Respiratory cost of leaf growth and maintenance in white oak saplings exposed to atmospheric CO2 enrichment

Atmospheric CO2 enrichment reportedly reduces respiration of mature leaves in a number of woody and herbaceous perennials. It has yet to be determined, however, whether these reductions reflect changes in maintenance respiration alone or whether CO2 might affect growth respiration as well. This possibility was examined in white oak (Quercusalba L.) seedlings that had been planted directly into the ground within open-top chambers and exposed to ambient, ambient +150 μL•L−1, and ambient +300 μL•L−1 CO2 concentrations over a 3-year period. In the spring of 1992, respiration rates were measured repeatedly during leaf expansion, and the growth and maintenance coefficients were determined using a two-component model. Specific respiration rates (mg CO2•g−1•h−1) were consistently lower for leaves of CO2-enriched saplings than for leaves of ambient-grown saplings. Partitioning these reductions in leaf respiration to either the growth or maintenance coefficients indicated a strong effect of CO2 on both components. The growth coefficient for leaves exposed to the ambient CO2 treatment was 964 mg CO2•g−1 compared with 849 and 664 mg CO2•g−1•for leaves from the two elevated CO2 concentrations, respectively. The maintenance coefficient was similarly reduced from a control rate of 114 mg CO2•g−1•d−1 to below 65 mg CO2•g−1•d−1 for leaves exposed to CO2 enrichment. Our results quantitatively describe the magnitude by which growth and maintenance respiration are affected by CO2 enrichment and as such should provide useful information for the future modeling of this phenomenon.