## 4 Discussion
### Physiological and Environmental Controls That Contribute to Elevated GPP and ET  
The achievement of high GPP and ET during the summer period, which indicates the resilience of black locust to xerothermic conditions, may be partially supported by the strategy of paraheliotropism, the movement of leaves so that the leaf lamina is oriented obliquely to the sun direct rays [67]. Such a mechanism is adopted by the leaves of black locust, and it allows the protection of the photosynthetic apparatus against high irradiance and temperature, thus enabling CO\({}_{2}\) fixation and transpiration in warm and dry summer days [68].  
GPP was also favoured by the increasing Rg during the summer and when soil water availability was adequate, but it saturated at SWC levels above 25% (Figure 6a,c). Under such water availability, Mediterranean-type ecosystems experience an optimal water status, illustrated by the absence of sensitivity to precipitation [60]. Increasing vegetation greenness during the summer, as expressed by EVI, also positively influenced GPP (Figure 6b), as similarly observed in other ecosystems with a Mediterranean climate ([60; 65]). In addition, the observed optimum GPP was probably supported by the time lag between GPP and Rg or VPD; we recorded an early achievement of maximum productivity at around 09:00 a.m., before the occurrence of high Rg and VPD (Figure 4a,c). GPP generally declines with increasing VPD, as has been evidenced across different ecosystems ([21; 69]), and it is attributed to photosynthetic limitation due to decreased stomatal conductance (e.g., [70]). In this context, increasing VPD after midday controlled GPP, in line with [52], and resulted in its c. 50% decrease at VPD above 20 hPa (Figure 5a). Moreover, the elevated VPD after midday caused a hysteresis which did not allow GPP to recover to morning levels, as similarly observed in a Mediterranean Eucalyptus site [53]. Nevertheless, although it declined, GPP was still considerable in the afternoon (Figure 4a), thus contributing to the overall high daily and annual GPP of the ecosystem.  
In line with the responses of GPP, ET was positively affected by Rg, EVI and SWC, but without being saturated at any threshold of these parameters (Figure 7a,c,d). On the contrary, no influence of VPD on ET was detected (Figures 4d, 5b and 7b). Plant transpiration increases with increasing VPD up to a certain threshold [22], while the ET response to elevating VPD may either be negative or positive, indicating a conservative or consuming water strategy, respectively [71]. However, ET was almost non-responsive to VPD in our study site, indicating the absence of stomatal regulation to corresponding vapor pressure deficit during the summer. This contradicts previous studies on the leaf level, which indicated that black locust exhibits an isohydric stomatal control and avoids an increase in transpiration due to summer evaporative demand and decreasing soil moisture [15; 33]. Thus, the increase in ET in mid-summer, independent of the elevated VPD, is probably associated with an, on average, adequate water supply. In the same context, it is reported that at high water availability black locust did not impose stomatal control on transpiration even at VPD exceeding the threshold of 20 KPa [72], above which GPP was impacted in our study (Figure 5a).  
However, as VPD continuously rises with the ongoing climate change [19; 20; 21; 22], it cannot be excluded that both the GPP and ET of black locust plantations may be impacted in the near future.