## 4 Discussion and Conclusions
Variables such as gross primary productivity (GPP), net primary productivity (NPP), leaf area index (LAI), fraction of photosynthetically active radiation (FAPAR), surface temperature (LST), albedo, and evapotranspiration are more directly indicative of major biogeochemical cycles, such as that of carbon and water, which underlie many ES [27; 37]. They are classically derived in the form of TERRA/AQUA MODIS time series, offering the possibility of continuous seasonal monitoring of the phenology of ecosystem functioning attributes and even detecting reversible or irreversible disturbances that can affect them [86; 87]. However, the relatively coarse spatial resolution of these sensors (between 500 m and 1 km) is not well suited for monitoring the ecosystems of the Ouled Hanneche forest. At the local scale of our study area, our experience has indeed shown us that the spatial resolution of LANDSAT probably corresponds to the upper limit of pixel size for which the measured spectral features could match the dimensions of our field surveys. Moreover, another advantage offered by LANDSAT is its long historical coverage [88]. At the desired spatial resolution (i.e., 30 m on a side), the archive of this Earth observation program is accessible back to the 1980s [88], allowing monitoring and comparisons of forest conditions by transferring our method to earlier dates, which is consistent with the concept of spectral feature variation [40; 86]. In perspective, methodological improvements are to be considered, especially first by applying them to our existing input dataset. The leaf area index (LAI), which provides information on canopy structure and is related to many functional processes, can be estimated from empirical relationships with reflectance or vegetation index values from LANDSAT images [89]. However, the empirical parameters need to be calibrated to the specificities of the ecosystem under study, and these estimates sometimes lack precision, especially when the vegetation cover is sparse [89]. An alternative to these approaches consists of inverting a Radiative Transfer Model (RTM), which can better return some of the biophysical and/or biochemical properties of forest ecosystems [90]. Among the set of RTMs available, one of the most popular and technically affordable is the PROSAIL model [91; 92], which combines the PROSPECT leaf optical properties model with the SAIL bi-directional canopy reflectance model. It allows us to describe, in the solar spectrum (400-2500 nm) and at the canopy level, the spectral features related to leaf biochemistry, mainly chlorophyll, water, and dry matter content, as well as those related to canopy architecture, such as the leaf area index (LAI), leaf angular distribution, and relative leaf size [91]. In addition, it is also possible to return the FAPAR, a key variable for primary production, and the albedo, an essential element for estimating the surface energy balance used to calculate evapotranspiration. Generally, the inversion of this RTM provides the most satisfactory results when applied to hyperspectral data but it can also be adapted to multispectral sensors [92]. More than for LANDSAT, PROSAIL is frequently inverted from SENTINEL-2 images; an inversion procedure using a regression algorithm derived from machine learning is specifically implemented in the SNAP software [93]. Given the spatial resolution of SENTINEL-2, 10 m, this approach should be seriously considered to study the properties of plant communities in the Ouled Hanneche forest. Ideally, most of the indicators described above could be retained, either as a replacement for or in addition to the variables created during this work, to carry out a decisional synthesis of the potential of the forest's provision of ES according to a method similar to that indicated in Section 2.9. Advances in the field of spatial multicriteria analysis (SMCA) [94] allow for a better representation of the expert opinions and trade-offs necessary to evaluate the multifunctionality of the forest and the services it provides [95].