The effect of elevated CO₂ on photosynthesis is modulated by nitrogen supply and reduced water availability in Picea abies.

This article investigates the interactive effects of elevated atmospheric carbon dioxide concentration ([CO₂]), nitrogen (N) supply, and water availability on physiological, morphological, and stoichiometric traits of Norway spruce (Picea abies) saplings under natural field conditions over two to three years. The study found that elevated [CO₂] enhances photosynthetic CO₂ assimilation and water-use efficiency primarily when combined with sufficient nitrogen supply and moderate water limitation; however, these stimulatory effects are substantially reduced under severe drought and low nitrogen availability. Photosynthetic downregulation under elevated [CO₂] was linked to decreased Rubisco carboxylation efficiency and electron transport rates, especially under nutrient and water stress. The findings highlight the importance of incorporating multifactorial environmental interactions into mechanistic models to accurately predict future forest growth and carbon sequestration in temperate coniferous ecosystems.