The interactive influences of elevated carbon dioxide, water stress, and ontogeny on carbon assimilation and biomass production were investigated in northern red oak, a species having episodic shoot growth characteristics. Seedlings were grown from acorns through three shoot-growth flushes (8–11 wk) in controlled-environment chambers at 400, 530 or 700 μmol mol−1 CO2 and under well watered or water-stressed soil-moisture regimes. Increasing CO2 growth concentration from 400 to 700 μmol mol−1 resulted in a 34% increase in net assimilation rate (A), a 31% decrease in stomatal conductance to water vapour (gs) and a 141% increase in water use efficiency (WUE) in well watered seedlings. In contrast, water-stressed seedlings grown at 700 μmol mol−1 CO2 demonstrated a 69% increase in A, a 23% decrease in gs, and a 104% increase in WUE. However, physiological responses to increased CO2 and water stress were strongly modified by ontogeny. During active third-flush shoot growth, A in first-flush and second-flush foliage of water-stressed seedlings increased relative to the quiescent phase following cessation of second-flush growth by an average of 115%; gs increased by an average of 74%. In contrast, neither A nor gs in comparable foliage of well watered seedlings changed in response to active third-flush growth. Whereas seedling growth was continuous through three flushes in well watered seedlings, growth of water-stressed seedlings was minimal following the leaf-expansion stage of the third flush. Through three growth flushes total seedling biomass and biomass allocation to root, shoot and foliage components were very similar in water-stressed seedlings grown at 700 μmol mol CO2 and well watered seedlings grown at 400 μmol mol−1 CO2. Enhancement effects of elevated CO2 on seedling carbon (C) assimilation and biomass production may offset the negative impact of moderate water stress and are likely to be determined by ontogeny and stress impacts on carbon sink demand.