Long-term response of photosynthesis to elevated carbon dioxide in a Florida scrub-oak ecosystem

The response of photosynthesis was analyzed during canopy closure in a Florida scrub-oak ecosystem exposed to elevated [CO₂] (704 μmol CO₂/mol air; concentration of CO₂). The species were measured on six occasions, covering different seasons, during the third and fourth year of exposure to elevated [CO₂]. The entire regrowth cycle of this community has been under elevated [CO₂], providing a rare opportunity to assess the differential responses of species during the critical phase of canopy closure. Measurements were taken in order to determine both season-specific and species-specific differences in the response of photosynthesis to elevated [CO₂]. Photosynthesis was measured with an open-gas exchange system, and in vivo rates of Rubisco carboxylation (V_c,max) and electron transport (J_max) were derived to assess changes in the photosynthetic capacity in the codominant, evergreen oak species. Quercus myrtifolia did not show any change in photosynthetic capacity with prolonged exposure to elevated [CO₂] during any season, and as a result the increase in photosynthesis due to the increased supply of CO₂ was sustained at 72%. The codominant, Q. geminata, showed a loss of photosynthetic capacity with growth at elevated [CO₂], such that during most measurement periods light-saturated photosynthesis in leaves grown and measured at elevated [CO₂] was no higher than in leaves grown and measured at ambient CO₂. A third oak, Q. chapmanii, showed a response similar to that of Q. myrtifolia. This suggests that at the critical phase of canopy closure in a woody community, elevation of [CO₂] causes a species-dependent and time-dependent change in the capacity of the codominants to acquire carbon and energy.