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

Abstract

Norway spruce and red oak trees were planted directly into the soil and exposed to 700 μmol mol^-1 CO₂ in 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 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 needle surface area was also developed.