## 3 Results  
VPD was clearly higher and SWC lower in 2021, compared to 2020, and the difference between the two years was significant (Figure S1b,c). The mean and the max values of VPD were 16.4 and 36.1 hPa, respectively, in 2021 and 10.7 and 22.1 hPa, respectively, in 2020. On the other hand, Rg and EVI were similar between these two years (Figures 2 and S1a,d). The peaks of both GPP and ET were higher in 2021 than in 2020 (Figure 3). However, 2020 was characterized by a longer period of higher mean daily GPP and ET, which finally resulted in significantly higher mean GPP and ET in 2020 vs. 2021 (Figure S1e,f). Figure 3: Seasonal fluctuation of daily GPP (**a**) and ET (**b**) values for the entire study period, expressed in 8-day averages. The grey bands in plots (**a**,**b**) represent the standard deviation of the means. The green and blue colour marking in plots (**a**) and (**b**), respectively, highlight the summer period (June–August). Figure 2: Seasonal fluctuations of environmental parameters during the study period: (**a**) Rg, (**b**) VPD, (**c**) SWC, expressed in 8-day averages, and (**d**) EVI, expressed in daily values, after the implication of the Loess smoothing function. The grey bands in plots (**a–c**) represent the standard deviation of the means. The green colour marking highlights the summer period (June–August). GPP peaked earlier in the day (around 09:00 a.m. local wintertime) in comparison to Rg (Figure 4a). This was controlled by VPD, as indicated by Figure 4c; increasing Rg resulted in increased GPP both before and after midday, but GPP was higher when VPD was lower, from the morning until midday. On the other hand, ET closely followed the diurnal variation of Rg (Figure 4b) independent of the diurnal variation of VPD (Figure 4d). To better illustrate the role of VPD on the relationship of Rg with GPP and ET, these relationships were tested within given VPD thresholds under optimal SWC > 20% (Figure 5). Optimal SWC was chosen based on the relationship between GPP and SWC shown at Figure S2c. For comparability reasons, the same SWC range was used for testing the relationship between ET and Rg (Figure 5b). Under these prerequisites, it was shown that at VPD levels higher than 20 hPa, GPP saturates at Rg around 500 W m\({}^{-2}\) and it is overall somewhat lower than at VPD below 20 hPa (Figure 5a). On the other hand, no effect of VPD on the relation of ET to Rg was found (Figure 5b). Figure 4: Diurnal fluctuation of mean instantaneous GPP (**a**) and ET (**b**) values in relation to the corresponding diurnal Rg patterns. The control of VPD on the effect of Rg on GPP and ET is shown in plots (**c**) and (**d**), respectively. Means of instantaneous values of the summer months (June to August) of the entire study period are presented. Instead of applying simple or multiple regression analyses which premise the linearity in the relationships among the independent and the dependent variables, we employed GAM analysis which allows non-linear data to be explained without the assumption of standard-shaped relationships, in order to assess the responses of daily GPP and ET to Rg, VPD, SWC and EVI. All independent parameters were predictors of GPP and ET at \(p<0.001\), except for VPD, which significantly controlled ET at \(p=0.016\) (Table 1). Both GAM models were quite strong, with R\({}^{2}\) values at 0.79 and 0.84 for GPP and ET, respectively. GPP increased almost linearly with increasing Rg until the level of c. 400 W m\({}^{-2}\) (Figure 6a), while the opposite response of GPP to increasing VPD was observed (Figure 6b); GPP declined sharply, as VPD increased above 10 hPa. Additionally, GPP became saturated when SWC and EVI reached 25% (Figure 6c) and 0.65 (Figure 6d), respectively.