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Satellite observations have revealed strong land surface “greening” (i.e., increases in vegetation greenness or leaf area index (LAI)) in the Northern Hemisphere over the past few decades. European terrestrial ecosystems are a greening hotspot, but how they respond to land surface greening, climate change, CO₂ fertilization, land use and land cover change (LULCC) and other factors is unclear. Here, we assessed how these interacting factors might be combined to alter terrestrial gross primary production (GPP) throughout Europe during the period of 2001 to 2016 using a process-based Farquhar GPP model (i.e., FGM). We found a more productive European terrestrial ecosystem and most of the GPP enhancement in Europe was explained by increases in LAI (62%) and atmospheric CO₂ concentration (29%). Spatially, the spatial signature of the LAI and GPP trends both suggested widespread (72–73% of the vegetated area) greening phenomena across Europe, among which 23.7% and 13.3% were statistically significant (p < 0.05). The interannual trend of GPP estimated by the FGM (0.55% yr⁻¹) was reasonable compared with other GPP products (0.47% yr⁻¹ to 0.92% yr⁻¹) and the observed LAI increasing rate (0.62% yr⁻¹). FGM factorial simulations suggested that land surface greening (+35.5 Pg C yr⁻², p < 0.01), CO₂ fertilization (+16.9 Pg C yr⁻², p < 0.01), temperature warming (+3.7 Pg C yr⁻², p < 0.05), and enhanced downwards solar radiation (+1.2 Pg C yr⁻², p > 0.05) contributed to the GPP enhancement, while the enhanced vapour pressure deficit (−5.6 Tg C yr⁻², p < 0.01) had significant negative impacts on GPP, especially in 2006 and 2012, when extreme droughts struck south-eastern Europe. Meanwhile, approximately 1.8% of the total area of Europe experienced LULCC from 2001 to 2016 and LULCC exerted a small but significant (−1.3 Tg C yr⁻², p < 0.01) impact on GPP due to decreases in the total number of vegetated pixels (−159 pixels yr⁻¹). Although the LULCC effect was negative, the largest increase occurred in forested land (+0.9% of total area). In addition, the increasing trends for the annual mean LAI (0.01 m² m⁻² yr⁻¹, p < 0.001) and total GPP (22.2 Tg C yr⁻², p < 0.001) of forests were more significant and higher than those of other vegetation types, suggesting that European forests may continue to play important roles in combating climate change in the future with long-lasting carbon storage potential. These results provide the first systematic quantitative analysis of the driving force of enhanced gross carbon assimilation by European ecosystems by considering variations in leaf physiological traits with environmental adaptations. Full article

The value of leaf photosynthetic capacity (V_cmax) varies with time and space, but state-of-the-art terrestrial biosphere models rarely include such V_cmax variability, hindering the accuracy of carbon cycle estimations on a large scale. In particular, while the European terrestrial ecosystem is particularly sensitive to climate change, current estimates of gross primary production (GPP) in Europe are subject to significant uncertainties (2.5 to 8.7 Pg C yr⁻¹). This study applied a process-based Farquhar GPP model (FGM) to improve GPP estimation by introducing a spatially and temporally explicit V_cmax derived from the satellite-based leaf chlorophyll content (LCC) on two scales: across multiple eddy covariance tower sites and on the regional scale. Across the 19 EuroFLUX sites selected for independent model validation based on 9 plant functional types (PFTs), relative to the biome-specific V_cmax, the inclusion of the LCC-derived V_cmax improved the model estimates of GPP, with the coefficient of determination (R²) increased by 23% and the root mean square error (RMSE) decreased by 25%. V_cmax values are typically parameterized with PFT-specific V_cmax calibrated from flux tower observations or empirical V_cmax based on the TRY database (which includes 723 data points derived from V_cmax field measurements). On the regional scale, compared with GPP, using the LCC-derived V_cmax, the conventional method of fixing V_cmax using the calibrated V_cmax or TRY-based V_cmax overestimated the annual GPP of Europe by 0.5 to 2.9 Pg C yr⁻¹ or 5 to 31% and overestimated the interannually increasing GPP trend by 0.007 to 0.01 Pg C yr⁻² or 14 to 20%, respectively. The spatial pattern and interannual change trend of the European GPP estimated by the improved FGM showed general consistency with the existing studies, while our estimates indicated that the European terrestrial ecosystem (including part of Russia) had higher carbon assimilation potential (9.4 Pg C yr⁻¹). Our study highlighted the urgent need to develop spatially and temporally consistent V_cmax products with a high accuracy so as to reduce uncertainties in global carbon modeling and improve our understanding of how terrestrial ecosystems respond to climate change. Full article