## 3 Results
### Climate and Drought Patterns  
As the main driver of GPP, we analyzed the seasonal pattern and between-year variability of climate variables registered at the flux tower on a half-hour basis. We also used the subsequent soil water status from a water budget model as an index of drought which is identified as the main constrain for carbon assimilation in the dry season for water limited ecosystems [38].  
Rainfall amounts and temperatures were used in the analysis for the 2001-2010 periods (Figure A1). Rainfall amounts present both high seasonal and inter-annual variations, with annual amounts varying between 1234 mm in 2003 and 570 mm in 2007. Mean rainfall in spring (184.3 mm) is higher than in summer (97.5 mm), with the most extreme dry summers occurring in 2006 (37 mm) and in 2003 (43.2 mm). Autumn and winter are the wettest seasons, with an extremely wet autumn in 2003 accounting for 82% of the total annual amount. Temperatures are also highly seasonal with hottest temperatures reached during the dry season (summer) and varying between 24.6 \({}^{\circ}\)C in 2003 and 20.6 \({}^{\circ}\)C in 2007. Absolute maximum daily mean temperature can reach up to 40 \({}^{\circ}\)C and even more, as in the extreme heat wave of summer 2003. Mean winter temperatures vary around 6 \({}^{\circ}\)C. Therefore the climate in the Puechabon forest is typical Mediterranean characterized by cool, wet winters and hot, dry summers and experiences a strong inter-annual variability of most climate variables. During the 2001-2010 periods, the most extreme events were excessively high temperatures as in the summer of 2003 and sustained moisture deficits in spring and summer of 2006, two key years for GPP anomaly analysis.  
We also analyzed vapor pressure deficit (VPD), minimum temperature (T\({}_{\rm min}\)) and photosynthetically active radiation (PAR), as drivers for LUE model calculation in MODIS GPP. The three variables show similar yearly patterns (Figure 3). The annual average value of VPD is 6.7 hPa, varying between 2.5 hPa in winter and 15.5 hPa in summer. T\({}_{\rm min}\) usually drops below 5 \({}^{\circ}\)C in winter to reach 18.6 \({}^{\circ}\)C in summer, a seasonal pattern in phase with _VPD_ variations. Table A2 presents the interannual variability of yearly mean and maximum PAR for the 2001-2010 periods, with a coefficient of variation of 10% showing little interannual variability. Regarding the seasonal pattern of PAR, we observed a time lag of 25 days in reaching the maximum values compared to T\({}_{\rm min}\). The maximum value of PAR is reached between DOY 150 and 200, while it is reached between days 175 and 225 for VPD and T\({}_{\rm min}\).  
Furthermore, we characterized daily drought intensity susceptible to affect ecosystem functioning by the soil relative water content (RWC) and the yearly integral of water stress (WSI) of _Quercus_ _i_ex ecosystems by capturing plant water deficit through daily estimates of predawn water potentials, and also integrating soil properties and the species functional traits controlling water extraction (leaf area index, root profile, stomatal conductance). When comparing the seasonal pattern of VPD controlling MODIS GPP algorithm with soil RWC (Figure A2), we observed a concomitant seasonal signal with highest VPD in summer when RWC is the lowest, but with a time lag \(>\) 20 days for the maximum values, except for 2003 and 2004 where the two peaks coincide in time. The VPD trend is, however, much smoother than RWC because heavy rainfalls at the end of the dry season lead to a rapid increase in the soil water content to values close its field capacity. The seasonal day of occurrence of maximum T\({}_{\rm min}\)maximum VPD and minimum RWC are presented in Table A3. We observe that the day of occurrence for the lowest value of tower GPP is generally correlated with the day of occurrence for the minimum RWC timing rather than both VPD and T\({}_{\text{min}}\), suggesting that RWC could be a better control for GPP estimates.