Inverting the maximum carboxylation rate (V cmax) from the sunlit leaf photosynthesis rate derived from measured light response curves at tower flux sites

Obtaining the maximum carboxylation rate ( V c m a x ) has been tedious and time consuming on the field. An algorithm of inverting V c m a x using the sunlit leaf photosynthesis rate was developed and validated at six Canadian flux sites. The sunlit leaf photosynthesis rate was calculated from the sunlit gross primary productivity (GPPsun) obtained through rectangular hyperbola light response curves constructed using the eddy covariance observations. The ability of the rectangular hyperbola method in GPP simulation and sunlit/shaded GPP separation was tested by 1) comparing the rectangular hyperbola simulated GPP (GPPLRC) to the half-hourly GPP derived from eddy covariance measurements (GPPflux) at six flux sites and 2) comparing the separated sunlit/shaded GPP to their counterparts simulated using the Boreal Ecosystem Productivity Simulator (BEPS). The results exhibited good agreements between the GPPLRC and the GPPflux (R2=0.74–0.93, and RMSE=1.71–3.27μmolm−2 s−1) for all six sites. The sunlit/shaded GPP separated using rectangular hyperbola method and BEPS also agreed closely in both magnitude and the seasonal and diurnal variations. The sunlit GPP obtained with incoming APAR bigger than 900μmolm−2 s−1 was selected to invert V c m a x through Farquhar’s classic photosynthesis model. The mean V m 25 ( V c m a x normalized to 25°C) values during the peak of growing season (day 152–day 243) agreed well with V m 25 values from previous studies at same sites. The four conifer sites shared similar V m 25 during the peak of growing season (22–29μmolm−2 s−1) though large difference between two deciduous sites was observed (32μmolm−2 s−1 and 47μmolm−2 s−1). The inverted V m 25 showed clear seasonal trends at two deciduous forest sites while little seasonal trend was observed at four conifer sites. Decreasing of V m 25 was observed during the drought period at the conifer site TP74 when volumetric soil water content (SWC) dropped to around 0.05m3/m3 but the controlling function of soil moisture on V m 25 remained unclear. Further studies on the temperature dependency of V c m a x demonstrated that a same set of parameters was enough in describing the temperature response of V c m a x for all sites. Our analysis revealed that the uncertainty of the inverted V c m a x were less than 25% for all the sites, indicating the algorithm developed in this study can be used to retrieve reliable V c m a x .