The Photosynthesis function is by default governed by Air Temperature, Nitrogen and Water Availability in the CoupModel in addition to the control of light interception and optionally CO2 concentrations in the atmosphere.
The control of Air Temperature is both a function of the current air temperature and a acclimation regulated by the Growing Temperature sum of the year.
The New version of the CoupModel will now optionally also use a new seasonal acclimation function based on a Green Index Forcing function instead of the acclimation function regulated by the Growing Temperature sum. This allow the photosynthesis to be different in the efficiency during the entire season not only the spring which is the case the growing temperature sum function.
Providing the Green Index forcing switch is put on
the model will read a time serie of the Green index from a separate PG-File defined in the module as Abiotic driving variables. The PG file will be read by linear interpolation so any time resolution of the data will be accepted.
The function is similar to the Function for Growing temperature sum using first a threshold parameter Min Frac Green Response to show the minium to which the temperature sesisivity will be reduced and the required lowest value of the Green Index for reaching Optimal response (Opt Green Index). Values of the Green Index below the , Opt Green Index, will be scaled as from the Min Frac Response to 1 by the ratio of Green Index and Opt Green Index parameter.
A value of Min Frac Green Response to zero implies that the photosynthesis can be reduced to zero because of the green index value in the input file. A value of the parameter as 1 implies that the function will always return 1 and will have not impact on the photosynthesis.
Preliminary results from testing the functions on data from the eddy-flux site at Zackenberg demonstrated the the performance of the model to describe the CO2-flux was positive related to low values of the threshold parameter and also to relative low value of the Opt Green Index parameter.
The Green Index may be a substitute to the nitrogen content of the leaf and can be recommended expecially if a fixed Nitrogen stess is assumed during the year.
Note that both CoupModel.exe and MSimCoup2.exe is updated to make use ot the new options.