Competition modifies effects of enhanced ozone/carbon dioxide concentrations on carbohydrate and biomass accumulation in juvenile Norway spruce and European beech

Elevated concentrations of carbon dioxide ([CO₂]) and ozone ([O₃]) affect primary metabolism of trees in opposite ways. We studied their potential interactions on carbohydrate concentrations and contents. Two hypotheses currently under debate were tested. (1) Stimulation of primary metabolism by prolonged exposure to elevated [CO₂] does not compensate for the adverse effects of O₃ on carbohydrate accumulation and biomass partitioning to the root. (2) Growth in a mixed-species planting will repress plant responses to elevated [O₃] and [CO₂] relative to conditions in a monoculture. To this end, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) saplings grown under conditions of intra- and interspecific competition were pre-acclimated for 1 year to ambient or elevated [CO₂]. In the following 2-year phytotron study, trees were exposed to factorial combinations of ambient and elevated [O₃] and [CO₂]. The total carbohydrate content (sugar and starch) of spruce was greater in plants exposed to elevated [CO₂] than in plants exposed to ambient [CO₂]. In beech, the opposite response was observed, especially when this species was grown in combination with spruce. Overall, the data did not support Hypothesis 1, because the adverse effects of O₃ were counteracted by elevated [CO₂]. Support for Hypothesis 2 was species-dependent. In beech saplings, reduction of carbohydrates by elevated [O₃] and stimulation by elevated [CO ₂] were repressed by competitive interaction with spruce. In contrast, in spruce, stimulation of carbohydrates by elevated [CO₂] was similar in mono- and mixed cultures. Thus Hypothesis 2 was supported for beech but not spruce. We conclude that, in juvenile beech and spruce, a 3-year exposure to elevated [CO₂] counteracts the adverse effects of O ₃ on carbohydrate concentrations and contents. For beech, sensitivity to elevated [CO₂] and [O₃] was high in monoculture but was largely repressed by interspecific competition with spruce. In contrast, the response of spruce to perturbations of atmospheric chemistry was not significantly affected by either intra- or interspecific competition.