Effects of photosynthetic parameterization on net ecosystem exchange (NEE) due to varying nitrogen allocation through the canopy: A sensitivity analysis with the Advanced Canopy Atmosphere Soil Algorithm (ACASA)

Land surface models (LSM) use different approaches to estimate terrestrial carbon and water exchanges. The most common photosynthesis model implemented to estimate these fluxes is that by Farquhar et al. (1980) along with the stomatal conductance model proposed by Ball et al. (1987), or some slight variation of their original versions. It has been demonstrated that these models are highly sensitive to parameterization. Furthermore, there still remain several uncertainties related to the effect of environmental variables on the biochemistry of photosynthesis, and the physiological control of stomatal conductance. Here, we use ACASA to analyze the sensitivity of net ecosystem exchange (NEE) and latent heat (LE) fluxes due to changes on the parameterization of the photosynthesis-stomatal conductance model, an enzymatic process highly dependent on nitrogen concentration. ACASA is an advanced ecosystem-atmosphere model that uses higher-order turbulence closure methods in a 10-layer plant canopy representation, linked to state-of-the-art plant physiological algorithms, soil transport algorithms, radiation transfer equations, and snow hydrology sub-models. We examine the NEE and LE response to different values for Vcmax (maximum rate of carboxylation limited by the amount, activity, and kinetics of Rubisco) in the vertical canopy profile, thus representing different nitrogen allocation strategies by the plant. Preliminary results show that a more accurate representation of canopy physiological processes depends on using actual nitrogen allocation patterns rather than assuming nitrogen allocation is linearly related to intercepted light. This optimal approach is usually assumed in biosphere-atmosphere interaction models. However, we found that NEE and LE fluxes can vary by about 30% depending on the photosynthesis-stomatal conductance scheme chosen and the parameterizations used.