Accounting for foliar gradients in Vc(max) and J(max) improves estimates of net CO2 exchange of forests

Vertical gradients in the canopy represent a major challenge for scaling from foliar photosynthesis to ecosystem-level CO2 fluxes. We tested whether accounting for independent gradients of carboxylation capacity (Vc(max)) and photosynthetic electron transport (J(max)) improves estimates of forest net ecosystem CO2 exchange (NEE). We modified the process-based Soil-Plant-Atmosphere (SPA) model to represent different gradients of foliar N allocation to Vc(max) and J(max) in the canopy, and inversely calibrated the model via Bayesian inference using eddy covariance measurements of NEE and evapotranspiration from a mixed-deciduous forest. Inversely calibrated N allocation resulted in highest Vc(max) at the top and highest J(max) at the bottom of the canopy, which is similar to N allocation gradients from measured foliar traits. These vertical gradients resulted in the best fit of simulated CO2 fluxes to measured NEE compared to alternative N allocation schemes, due to a higher, more realistic foliar CO2 uptake in the lower canopy. Canopy gradients of Vc(max) and J(max) are thus important drivers of NEE and need to be considered to improve simulations of ecosystem-level CO2 fluxes of forests.