## 1 Introduction  
Since the early 1950s, a continuous increase in the atmospheric CO\({}_{2}\) concentration has been reported [1]. The rapid release of greenhouse gases into the atmosphere has been associated with global climate warming [2] and anthropogenic climate change. Climate change is rapidly altering the vegetation structure, plant phenology, and carbon uptake of the terrestrial ecosystems [3]. There are serious concerns that sustained global climate change in the European continent may lead to significant impacts on plant distribution, growth and productivity [4]. According to the national reports of the conservation status of species and habits by the EU Habitats Directive [5], 14% of habitats are already under pressure, and 33% of the habitats are projected to be continued threatened by climate change in the near future. Thus, European terrestrial ecosystems are particularly sensitive to climate change and deserve special attention [6]. Thus, a better understanding of the response of carbon dynamics in terrestrial ecosystems across Europe is needed. Satellite observations have indicated that the Earth has been greening during the past three decades [7; 8; 9], especially in the middle and high latitudes in the Northern Hemisphere [10; 11]. Greening refers to increases in vegetation greenness based on spectral vegetation indices (e.g., the normalized difference vegetation index) or leaf density based on vegetation the leaf area index (LAI). Although identifying the drivers of the observed greening is complicated, recent studies have made advances using model simulation methods. On the global-scale, CO\({}_{2}\) fertilization effects explain most of the greening (approximately 70%) [8]. However, there are some differences in the key factors driving the greening phenomenon on a regional scale due to heterogeneous land surfaces impacted by human activities. For example, China and India lead the greening process [6], mainly due to increases in forest area caused by afforestation and increases in farmland due to agricultural intensification, respectively. Thus, land use and land cover change (LULCC) is recognized as the main driver of the observed greening trend [7] and the enhanced terrestrial gross primary production (GPP) in these countries [12]. LULCC is often accompanied by changes in LAI and vegetation structure. Thus, it is difficult to distinguish the contributions of LULCC and LAI to GPP in regions where intense land use and management exist (e.g., China and India). Europe is among the regions with the largest greening trends, and climate change is the major factor driving land surface greening throughout Europe [8]. Part of the observed greening in northern Europe is due to temperature warming [13], which can prolong the growing season length and allow biochemical and physiological processes to proceed at faster rates [14]. In regions where widespread farmland abandonment has occurred, such as Eastern Europe and southern and central Europe, woody regrowth also contributes to the observed vegetation greening [15]. Compared to India and China, European vegetation cover is relatively high (approximately 97%) and stable without strong correlation between land surface greening and LULCC. Thus, quantifying the driving factors of GPP dynamics in Europe can help us better understand the climate change feedback of the terrestrial ecosystems [7]. GPP is a product of photosynthesis, which is regulated by both environmental factors and biological processes on different temporal and spatial scales. On the leaf-scale, the photosynthetic rate and stomatal conductance can adjust in response to environmental changes within seconds. On the stand scale, individual plants may adjust anatomically to changes in environmental factors due to acclimation and adaptation on longer time scales [16].