## 1 Introduction  
The eddy covariance method is a powerful tool for evaluating fluxes of carbon dioxide and water between terrestrial ecosystems and the atmosphere over different temporal scales (e.g., [1]). Among other parameters, eddy covariance is used for the estimation of a forest ecosystem's gross primary production (GPP) and evapotranspiration (ET). GPP represents total ecosystem CO\({}_{2}\) uptake, while ET is the sum of evaporation from soil, canopy and litter perception, and plants' transpiration. On average, ET is responsible for the return of 70% of precipitation to the atmosphere globally [2], and for the majority of water leaving semi-arid regions [3]. On a global scale, the annual GPP of forest ecosystems is on average approximately 53.7 \(\pm\) 4.8 Pg C yr\({}^{-1}\)[4]. Both parameters are key variables to understand the energy, water and carbon cycles in forests and to link them with ecosystem functioning and climate feedbacks [5, 6]. The coordinated responses of GPP and ET to environmental conditions are vital for ecosystems' performance. While elevated ET may increase ecosystem vulnerability, particularly in drought-prone regions, improved GPP and water use efficiency can enhance the carbon fixation and storage of an ecosystem. Meteorological and soil water conditions may strongly affect ecosystem water and carbon fluxes through their control of canopy transpiration and evapotranspiration [7, 8, 9], as well as ecosystem productivity [10], which is driven by plant photosynthesis. At a short temporal scale, the optimum equilibrium between maximizing photosynthesis and minimizing water losses through transpiration is governed by stomatal responses to solar radiation and vapor pressure deficit--VPD [8; 11]. Net radiation may be a strong determinant of ET and GPP on a monthly basis [12]. In the long-term, ET has been found to follow the variations in precipitation [12; 13], but this was not the case for GPP [14]. Moreover, the effect of air temperature is critical for ecosystem evapotranspiration, particularly at sites characterized by warm and dry summers [15]. There, the summer evapotranspiration deficit may negatively impact ecosystem productivity [16]. Drought effects on ET and GPP are also the result of a combination of limited soil water supply and enhanced VPD. Intense and prolonged drought events have the strongest influence on GPP [10], while in extremely dry years soil water may control both the water and carbon dynamics in temperate forests; declining GPP and ET were evident with decreasing soil water, particularly so in broadleaf rather than in conifer forests [17]. However, the effect of soil water availability is complex, being controlled by the root depth of the trees [18]. On the other hand, the exact effect of VPD on both carbon and water fluxes at an ecosystem level is still a topic of debate. Additionally, VPD is strongly coupled with soil water content (SWC), making the distinction of the drought effects on ecosystem fluxes quite difficult. In the context of ongoing climate change and the established increase in VPD [19; 20; 21; 22], it is crucial to assess in detail the effect of VPD on ecosystem fluxes. _Robinia pseudoaccia_ L. (black locus) is native to the NE Unites States, and was introduced to Europe in the early 17 th century [23]. Nowadays, it also occurs in Asia, southern Africa and Australia and it is the second-most-often planted tree species worldwide [24]. Black locust exhibits great plasticity under diverse climatic conditions, indicating a high acclimation potential for climate change [25]. The species is extensively used for the establishment of plantations for soil and landscape reclamation at degraded sites, due to its advantageous traits. It is a light-demanding, fast-growing and drought-tolerant legume tree [26] that regenerates and spreads rapidly with root-suckers [27].