## Materials and Methods
### JSBACH Simulations  
#### 2.4.1 The JSBACH Model  
The biosphere model JSBACH [34; 44] is used for the modelling of GPP. It is part of the Max Planck Institute's (MPI) Earth System Model and implemented as a subroutine of its atmospheric component ECHAM [45]. JSBACH is used in global simulations, but it can also be applied at regional [46; 47; 48] and at site level scales [49], as is done in this work. JSBACH is a process-based model and calculates the exchanges of carbon, water and energy between the land surface and the atmosphere. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Site Name** & **Location** & \multicolumn{2}{c}{**Phyogeographical Zone**} & \multicolumn{2}{c}{**Altitude m a.s.l. **} & \multicolumn{2}{c}{**Number of Pixels 1**} \\ \hline  & & & & **Site** & **Median for Pixels 2** & **SD 3** & \\ \hline Aulanko & 61\({}^{\circ}\)01’N & 24\({}^{\circ}\)27’E & Southern boreal & 145 & 99.8 & 20.5 & 10 \\ Vesijako & 61\({}^{\circ}\)23’N & 25\({}^{\circ}\)03’E & Southern boreal & 110 & 109.6 & 9.3 & 12 \\ Koli & 63\({}^{\circ}\)30’N & 29\({}^{\circ}\)49’E & Southern boreal & 135 & 139.1 & 56.1 & 17 \\ [PERSON] & 63\({}^{\circ}  
The phenology model (Logistic Growth Phenology; LoGro-P) of JSBACH provides leaf area index (LAI) at each time step to compute photosynthetic production. The model describes the phenological dynamics based on three subsequent phases; a growth (spring), a vegetative (summer) and a rest phase (autumn and winter). For evergreen and deciduous forest, the phenological cycle is dependent on air temperature. For the shift from dormancy to the growth phase in spring, the \"alternating model\" of [PERSON] et al. [PERSON] et al. (2016) is employed in JSBACH. It consists of a heat sum approach (also often referred to as growing degree day approach) combined with a chilling condition. The counting of chilling days starts from the last autumn event (shift from vegetative phase to dormancy phase). The critical temperature, above which temperature contributes to heat sum and below which chilling days are counted, was set to 4 \({}^{\circ}\)C in both forest types. A detailed description of the JSBACH LoGro-P phenological model and the parameter values used in this work is given in Appendix A. The shift to the growth phase corresponds to the start of development of new shoots in ENF and to bud break in DBF. Because ENF have needles present throughout the year (and non-zero LAI in the model), photosynthesis can start before the shift to the growth phase and defines the start of season in ENF. The photosynthetic uptake increases with temperature and is enhanced by the increase of LAI after the beginning of the growth phase. The photosynthesis model in JSBACH is based on the biochemical model developed by [PERSON] et al. [PERSON] et al. (2016), as are many of the large-scale models. The temperature responses of photosynthesis model parameters describing the potential electron transport rate and maximum carboxylation rate stay the same throughout the year. The photosynthesis is coupled to the stomatal conductance, which is constrained at low soil water content ([PERSON] et al., 2016). As a part of the water and energy balances, the model simulates snow cover. Fractional Snow Cover is calculated according to [PERSON] et al. [PERSON] et al. (2016). Different vegetation types are described as Plant Functional Types (PFTs). Each PFT has specific physiological and physical properties, such as photosynthesis parameters, aerodynamic roughness and canopy albedo. Four different PFTs were described for each grid cell in our simulations and based on the Coordinated Information on the European Environment (CORINE) Land Cover data. In this work, only the tiles containing ENF and DBF are considered. As ENF is the most abundant PFT in Finland, for the vast majority of grid cells there was representation of ENF, and thus an estimate was available for the whole Finnish territory except for the very northern areas.