Drought overrides CO2-driven carbon sink enhancement in Moroccan Atlas cedar forests

Increasing drought and extensive forest die-off may override the fertilization effect of rising atmospheric CO 2 (C a ), limiting the contribution of forests as carbon sinks. Tree-rings-based estimates of intrinsic water use efficiency (iWUE) have been recently improved, including the effects of diffusion through stomata and mesophyll, carboxylation and photorespiration. However, it remains unclear whether increasing iWUE is attributable to carbon gain or water shortage. We tested whether leaf gas-exchange regulation, inferred from tree-ring δ 13 C, translates into actual growth or reflects drought responses. Wood δ 13 C and basal area increment were analysed to investigate the effect of rising C a and drought stress on growth and iWUE of Atlas cedar ( Cedrus atlantica ) forests, spanning seven centuries across a latitudinal gradient in Morocco. iWUE remained stable (∼50 μmol CO₂ mol⁻¹ H₂O) across sites until the onset of the 20th century, whereas long-term growth showed greater variability. Since the 1980s, sequential increases in drought sensitivity and growth decline were observed, starting by the southernmost dry regions but extending northwards. Nonetheless, iWUE maintained rising trends, with an average C i /C a ratio of 0.72 and C a sensitivity of 0.18 ppm ppm −1 . Growth and iWUE became increasingly decoupled in southern populations, suggesting that rising C a no longer enhances growth under intensified drought. Local extinction risk is expected to increase at the High Atlas (trailing edge), while the highest conservation priority must be in the Rif (leading edge). We conclude that, whereas C a and drought have synergistic effects on iWUE, increasing VPD and decreasing soil water availability prevail as drivers of the progressive decoupling between growth and iWUE. • Increasing drought offsets CO 2 improvement on Atlas cedar growth. • iWUE increased since 1800 but uncoupled from tree growth trends. • Average CO 2 sensitivity of iWUE was steady at 0.18 ppm ppm −1 . • Latitudinal patterns of growth and climate sensitivity reflect contrasting vulnerability.