Sensitivity of leaf photosynthesis to CO₂ concentration is an invariant function for C₃ plants: A test with experimental data and global applications

Rising atmospheric CO₂ concentration (C_a) may alter two components (sensitivity and acclimation) of global photosynthetic carbon influx into terrestrial ecosystems (P_G). Most existing global models focus on long-term acclimation. We have developed a leaf-level function (ℒ) to quantify short-term increment of P_G associated with sensitivity. The ℒ function is the normalized response of leaf photosynthesis to a small change in C_a and has been suggested to be an invariant function for C₃ plants grown in diverse environments. This paper tests the hypothesis that ℒ is an invariant function. We calculated values of ℒ from 9 sets of experimental data which incorporated photosynthetic responses of 12 plant species to measurement conditions of light and temperature and to growth in different light, temperature, nitrogen, phosphorus, water stress, and CO₂ concentration. Absolute rates of leaf photosynthesis differed by more than tenfold due to species differences and environmental variation. However, ℒ values derived from these data sets converged into a narrow range defined by two equations of the ℒ function, confirming that ℒ was insensitive to differences in photosynthetic capacity among species and between plants acclimated to different growth environments. Using the ℒ function, we predict that a yearly increase of 1.5 parts per million (ppm) in C_a will induce an increase in P_G by 0.18 to 0.34 Gt (1 Gt = 10¹⁵ g) C yr^-1 in 1993, provided that (1) P_G = 120 Gt C yr^-1, (2) 85% of P_G is generated by C₃ plant assimilation, and (3) the 1.5-ppm increase in C_a will not induce significant photosynthetic acclimation.