Canopy Carbon Discrimination in a Dense Forest Estimated From a Multi-Layer Biophysical Model

Carbon isotope discrimination during photosynthesis of plant canopies has been used to constrain global carbon models and to partition ecosystem fluxes into its components. Often carbon discrimination is estimated based on a big-leaf model assuming constant environmental forcing for the entire canopy. In dense and tall forest canopies, however, microclimatic conditions and therefore stomata control on discrimination can strongly vary vertically within the canopy. Numerous studies found a large vertical gradient in bulk leaf carbon isotope values indicating the importance of microclimate on discrimination. It remains unclear how well big-leaf models represent actual canopy discrimination considering microclimatic variations throughout the canopy. Here we use a multi-layered biophysical canopy model to estimate discrimination for each layer and to obtain flux-weighted canopy discrimination for the entire canopy. The model consists of 40 layers, each distinguishing sunlit and shaded leaves. Leaf energy balance, leaf transpiration and photosynthesis are calculated for each layer based on turbulence inside the canopy and light penetration through the canopy. The model showed very good agreement with carbon, water and energy fluxes measured with the eddy covariance technique. The modeled vertical gradient in carbon discrimination matches well with observations made in bulk material and sugars from leaves indicating the importance of microclimatic gradients on canopy discrimination in dense forests.