The use and misuse of Vc,max in Earth System Models

Earth System Models (ESMs) aim to project global change. Central to this aim is the need to accurately model global carbon fluxes. Photosynthetic carbon dioxide assimilation by the terrestrial biosphere is the largest of these fluxes, and in many ESMs is represented by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. The maximum rate of carboxylation by the enzyme Rubisco, commonly termed Vc,max, is a key parameter in the FvCB model. This study investigated the derivation of the values of Vc,max used to represent different plant functional types (PFTs) in ESMs. Four methods for estimating Vc,max were identified; (1) an empirical or (2) mechanistic relationship was used to relate Vc,max to leaf N content, (3) Vc,max was estimated using an approach based on the optimization of photosynthesis and respiration or (4) calibration of a user-defined Vc,max to obtain a target model output. Despite representing the same PFTs, the land model components of ESMs were parameterized with a wide range of values for Vc,max (−46 to +77 % of the PFT mean). In many cases, parameterization was based on limited data sets and poorly defined coefficients that were used to adjust model parameters and set PFT-specific values for Vc,max. Examination of the models that linked leaf N mechanistically to Vc,max identified potential changes to fixed parameters that collectively would decrease Vc,max by 31 % in C3 plants and 11 % in C4 plants. Plant trait data bases are now available that offer an excellent opportunity for models to update PFT-specific parameters used to estimate Vc,max. However, data for parameterizing some PFTs, particularly those in the Tropics and the Arctic are either highly variable or largely absent.