Coupling a detailed photosynthetic model with foliage distribution and light attenuation functions to compute daily gross photosynthesis in sugar maple (Acer saccharum Marsh.) stands

Canopy multilayer models for forest stands to scale from leaf to canopy have generally focused on developing relatively detailed photosynthetic active radiation (PAR) characterization functions within canopies, but with much simplified photosynthetic production functions. This study aimed at developing a multilayer model based on detailed foliage distribution, PAR interception and photosynthesis components. Allometric, physiological and meteorological data collected in two sugar maple (Acer saccharum Marsh.) stands that differed in climatic conditions, stand structure and fertility were used to calibrate the model. In the leaf photosynthesis model, photosynthetic rate is limited by the ribulose-bisphosphate (RUBP) concentration or the activity of RUBP carboxylase/oxygenase. The Rubisco potential capacity for CO2 fixation, Vcmax, and the potential electron transport rate, J, were related to temperature and leaf nitrogen and soluble and insoluble protein contents. The Weibull distribution function was used to represent leaf area and biomass distribution within the canopy. PAR was computed in different layers of the canopy using a radiative transfer approach. There was fairly good agreement between measured and predicted photosynthetic rate at the individual leaf level, which indicated that the leaf photosynthesis model accounted for variation in PAR, temperature and foliage nitrogen content. The pattern of foliage nitrogen variation at different levels of the canopy was similar for both sites. However, foliage area and biomass distribution functions were characterized by different patterns between both sites. Simulations showed that differences in canopy properties represented by the site-specific functions were essential to obtain good agreement between predicted and measured PAR below the canopy, as both stands had relatively close values in leaf area index (LAI) and leaf biomass. Sensitivity analysis indicated that the coupled multilayer model derived accounted for relatively small variation in LAI and foliage nitrogen concentration.