Growth of mature boreal Norway spruce was not affected by elevated [CO2] and/or air temperature unless nutrient availability was improved

The growth responses of mature Norway spruce (Picea abies (L.) Karst.) trees exposed to elevated [CO₂] (C_E; 670-700ppm) and long-term optimized nutrient availability or elevated air temperature (T_E; +3.9 °C) were studied in situ in northern Sweden in two 3 year field experiments using 12 whole-tree chambers in ca. 40-year-old forest. The first experiment (Exp. I) studied the interactions between C_E and nutrient availability and the second (Exp. II) between C_E and T_E. It should be noted that only air temperature was elevated in Exp. II, while soil temperature was maintained close to ambient. In Exp. I, C_E significantly increased the mean annual height increment, stem volume and biomass increment during the treatment period (25, 28, and 22%, respectively) when nutrients were supplied. There was, however, no significant positive C_E effect found at the low natural nutrient availability. In Exp. II, which was conducted at the natural site fertility, neither C_E nor T_E significantly affected height or stem increment. It is concluded that the low nutrient availability (mainly nitrogen) in the boreal forests is likely to restrict their response to the continuous rise in [CO₂] and/or T_E.