Reconciling leaf physiological traits and canopy flux data: Use of the TRY and FLUXNET databases in the Community Land Model version 4

Biases in the multilayer canopy with observed Vcmax are similar, or improved, compared with the standard two-leaf canopy and its low Vcmax, though the Amazon is an exception. The difference relates to light absorption by shaded leaves in the two-leaf canopy, and resulting higher photosynthesis when the canopy scaling parameter Kn is low, but observationally constrained. Larger Kn decreases shaded leaf photosynthesis and reduces the difference between the two-leaf and multilayer canopies.

The low model Vcmax is diagnosed from nitrogen reduction of GPP in simulations with carbon-nitrogen biogeochemistry. Our results show that the imposed nitrogen reduction compensates for deficiency in the two-leaf canopy that produces high GPP. Leaf trait databases (Vcmax), within-canopy profiles of photosynthetic capacity (Kn), tower fluxes, and empirically upscaled fields provide important complementary information for model evaluation.