## 4 Discussion
### Modelling of Springtime Development and the Start of Season in JSBACH  
The inspection of the modelled GPP time series (Figures 4 and 5) showed that the JSBACH model instantaneously responds to the increase of air temperature above zero \({}^{\circ}\)C resulting in a higher GPP than obtained from the CO\({}_{2}\) flux measurements and, consequently, earlier ENF_SOS\({}_{\text{mod}}\) than ENF_SOS\({}_{\text{EC}}\). The photosynthesis module in JSBACH has the same temperature response for parameters describing the potential electron transport rate and maximum carboxylation rate for the whole year, thus indicating that the simulated vegetation is ready to immediately photosynthesize in spring with increasing air temperatures without any recovery period required, providing there is foliage present as in the case of ENF. However, the boreal coniferous forests have a recovery period in spring after the winter dormancy before they reach their full summertime photosynthetic capacity [89; 90]. This may explain the differences between observed GPP at Sodankyla and simulated GPP using local weather observations (Figures 4 and 5). Overestimation of the spring carbon uptake (a too large GPP) by JSBACH was also found by [PERSON] and [PERSON] [21] in a study that evaluated, among others, the phenology of the JSBACH model. Furthermore, because of generally lower air temperatures from the regional climate model than from the site measurements, the ENF_SOS\({}_{\text{mod,loc}}\) was earlier than ENF_SOS\({}_{\text{mod}}\). The observed early bias of three days in ENF_SOS\({}_{\text{mod}}\) from JSBACH is relatively small in comparison to results by [PERSON] et al. [14] for the simulated start of photosynthetic uptake from 14 biosphere models (mean bias of \(-11\pm 15\) days) and is in the same range as for the best-performing models in the evaluation. The increase in the negative bias of ENF_SOS\({}_{\text{mod}}\) from south to north (Figure 8, Supplementary Materials: Table S2) found here, agrees with higher temperatures observed at time of ENF_SOS at northern sites compared to southern boreal sites [4; 18; 33] and suggests that the response of air temperature for ENF follows a climate gradient in the boreal zone. For the performance of the JSBACH model regarding the estimation of ENF_SOS, the photosynthesis model plays the dominant role and deviations from ENF_SOS\({}_{\text{sat}}\) are not due to the LoGro-P model. Improvements to the model performance during the start of season of ENF could be achieved by implementing seasonality to temperature response of JSBACH photosynthesis parameters, and possibly also accounting for photoinhibition in coniferous forests, a phenomenon observed during spring at high light levels and freezing temperatures [91]: the light receiving apparatus in chloroplast gets damaged at high light levels, and for low temperature conditions the photosynthetic capacity may be notably reduced. The description of effects of soil water freeze on carbon uptake could also be improved. According to observations [18; 92], the coniferous trees are able to photosynthesize before the soil thaw, but lack of liquid water from soil soon reduces the carbon uptake. In the model, the actual simulated proportions of liquid and frozen water in soil layers could be used to limit the carbon exchange. There are advances related to expressing frozen soil in JSBACH northern permafrost regions [93]. The modelled FSC by JSBACH does not serve as a reliable predictor of ENF_SOS (Section 3.3), as too little snow and too early snow melt is simulated for our study area. Having bias-corrected temperature and precipitation data does not guarantee precise snow cover, as the formulation of snow accumulation relies on a universal temperature threshold and parameterizations of snow processes are not adjusted for region and drivers. In contrast to results for ENF, we found a late bias of DBF_SOS\({}_{\text{mod}}\) (Figure 7, Supplementary Materials: Table S2).