The growth and gas exchange response of soil‐planted Norway spruce [Picea abies(L.) Karst.] and red oak (Quercus rubraL.) exposed to elevated CO₂and to naturally occurring drought

SUMMARY Norway spruce and red oak trees were planted directly into the soil and exposed to 700 μmol mol ‐1 CO 2 in open‐top chambers. There were large inter‐specific differences in response to naturally occurring drought during the second year of exposure to elevated CO 2 . Both species had decreased assimilation rates. CO 2 ‐treated red oak had no loss of photosynthetic enhancement when undroughted, whereas CO 2 ‐treated Norway spruce showed a relative increase in assimilation rates only when draughted. The effect of CO 2 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 CO 2 . Stomatal density and chlorophyll content were largely unaffected by the CO 2 treatment. A precise method for measuring Norway spruce needle surface area was also developed.