Abstract: A concept is presented for analysing and quantifying the competitiveness of woody plants. Efficiency ratios are defined that relate resource investments and gains to above- and belowground space sequestration. Such efficiencies are exemplified in comparisons between young and mature trees of European beech (Fagus sylvatica) and Norway spruce (Picea abies), resulting in remarkable consistencies across plant age and species. The demonstration extends to assessments of the inter- and intra-specific competition between young beech and spruce plants in a phytotron study, using CO2/O3 regimes as disturbants for examining the sensitivity of the efficiency ratios. In mixed plantations, the growth of spruce was enhanced and that of beech reduced as compared with the corresponding monocultures. This effect related to a high efficiency of spruce in aboveground space sequestration per resource investment, whereas the efficiency in carbon gain (return per unit of crown volume) was of minor importance. In contrast, beech displayed advantages in belowground space sequestration. However, competitiveness of young trees was hardly affected by the applied CO2/O3 treatments. The concept proves to be adequate for quantitatively comparing competitiveness across species, age and growth conditions. One further efficiency ratio, i.e., volume-related respiratory and transpiratory costs for sustaining functionality in resource sequestration, deserves attention in achieving a mechanistic understanding of competitive behaviour, in particular, in woody plants.