# Forest Modelling Lab.

## Photosynthesis and Net Primary Production

The carbon flux is estimated into 3D-CMCC-FEM by two different, mutually independent, methods: the Light Use Efficiency (LUE) emprical approach (Monteith 1977) (see parag. “Light and water competition”) and the biochemical model of Farquhar, von Caemmerer and Berry (FvCB, 1980) model as adapted by DePury and Farquhar (1997) for sun and shaded leaves.

In the LUE version, the Gross Primary Production (GPP; gC m-2 day-1) is computed as:

The parameters epsx and alfax are the prognostic potential light use efficiency (in grams of Carbon or moles of Carbon per moles of PAR, respectively). APAR is the Absorbed Photosynthetic Active Radiation (molPAR-2 day-1). Modx,k (dimensionless values ranged from 0 to 1) are the modifiers related to several environmental factors, such as vpd, minimum and maximum air temperature, soil water content and site nutrient status and including effects of stand age and atmospheric CO2 concentration.

Referring to the understories, APAR is computed by the following equation:

where LAI is the Leaf Area Index (m2 m-2) and kx is the specie-specific light extinction coefficient. PAR for understories is computed as:

where PAR0 is the photosynthetically active radiation above the overstorey and CCzoverstorey is the Canopy Cover of the overstorey.

Leaf Area Index is calculated as:

where Wf (MgC ha-1 day-1) is the daily biomass allocated to the leaf compartment, deltax is the Specific Leaf Area (SLA; m2 KgC-1).

In the fully-biogeochemical version (BGC version) the 3D-CMCC-FEM adopts the Farquhar et al. (1980) approach as modified for sun and shaded leaves by dePury & Farquhar (1997). Hence, the effect of atmospheric CO2 concentration is directly embedded in the photosynthesis routine. For the temperature dependence of the Michaelis-Menten coefficient for Rubisco and the CO2 compensation point without mitochondrial respiration we use the parameterization described in Bernacchi et al. (2001, 2003). Temperature acclimation of leaf photosynthesis to increasing temperature is accounted following Kattge & Knorr (2007).

In the DePury and Farquhar two-leaf model, the rate of photosynthesis depends on nitrogen content in the sun and shadede leaves and Rubisco, the temperature leading enzyme kinetics, the Maintenance Respiration and the difference between internal and external partial pressure of CO2.

The Net Primary Production (NPP; gC m-2 day-1) is finally calculated subtracting the Autotrophic Respiration (AR; gC m-2 day-1) to the Gross Primary Production (GPP):

Simulations of daily absorbed PAR and canopy photosynthesis for a hypothetical stand of Pinus radiata using the BIOMASS model. Reprinted from ‘‘Crop Photosynthesis: Topics in Photosynthesis, Vol. 12, Wang, Y.P., McMurtrie, R.E., Landsberg, J.J. Modeling canopy photosynthesis productivity, pp. 43–67 (1992).

More info about the 3D-CMCC-FEM can be found at PUBLICATIONS page