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 r(2)value of0.98 and a root-mean-squared error (RMSE) of 8.19molm(-2)s(-1). However, R-day typically must be estimated. Estimating R-day as 1.5% of V-cmax,V- we found that V-cmax could be estimated with an r(2)of0.95 and an RMSE of 17.1molm(-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.