Instantaneous gross assimilation

After the calculation of solar radiation absorption by the shaded and sunlit leaves (see topic Solar radiation absorption), the calculation of instantaneous gross assimilation for each canopy layer can be done.

For the computation of instantaneous gross assimilation of the shaded leaves for each layer, this equation is applied:

where Asssh is instantaneous gross assimilation of the shaded leaves (Kg ha-1 h-1), Amax is maximum leaf CO2 assimilation rate (kg ha-1 h-1), LUE is light use efficiency (kg ha-1 h-1/J m-2 s-1) and Fluxsh is absorbed radiation flux for shaded leaves (W m-2).

The instantaneous gross assimilation of the sunlit leaves is calculated for each layer with the equation:

where Asssun is instantaneous gross assimilation of the sunlit leaves (Kg ha-1 h-1), Amax is maximum leaf CO2 assimilation rate (kg ha-1 h-1), Asssh is instantaneous gross assimilation of the shaded leaves (Kg ha-1 h-1), Fluxsun is absorbed radiation flux for sunlit leaves (W m-2) and LUE is light use efficiency (kg ha-1 h-1/W m-2).

Then the fraction of sunlit leaf area is calculated with the equation:

where Areasun is fraction of sunlit leaf area (unitless), Extdir is extinction coefficient of direct light (unitless) and LAIl is leaf area index for the layer l (m2 m-2).

This sunlit leaf area is used then for the calculation of the local assimilation rate of each layer, calculated by the equation:

where Asslocal is local assimilation rate of each layer (Kg ha-1 h-1), Areasun is fraction of sunlit leaf area (unitless), Asssun is instantaneous gross assimilation of the sunlit leaves (Kg ha-1 h-1) and Asssh is instantaneous gross assimilation of the shaded leaves (Kg ha-1 h-1).

Finally, Gaussian integration is used for deriving the daily total assimilation rate from the assimilation rates of the three canopy layers.

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