Seedlings of six major European temperate forest tree species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba, Pinus sylvestris) were exposed to 360, 500, and 660 μL CO2 L−1 in the understorey of a 120-y-old forest over two growing seasons. Seedlings rooted in the natural forest soil within 36 open-top chambers (12 OTCs per CO2 treatment), each with a different known quantum flux density (QFD) ranging from 0.36 to 2.16 mol m−2 d−1 (= 0.8% to 4.8% of full sun). In contrast to a frequent assumption the natural CO2 concentration in the understorey is close to the ambient concentration in the free atmosphere during daytime. The CO2-effect on seedling growth differed greatly among species and was strongly codetermined by microsite-specific QFD. Biomass production in the deep-shade tolerant species Fagus and Taxus increased by 73% and 37% under elevated CO2 in low QFD microsites but was not significantly different among CO2-treatments in high QFD microsites. The less shade-tolerant species Acer, Quercus, and Abies showed no significant response to elevated CO2 in low QFD microsites, but increased their biomass by 39%, 25%, and 55% in high QFD microsites. In the shade-intolerant Pinus, seedling survival was too low for a safe conclusion. Our data showed that the largest relative responses to increasing CO2 occurred at a comparatively small increase from 360 to 500 μL L−1 with only small and non-significant changes with a further increase to 660 μL L−1. Subtle shifts in the availability of light can totally reverse interspecific differences in the CO2 response. Given these different responses, we conclude that increasing atmospheric CO2 is likely to induce changes in species composition of temperate forests due to altered chances of recruitment. However, these shifts will depend on light patterns in the understorey, and thus on canopy structure, disturbance patterns and forest management.