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

Abstract
SUMMARY: Norway spruce and red oak trees were planted directly into the soil and exposed to 700 μmol mol‐1CO2in open‐top chambers. There were large inter‐specific differences in response to naturally occurring 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 draughted. The effect of CO2on 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 CO2treatment. A precise method for measuring Norway spruce needle surface area was also developed.