## 4 Discussion
### Soil Drought as the Main Driver of GPP Anomalies  
The comparisons of EC GPP and MODIS GPP have already been performed across a large range of climate, land use, and vegetation class and have been found to provide rather good agreements [10, 54, 55]. Discrepancies have been explained by the validity, or not, of internal hypothesis concerning either the maximum LUE derived from the BPLUT or the response sensitivities to both scalars T\({}_{\rm min}\) and VPD. In our case we observed good agreements between the two cool wet seasons. The underestimations of FPAR during these periods (Figure 2) may explain why EC is slightly greater than MODIS. The T\({}_{\rm min}\) scalar seems effective. It increases linearly from zero at \(-8\)\({}^{\circ}\)C to reach 1 at 9.1 \({}^{\circ}\)C. This scalar is rarely questioned as a potential source of error in the literature. The VPD scalar does not act during these periods because it begins to control GPP at only 11 hPa and across the cool periods' 8-day mean, VPD were always lower than this threshold (Figure A2). At the peak of productivity, EC GPP was always lower than MODIS GPP. We suspected first an overestimation of LUE\({}_{\rm max}\). These over or underestimations have been already discussed as the main source of errors in [55], [10] or in [56]. [PERSON] _et al. [36]_ observed this overestimation by comparing multiyear EC measurements between two contrasted ecosystems in Australia, an _Eucalyptus_ spp. dense forest and a tropical savanna both ecosystems dominated by broadleaved evergreen tree species. [PERSON], [PERSON] _et al._[57] carried out an extensive analysis showing the strong control of rainfall on the inter-annual variation of GPP as estimated using a LUE-based GPP model in a water-limited region. As [38] did further, they suggested applying site-specific estimates. [PERSON], [PERSON] _et al._[56] proposed a modifier based on evaporative fraction (EF) to replace VPD in water limited regions. They found good agreement between tower GPP and GPP estimated using site specific parameters and EF via a LUE model. In our case, for the two well-watered, highest WSI years, 2002 and 2004, the lowest discrepancy was observed in 2002. This year displayed a very limited effect of VPD and an insignificant control by soil water limitation (Figure 5). At a VPD of 15 hPa the scalar is affected by only 14%. At 25 hPa it declines by half. The more drastic effects on peak GPP was observed in 2006 the driest year of our study period with a \(\mathrm{WSI}=-358.6\) MPa day. This year revealed how early severe drought that started in February is not taken into account by the VPD scalar. Interestingly, in 2003, a heat wave took place from June to August. It was associated with a period of high VPD in phase with the lower relative soil water storage. At this level of VPD, the scalar drastically declined by 80%, but insufficiently to closely mimic EC GPP. In a MT climate, there exists a substantial delay between peak air VPD and lower soil \(RWC\) (Table 4). As a consequence, this delay, even changing the threshold values in the VPD scalar, prevents the direct use of VPD as an efficient surrogate of soil water limitation. Among others, there is an increasing debate about the use of MOD15A2 FPAR, both in terms of quality of this dataset when compared to other available products, and in term of effective radiation absorbed by chlorophyll for photosynthesis (see for a substantial account [58, 59, 60, 61, 62] and also [63, 64, 65]). The widely used FPAR is in turn a canopy level index, accounting for both the photosynthetic active vegetation and the non-photosynthetic parts including stems, branches or senescent leaves. This debate also applies to ground estimates of the leaf area index [66] and biases linked to remote sensing signals in general [67]. Increasing efforts are being devoted to obtaining more accurate information concerning the vegetation photosynthetic capacity retrieved from space borne measurement [68, 69, 70, 71].