Turning point in forest productivity revealed from 40 years of national forest inventory data

Aim: Global changes, such as changing climate or disruption in biogeochemical cycles, are affecting forest productivity worldwide. Trends in productivity are depending on the focal spatial scale and on the considered time window, stable trends at large spatial scale can mask divergence at smaller scale while short time windows limit the capacity to reveal non-linear trends such as turning points. Capitalizing on 40 years of national forest inventory data from more than 100 tree species we explored trends in forest productivity at the regional level across 4 biogeographical regions. Location: France Time period: 1978-2022 Major taxa studied: 146 tree species Methods: We fitted two classes of models, a first one explicitly estimating temporal trends and a second one including no temporal components but climatic variables reflecting changing temperature and water availability. Results: We find a decrease in productivity in 95% of the regions and a high contrast in trend shapes between regions over the period studied: lowland regions with average temperature above 11.9°C showed linear negative trends in productivity since 1985 while colder lowland regions showed hump-shaped trends with turning points between 1985 and 2005, followed by declines in productivity. In mountainous regions, average climate did not appear to be a strong mediator of trend shapes. The temporal trends were reconstituted with high fidelity from the model including only climatic variables implying that changes in temperature and water availability are likely drivers of the reported trends. Main conclusion: These results illustrate the progression during the last decades of the adverse effects of climate change on forest productivity over the European forests. They suggest the expected changes over the 21st century that will put further pressure on forest productivity, impacting forest carbon sink potential and reducing sustainable rate of timber extraction. ### Competing Interest Statement The authors have declared no competing interest.