Response of Fagus sylvatica and Picea abies to the interactive effect of neighbor identity and enhanced CO2 levels

The interest in adaptive forest strategies to overcome predicted scenarios of climate change is increasing worldwide. An example of these strategies is the introduction of native species into mono-specific plantations. However, to fully consider this option/strategy, a higher understanding of the responses of forest tree species to concurrent biotic and abiotic factors is needed. The aim of the present study was to assess nutritional and morphological adjustments of individuals of European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst] growing at enhanced levels of CO2 and with different proportions of con-specific individuals in its vicinity. Individuals that grew at elevated CO2 levels showed higher values of relative growth rate (RGR), total twig dry biomass and root biomass, and lower values of leaf area ratio, leaf N and Mg concentrations and soil nutrient concentrations. Individuals of Norway spruce growing in the vicinity of high proportions of European beech showed a reduction in the allocation of biomass to foliar tissue, and lower values of RGR and root biomass. European beech, by contrast, showed a limited response to Norway spruce presence and higher capacity in the exploitation of space both above- and below-ground. In conclusion, the lower response of European beech to both environmental factors suggests that the introduction of European beech into Norway spruce stands could be a feasible option in current forest transition strategies