## 4 Discussion
### Correlations, Impacts, and Anomalies of GPP and mTVWI  
Significant correlations between GPP and mTVWI were found in most of the European tree species. The highest positive coefficients were found for two Mediterranean species, _Q.__ilex_ and _P.__pinaster_. Greater water availability allows higher stomatal conductance and thus a positive relationship between mTVWI and GPP [76]. Conversely, we found negative GPP-mTVWI relationships in other species (e.g., _P.__sylvestris_, _P.__remula_) mainly growing in Central-Northern Europe. These results were well corroborated by [PERSON] et al. [77] in 2011, where they found a strong association between tree growth and a drought index (the standardized precipitation index) in _P.__sylvestris_. On the other hand, other studies in Northern European countries did not find any association between SPI and crown defoliation (e.g., [78]). The different responses to drought from tree species are related to different strategies in coping with drought and local conditions of the forest stands [77]. As a function of site location and tree species, mTVWI negatively affected GPP from 20 to 80%. We found that broad-leaf evergreen (_Q.__ilex_) and conifer species showed higher GPP reduction than broad-leaf deciduous species. A generalized increase in defoliation and mortality occurring was observed during 1987-2007 in Southern European forests (principally in _Q.__ilex_ stands), largely related to a severe drought regime [79], thus confirming our results. In agreement with our results, a high sensitivity to drought was found for broad-leaf Mediterranean forests [24]. _P.__pinaster_, a Mediterranean species, showed high GPP reductions (50-80%) over the years, which are in line with earlier studies suggesting a high dependence of water availability in mixed pine-oak Mediterranean forests [80; 81; 82; 22]. Probably, forests located in sites experiencing strong drought events (e.g., Southern European forests) could reduce their carbon sink efficiency and contribute to the reduction in carbon sink in the Northern Hemisphere [83; 82]. However, the response of GPP to water availability significantly varied among species and locations. For example, _B.__pendula_ and _B.__pubescens_, mainly distributed in northern temperate regions [84], showed low GPP reductions (10-30%). Conversely, _F.__excelsior_ was characterized by moderate-to-high GPP reductions (30-60%) being a water-demanding species suffering severe damages under increasing temperature [85]. Similarly, _F.__sylvatica_ showed high GPP reductions (up to 70%) likely due to its high sensitivity to drought [24]. Furthermore, an increase of crown defoliation was reported for the beech, especially in the Mediterranean area [50]. In fact, sensitivity to extreme temperature in _F.__sylvatica_ increased with the decreasing of soil water availability in the Mediterranean area [86].  
Anomalies analysis showed a similar pattern of mTVWI and GPP anomalies over the time period 2000-2010, highlighting that negative anomalies of mTVWI (more aridity) induced a reduction of GPP. This result is in line with several studies (e.g., [87; 88]) that reported a reduction of GPP NPP related to drought. Taking into account the 2003 heat wave, we found a highly negative deviation from the grand mean of GPP, whereas mTVWI did not show a corresponding variation. However, it should be noted that mTVWI was calculated considering temperature of the overall year (2003), whilst the peak temperatures in 2003 were found only during the growing season [68], thus impacting mainly GPP.