and harvested trees. • Regional trend shapes were contrasted. Trends were decreasing for the whole period in regions with warm and dry climates, but increasing and then decreasing in colder and wetter regions where climate was historically favourable for productivity and where increases were observed until the end of the 20th century. These declines are attributed to rising temperatures and exacerbated water deficits. • Our findings suggest that anticipated climatic changes in the 21st century will put further pressure forest productivity, with significant implications for ecosystem dynamics, forest carbon sequestration potential, and the sustainable harvesting of timber. • Forest productivity trends between 1978 and 2023 were derived. • Analyses were conducted using the French national forest inventory database. • Forest productivity reached a turning point in western European forests. • Productivity trends varied across space and were linked to climatic gradients. • Warmer temperature and drier soils led to declines in forest productivity.