This paper provides responses in nitrogen productivities for 12 important tree species to elevated CO 2. The increasing atmospheric carbon dioxide concentration is expected to increase plant productivity. However, the strength of the response depends on the interaction with other limiting factors, of which nitrogen has been identified as one of the most important. This study analyzed the effects of increasing the CO2 concentration from 380 ppm (ambient) to 1000 ppm (elevated) on nitrogen productivity (incl. biomass allocation and nutrient concentration of plant organs) in nine deciduous and three conifer tree species. No clear effects on biomass allocation were observed, but leaf nitrogen concentration decreased. Nitrogen productivity increased by 28% over all species, with the strongest response in deciduous trees (34%) and the weakest in conifers (8%). Although these changes are statistically not significant, we conclude that nitrogen productivity provides an integrative and robust concept to assess the effect CO2 fertilization effects on tree growth under varying nitrogen availability, while more studies are required to firmly establish the magnitude of the response.