Species mixing effect on Norway spruce response to elevated CO2 and climatic variables: root and radial growth response

A 7-year study was conducted to examine the growth (diameter and root) response of Norway spruce (Picea abies (L.) Karst.) seedlings to elevated CO₂ (CO_2ELV, 770 μmol (CO₂) mol^?1) in different mixture types (monospecific (M): a Norway spruce seedling surrounded by six spruce seedlings, group-admixture (G): a spruce seedling surrounded by three spruce and three European beech seedlings, single-admixture (S): a spruce seedling surrounded by six beech seedlings). After seven years of treatments, no significant effect from elevated CO₂ was found on the root dry mass (p?=?0.90) and radial growth (p?=?0.98) of Norway spruce. Neither did we find a significant interaction between [CO₂]?×?mixing treatments (p?=?0.56), i.e. there was not a significant effect of CO₂ concentrations [CO₂] in all the admixture types. On the contrary, spruce responses to admixture treatments were significant under CO_2AMB (p?=?0.05), which demonstrated that spruce mainly increased its growth (diameter and root) in M and neighbouring with beech was not favourable for spruce seedlings. In particular, spruce growth diminished when growing beside high proportions/numbers of European beech (S). Here, we also evaluated the association between tree-ring formation and climatic variables (precipitation and air temperature) in different admixture types under elevated and ambient CO₂ (CO_2AMB, 385 μmol (CO₂) mol^?1). Overall, our result suggests that spruce responses to climate factors can be affected by tree species mixing and CO₂ concentrations, i.e. the interaction between climatic variables?×?admixture types?×?[CO₂] could alter the response of spruce to climatic variables.