Declining tree growth rates despite increasing water-use efficiency under elevated CO₂ reveals a possible global...

Though rising atmospheric CO₂ concentrations (C_a) harm the environment and society, they may also raise photosynthetic rates and enhance intrinsic water-use efficiency (iWUE). Numerous short-term studies have investigated tree growth under elevated CO₂ (eCO₂) conditions, but no long-duration study has investigated eCO₂ impacts on tree growth and iWUE under natural conditions. Utilizing a new dendrochronological experimental design in a heavily-touristed nature preserve in Southwest China (Jiuzhaigou National Nature Reserve), we compared tree growth (e.g., basal area increment) and iWUE in two biophysically and environmentally similar valleys with contrasting anthropogenic activities. Trees in the control valley with ambient CO₂ benefited from increasing C_a, possibly due to the CO₂ fertilization effect and optimal environmental conditions. However, trees in the treatment valley with intensive tourism experienced comparatively higher localized eCO₂ and growth rate declines. While iWUE increased (1959-2017) in the control (25.3%) and treatment sites (47.8%), declining tree growth rates in the treatment site was likely because comparatively extreme CO₂ exposure levels encouraged stomatal closures. As the first long-term study investigating eCO₂ impacts on tree growth and iWUE under natural conditions, we demonstrate that increased forest iWUE is unlikely to overcome negative drought stress and rising temperature impacts. Thus, forest potential for mitigating eCO₂ and global climate change is likely overestimated, particularly under dry temperate conditions.