## 3 Results
### Regional Assessment of _I_SBACH-Modelled Start of Season against Satellite Observations  
The earliest dates of ENF_SOS\({}_{\text{sat}}\) occurred in the south-western coastal areas and the latest dates were found in the north-east, ranging from the end of March to the beginning of May (Figure 6). DBF_SOS\({}_{\text{sat}}\) occurred about one month later than ENF_SOS\({}_{\text{sat}}\) and ranged from the beginning of May to mid-June. For the regional assessment of JSBACH SOS, we first investigated the spatial agreement between the multi-year averages of ENF_SOS\({}_{\text{mod}}\) and DBF_SOS\({}_{\text{mod}}\) for the period 2003-2010 with satellite observations. The modelled mean dates were highly correlated with the respective remote sensing observations (Figure 7, Supplementary Materials: Table S2) showing, however, an early bias for ENF_SOS\({}_{\text{mod}}\) and a late bias for DBF_SOS\({}_{\text{mod}}\). The RMSEs were five and six days for ENF_SOS\({}_{\text{mod}}\) and DBF_SOS\({}_{\text{mod}}\), respectively. For DBF_SOS\({}_{\text{mod}}\), this is a bit larger than the accuracy of the remote sensing estimates when considering the site-means (Table 2). For the boreal sub-regions, the highest spatial correlations between the mean start of season dates for the period 2003-2010 was observed in the middle boreal zone (R\({}^{2}\) values \(\geq\) 0.79) for both, ENF_SOS\({}_{\text{mod}}\) and DBF_SOS\({}_{\text{mod}}\), while R\({}^{2}\) was low in the northern boreal zone (<0.5). The bias varied regionally; it increased for ENF_SOS\({}_{\text{mod}}\) and decreased for DBF_SOS\({}_{\text{mod}}\) from the southern to the northern boreal zone (Supplementary Materials: Table S2). For ENF, it was largest (\(\geq\)10 days) in the south-western coastal areas (hemiboreal zone, late bias of ENF_SOS\({}_{\text{mod}}\)) and south-western Lapland (early bias of ENF_SOS\({}_{\text{mod}}\)) (Figure 8a). It has to be noted that the satellite estimate based on FSC may be less reliable for the hemiboreal zone in winters with low snow depth [33]. Only a few grid cells in the hemiboreal zone and the most northern grid cells showed a larger bias than 10 days for DBF_SOS\({}_{\text{mod}}\) (Figure 8b) and the bias was smallest for the transient region between the middle boreal and northern boreal zone. Throughout Finland, the multi-year average ENF_SOS\({}_{\text{mod}}\) took place 20-40 days earlier than the simulated date of the JSBACH start of \"growth phase\" of ENF. The time lag between these two  
Figure 5: **(a–c)** Daily GPP, mean and minimum air temperature and snow depth for Sodankylá in spring 2008. Vertical lines indicate the start of season date: JSBACH local (magenta) refers to ENF_SOS\({}_{\text{mod,loc}}\); JSBACH regional (cyan) is ENF_SOS\({}_{\text{mod}}\). Note that ENF_SOS\({}_{\text{EC}}\) occurred on the same day as ENF_SOS\({}_{\text{mod}}\) shown with cyan line. -phenological events decreased from south-west to north-east and increased again for northern areas of Lapland. In the second step, we looked into year-wise comparisons of the start of season dates (Figure 9, Supplementary Materials: Table S3). The correspondence between the annual dates from JSBACH and from satellite observations showed large variations in R\({}^{2}\), RMSE and bias. As shown in Figure 9a,c, for many adjacent grid cells ENF_SOSmod occurred on the same day (clumping), whereas we found a more gradual change from south to north in ENF_SOSsat. In order to assess the reasons for clumping, we analysed the change in 5-day mean temperature, which is a good predictor of spring recovery in ENF, according to [PERSON] et al. (2018), and the 5-day mean temperature often exceeded zero degrees concurrent with ENF_SOSmod. Thus, a change in weather pattern may simultaneously trigger the start of season in a large area. Clumping was not observed for DBF_SOSmod (Figure 9; lower panel). Figure 6: Spatial distribution of mean start of season dates in Finland for the period 2003–2010.