Photosynthesis and Net Primary Production
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The carbon flux is estimated by two different methods: the Light Use Efficiency (LUE) approach (See Par. “Light and water competition”) and the DePury and Farquhar two-leaf model.
In the LUE approach, the Gross Primary Production (GPP; gC/m2/day) is computed as:
The parameters epsx and alfax are the prognostic potential radiation use efficiency (grams of dry matter MJ-1) and maximum canopy quantum use efficiency (molCO2 mol-1PAR) 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.
Referring to the understories, APAR is computed by the following equation:
where PAR is the Photosynthetic Active Radiation, LAI is the Leaf Area Index (m2/m2) and kx is the specie-specific light extinction coefficient. PAR 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 (Mg of dray matter/ha/day) is the daily biomass allocated to the leaf compartment, deltax is the Specific Leaf Area (SLA; m2 Kg-1).
The Net Primary Production (NPP; gC/m2/day) is calculated subtracting the Autotrophic Respiration (AR; gC/m2/day) to the Gross Primary Production (GPP):
In the DePury and Farquhar two-leaf model, the rate of photosynthesis depends on nitrogen content in the leaves and Rubisco, the temperature leading enzyme kinetics, the Maintenance Respiration and the difference between internal and external partial pressure of CO2.