Climate sensitivity of radial growth in Norway spruce (Picea abies (L.) Karst.) under different CO₂ concentrations

This investigation examined the effects of two different carbon dioxide concentrations ([CO₂]): Ambient (A, 385 μmol (CO₂) mol⁻¹) and elevated (E, A+385 μmol (CO₂) mol⁻¹)) on the tree-ring width and early to latewood proportion in Norway spruce for seven years (2006-2012). Further, to improve our understanding of the influence of climatic variables, we assessed the effects of precipitation and temperature. Our observations showed that spruce trees growing under elevated CO₂ (EC) formed less early (p > 0.05) and latewood (p < 0.05) and hence smaller annual increments (p > 0.05) than trees in ambient CO₂ (AC). Early to latewood proportion was nearly 73% and 75% in AC and EC, respectively. In both CO₂ concentrations, the largest tree-rings and earlywood width was observed during 2009 and 2010, which is coincident with the highest precipitation in May (2010) and the highest air temperature in April (2009). Moreover, to determine the association between the latewood formation and air temperature during the second half of the growing season, and correlation between the earlywood formation and precipitation during the first half of the growing season we run Spearman’s correlation test, the determination coefficient values for latewood formation were r = 0.45 (AC) and r = 0.68 (EC), and for earlywood formation were r = 0.53 (AC) and r = 0.42 (EC), although coefficient values were not statistically significant (p > 0.05). Also, our study indicated that temperature had stonger influence than precipitation in EC, but in AC precipitation had the strongest effect on radial growth.