A test of the 'one-point method' for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis

Simulations of photosynthesis by terrestrial biosphere models typically need a specification of the maximum carboxylation rate (V_cmax). Estimating this parameter using A-C_i curves (net photosynthesis, A, vs intercellular CO₂ concentration, C_i) is laborious, which limits availability of V_cmax data. However, many multispecies field datasets include net photosynthetic rate at saturating irradiance and at ambient atmospheric CO₂ concentration (A_sat) measurements, from which V_cmax can be extracted using a 'one-point method'. We used a global dataset of A-C_i curves (564 species from 46 field sites, covering a range of plant functional types) to test the validity of an alternative approach to estimate V_cmax from A_sat via this 'one-point method'. If leaf respiration during the day (R_day) is known exactly, V_cmax can be estimated with an r² value of 0.98 and a root-mean-squared error (RMSE) of 8.19 μmol m^-2 s^-1. However, R_day typically must be estimated. Estimating R_day as 1.5% of V_cmax, we found that V_cmax could be estimated with an r² of 0.95 and an RMSE of 17.1 μmol m^-2 s^-1. The one-point method provides a robust means to expand current databases of field-measured V_cmax, giving new potential to improve vegetation models and quantify the environmental drivers of V_cmax variation.