Tree ring responses to elevated CO₂ and increased N deposition in Picea abies

Four‐ to seven‐year‐old spruce trees ( Picea abies ) were exposed to three CO 2 concentrations (280, 420 and 560 cm 3 m −3 ) and three rates of wet N deposition (0, 30 and 90 kg ha −1 year −1 ) for 3 years in a simulated montane forest climate. Six trees from each of six clones were grown in competition in each of nine 100 × 70 × 36 cm model ecosystems with nutrient‐poor natural forest soil. Stem dises were analysed using X‐ray densitometry. The radial stem increment was not affected by [CO 2 ] but increased with increasing rates of N deposition. Wood density was increased by [CO 2 ], but decreased by N deposition. Wood‐starch concentration increased, and wood nitrogen concentration decreased with increasing [CO 2 ], but neither was affected by N deposition. The lignin concentration in wood was affected by neither [CO 2 ] nor N deposition. Our results suggest that, under natural growth conditions, rising atmospheric [CO 2 ] will not lead to enhanced radial stem growth of spruce, but atmospheric N deposition will, and in some regions is probably already doing so. Elevated [CO 2 ], however, will lead to denser wood unless this effect is compensated by massive atmospheric N deposition. If can be speculated that greater wood density under elevated [CO 2 ] may alter the mechanical properties of wood, and higher ratios of C/N and lignin/N in wood grown at elevated [CO 2 ] may affect nutrient cycles of forest ecosystems.