Derivation of temporally continuous leaf maximum carboxylation rate (Vcmax) from the sunlit leaf gross photosynthesis productivity through combining BEPS model with light response curve at tower flux sites

Photosynthesis plays an important role in terrestrial carbon cycle, and simulation of photosynthesis through terrestrial biosphere models usually requires a specification of maximum carboxylation rate (V-cmax). However, estimating V-cmax by gas-exchange experiments is laborious and tedious, resulting in a general paucity of V-cmax data. In this study, a reliable assimilation scheme to derive temporally continuous V-cmax from measured sunlit gross photosynthesis productivity (GPP) was proposed and validated at three Canadian tower flux sites. To fulfill this purpose, the Boreal Ecosystem Productivity Simulator (BEPS) and light response curve were combined to separate the eddy covariance GPP data into sunlit GPP and shaded GPP, due to the lack of measured sunlit GPP. The reliability of V-cmax was validated through the correlation analysis between the normalized difference vegetation index (NDVI) and V-cmax. To illustrate the advantages of the proposed scheme in our study, BEPS simulated GPP using the derived from sunlit GPP in our study, the derived V-cmax from canopy GPP and the reference V-cmax as constants were respectively compared with corresponding eddy covariance GPP measurements. Results showed that (1) the proposed assimilation scheme based on sunlit GPP can be used to derive reliable temporally continuous V-cmax. The V-cmax obtained in our study was significantly correlated with NDVI (R-2 = 0.77); (2) the seasonal variations in V-cmax could significantly improve the accuracy of photosynthesis simulation. The GPP difference between eddy covariance GPP and BEPS simulated GPP using derived V-cmax from sunlit GPP (mean = 0.12, std = 0.92) were significantly smaller than those between eddy covariance GPP and BEPS simulated GPP using reference V-cmax as constants (mean = 1.37, std = 1.37); (3) the sunlit GPP is able to obtain the relatively accurate V-cmax that can improve the simulation of photosynthesis. GPP difference between eddy covariance GPP and BEPS simulated GPP using derived from sunlit GPP (mean = 0.12, std = 0.92) showed lower mean values and standard deviations than those between eddy covariance GPP and BEPS simulated GPP using derived V-cmax from canopy GPP (mean = 0.67, std = 1.00). Our study illustrated the reliability that sunlit GPP can be used to derive relatively accurate temporally continuous V-cmax, and the assimilation scheme gives new potential to improve the simulation of photosynthesis through terrestrial biosphere models.