Norway spruce and red oak trees were planted directly into the soil and exposed to 700µmol mol-1 CO2 in open-top chambers. There were large inter-specific differences in response to naturally occuring drought during the second year of exposure to elevated CO2. Both species had decreased assimilation rates. CO2 treated red oak had no loss of photosynthetic enhancement when undroughted, whereas CO2 treated Norway spruce showed a relative increase in assimilation rates only when droughted. The effect of CO2 on radial growth of both species was less marked in the second growing season, but this may have been a result of different biomass partitioning as Norway spruce shoot extension had a different pattern of growth in elevated CO2. Stomatal density and chlorophyll content were largely unaffected by the CO2 treatment. A precise method for measuring Norway spruce nneedle surface area was also developped.