## 1 Introduction  
Climate change results in rising air temperature and changes in the frequency and distribution of precipitation [1]. More-frequent extreme events are projected for the next decades in many regions around the globe [2]. Earlier studies have suggested a significant increase in frequency and severity of droughts [1; 3], especially in the Mediterranean region [4; 5; 6; 7; 8; 9]. Knowledge of mechanisms through which climate variations affect forest ecosystem processes, such as productivity, phenology, regeneration, and mortality, is still partial [10; 11; 12]. Consequently, there is an urgent need to realize a permanent assessment of forest sensitivity to climate change [13; 14]. Mediterranean forests are considered particularly susceptible to future warming and drought conditions due to their location within the transition zone between arid and temperate climates [2; 15; 16; 17]. Several studies have reported increased plant mortality rates and die-off events, reduced seedling recruitment, long-term shifts in vegetation composition, reduced radial growth, and increased crown defoliation responses [18; 19; 20; 21; 22]. Similarly, studies carried out in boreal forests, located in regions less constrained by drought, have reported reductions in productivity and widespread increases in tree mortality because of increasing drought stress [23]. On the one hand, warming conditions might promote drought stress, reducing gross primary production (GPP, representing the sum of gross carbon uptake by plant photosynthesis) and enhancing root and tree mortality [24; 25; 26; 27]. On the other hand, warming conditions could increase: (i) forest growth from CO\({}_{2}\) fertilization, (ii) water use efficiency, and (iii) growing seasons duration [28; 29; 30; 31; 32]. Soil moisture plays a pivotal role in vegetation dynamics [8; 33], considering that soil water availability is a crucial limiting factor for plant photosynthesis [34; 35]. However, soil water content (SWC) is a critical variable depending on many factors such as precipitation amount, the duration and intensity of drought, level of soil erosion, surface runoff impacts, and rate of evapotranspiration [36; 37]. Consequently, SWC is difficult to estimate both with field measurements and model estimations [38]. There are different methods to estimate SWC, such as volumetric and gravimetric methodologies [39], or remote sensing techniques [40; 41]. The Soil Moisture and Ocean Salinity (SMOS) mission conducted by the European Space Agency (ESA) has the capacity to estimate soil moisture globally and routinely based on a microwave radiometer system [42]. To date, SMOS soil moisture data have provided particularly suitable long-term records of global soil moisture, even if limited to shallow soil [43]. Remote sensing can be used to generate a vast amount of information about the Earth's surface and can be useful for suitable land-management planning. A strong relationship was found between SWC and values derived from surface temperature-vegetation index (Ts-VI) methods using optical and thermal remote sensing data as inputs [44]. Based on this method, a temperature-vegetation wetness index (TVWI) was developed to estimate SWC in a forest-dominated and topographically variable landscape in eastern Canada [45]. The integration of the normalized difference vegetation index (NDVI) and potential surface temperatures (\(\theta\)s) was used for an indirect assessment of soil moisture [45]. Following the concepts described above, a modified version of the temperature vegetation wetness index (mTVWI) was proposed using the complementary value of crown defoliation instead of NDVI [46]. In Europe, tree crown defoliation is monitored by the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) and is regarded as morphological damage affecting the green biomass of plants [47].