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

Abstract

Increasing drought and extensive forest die-off may override the fertilization effect of rising atmospheric CO2 (Ca), limiting the contribution of forests as carbon sinks. Tree-rings-based estimates of intrinsic water use effi-ciency (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 δ13C, translates into actual growth or reflects drought responses. Wood δ13C and basal area increment were analysed to investigate the effect of rising Ca 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 Ci/Ca ratio of 0.72 and Ca sensitivity of 0.18 ppm ppm􀀁-1. Growth and iWUE became increasingly decoupled in southern populations, suggesting that rising Ca no longer enhances growth under intensified drought. Local extinction risk is expected to increase at the High Atlas (trailing edge), while the highest con-servation priority must be in the Rif (leading edge). We conclude that, whereas Ca 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.