## 4 Discussion
### The Studied Black Locust Plantation Is Characterized by High GPP and ET  
Given that no data have been published on the GPP and ET of other black locust ecosystems, comparisons can only be made with other forest ecosystems that prevail in the Mediterranean area or are characterized by a Mediterranean climate. Overall, the annual GPP in the studied plantation was 1914.7 and 1445.0 g C m\({}^{-2}\) yr\({}^{-1}\) in 2020 and 2021, respectively (Table S1), and it was similar to the mean annual GPP of 11 other deciduous broadleaf forest sites in Europe within the Fluxnet network (on average 1644.4 \(\pm\) 245.12 g C m\({}^{-2}\) yr\({}^{-1}\); [58]). However, the GPP of black locust plantations was higher compared to that, either measured by an eddy tower or estimated by models, of other Mediterranean-type ecosystems or ecosystems with a Mediterranean climate, dominated by _Quercus ilex_. [59] and _Pinus brutia_[37], as well as Chestnut, European beech and other forest species [36]. Furthermore, the GPP of the studied black locust peaked at higher levels (approximately 12-14 gC m\({}^{-2}\) d\({}^{-1}\)) compared to the respective maximum values recorded in a deciduous oak ecosystem with a Mediterranean climate during a 12 yr period (c. 8 gC m\({}^{-2}\) d\({}^{-1}\); [60]). Although these GPP estimations generally refer to mature ecosystems, the differences in GPP may also, to some extent, be due to age effects. Nevertheless, the elevated GPP rates of black locust are in line with the species' high growth rate, particularly at a young age [24; 44], even though the studied plantation grows on particularly infertile post-mining depositions [31]. However, an important contributor to the ecosystem's GPP, apart from black locust, is the understorey grasses vegetation which performs a considerable CO\({}_{2}\) fixation in the months prior to the initiation of leaf expansion in black locust (January to March), and its contribution to GPP continues until the dry summer period, when it wilts and is quickly decomposed, as similarly observed by others [34]. To assess the seasonal dynamics of the grasses understorey and estimate its GPP, light use efficiency models with phenological indices were developed in an Australian savanna ecosystem [61], but this approach has not been applied at the study site yet. The annual ET at the studied site ranged from 578.5 to 739.6 mm yr\({}^{-1}\) (Table S1) and it was comparable to that of 11 Fluxnet broadleaf deciduous European forests (622.7 \(\pm\) 133.2; [58]), mixed forests with Mediterranean climate in California, USA [62], and with holm oak and Aleppo pine ecosystems with similar precipitation in Spain [63; 64]. With regard to the seasonal fluctuation of ET, [PERSON] et al. [65] recorded the ET minima and maxima of a Mediterranean evergreen coniferous forest in winter and summer months, respectively, in line with the seasonal ET pattern we observed (Figure 3b). In two forest ecosystems with a Mediterranean climate and similar precipitation and mean annual temperature as in oursite, ET peaked earlier than in the present study, in June, and decreased in the following summer months [62]. Similarly, tree transpiration and the water requirements (transpiration and vegetation and soil evaporation) in black locust ecosystems were maximum around June [33; 66]. On the other hand, in the studied black locust ecosystem, ET was maintained at high levels until July in both study years and even peaked during the xerothermic period in 2020 (Figure 3b), indicating a non-conservative water use. In the same context, it was observed that the ET of the tree layer of a mixed _Olea sylvestris-Quercus super_ Mediterranean ecosystem was not greatly affected by summer drought and that the wilting of the grasses understorey in the summer had an influence on ecosystem ET [34]. Although grasses' wilting and replacement by bare ground was evident in the xerothermic period in the studied plantation, we were not able to distinguish its effect on evapotranspiration at the ecosystem level.