Increased water‐use efficiency does not lead to enhanced tree growth under xeric and mesic conditions

Higher atmospheric CO₂ concentrations (cₐ) can under certain conditions increase tree growth by enhancing photosynthesis, resulting in an increase of intrinsic water‐use efficiency (ᵢWUE) in trees. However, the magnitude of these effects and their interactions with changing climatic conditions are still poorly understood under xeric and mesic conditions. We combined radial growth analysis with intra‐ and interannual δ¹³C and δ¹⁸O measurements to investigate growth and physiological responses of Larix decidua, Picea abies, Pinus sylvestris, Pinus nigra and Pseudotsuga menziesii in relation to rising cₐ and changing climate at a xeric site in the dry inner Alps and at a mesic site in the Swiss lowlands. ᵢWUE increased significantly over the last 50 yr by 8–29% and varied depending on species, site water availability, and seasons. Regardless of species and increased ᵢWUE, radial growth has significantly declined under xeric conditions, whereas growth has not increased as expected under mesic conditions. Overall, drought‐induced stomatal closure has reduced transpiration at the cost of reduced carbon uptake and growth. Our results indicate that, even under mesic conditions, the temperature‐induced drought stress has overridden the potential CO₂ ‘fertilization’ on tree growth, hence challenging today's predictions of improved forest productivity of temperate forests.