## 4 Discussion  
Mediterranean-type ecosystems cover a small fraction of the terrestrial component of the Earth. They are often neglected in Global modeling exercises or included in vegetation classes that comprise various vegetation types growing under contrasted abiotic and biotic constraints. Nevertheless, Mediterranean-type ecosystems remain a good system model for studying broadleaved evergreen plant covers that have to tackle recurrent severe droughts during their vegetative season and under the threats of the on-going climate changes. Among the dominant key-species in this area, _Quercus__i_lex trees are present across more than 6 Mha around the Mediterranean Sea. They occur in dense woodlands, such as the one we analyze here, as well as open woodlands or even savanna-like formations.  
Concerning the climate constraints, all simulations performed with high resolution coupled atmosphere-ocean regional climate models or with coarser resolution GCMs under SRES A2 and A1B scenarios gave a collective picture of a substantial drying and warming of the regions surrounding the Mediterranean Sea, especially in the warm season [4, 39, 40]. The comparative analysis of [41] placed this region among the most responsive regions to global climate change, and one of the main climate change \"hotspots\". This is also true for California. Global changes have the potential to deeply modify local climate patterns mostly by lowering spring soil water availability leading to reduced summer  
Figure 4: Nonlinear regression between the relative soil water content and the weekly relative error of GPP between MODIS estimates and measurements at the tower flux.  
convection and large land-sea contrasts in warming. All this is leading to increased air dryness and decreased precipitation [42]. As a result, we are expecting a warming from +3.2 \({}^{\circ}\)C in Winter (DJF) and exceeding 5 \({}^{\circ}\)C (+5.2 \({}^{\circ}\)C) in Summer (JJA). For our study site, precipitation projections yielded decreases of ca. 0.1, 0.4, 0.5 and 0.35 mm\(\cdot\)day\({}^{-1}\) for DJF, MAM, JJA and SON, respectively [4]. The consequence for the Spring MMA rains will be a significant decline of 18%. More drastically, the Summer JJA amount, currently 108 mm, will lose 45 mm with expected negative impacts on ecosystem functions or more frequent perturbations and particularly fires [43, 44, 45]. Inter-annual variability is projected to increase as the occurrence of extreme heat and drought events [46, 47]. Return periods for drought durations lower than 4-6 months will be multiplied by 3 while return periods for drought durations longer than one year will be multiplied by 7 [3, 48].  
In such an already water-limited climate going to drier conditions, we have to focus on the mechanisms that facilitate or constrain the ability of Mediterranean terrestrial ecosystems to cope, adapt or recover from various disturbing forces and particularly to severe droughts. Factors related to climate change are likely to play a key role in predisposing ecosystems to tipping points [49, 50, 51], while vulnerable ecosystems are already being influenced by multiple environmental drivers. This reinforces a general view that synergisms among stressing elements can be extremely important, predisposing ecosystems to serious environmental changes. GPP is the first step of C-use cycle which has been shown to be drastically affected by heat waves [9] and drought [16, 52, 53]. GPP may be useful for evaluating regional carbon budget while also contributing to detect early warning sign of dysfunctions.