## 1 Introduction  
Terrestrial gross primary productivity (GPP) and evapotranspiration (ET), two critical components of the terrestrial carbon and water cycles, are driven by solar radiation and limited by soil moisture (and nutrient) availability [1, 2, 3, 4, 5, 6, 7]. In Africa where 50% of the land areas are covered by arid and semi-arid ecosystems [8], the limiting factors of GPP and ET include precipitation, which controls soil moisture available for plants, and nutrient availability [9, 10, 11, 12, 13]. Here, we investigate two widely used vegetation resource use variables, which offer valuable insight into the representation of carbon and water coupling in ecosystem process models: Light Use Efficiency (LUE) and inherent Water Use Efficiency at the ecosystem level (iWUE*). LUE is defined as the ability of the vegetation to use GPP per unit of Absorbed Photosynthetically Active Radiation (APAR) that is limited by temperature and water shortage [14, 15]. This definition integrates limiting environmental factors through the fraction of absorbed photosynthetically active radiation (fAPAR) and provides LUE value below its theoretical potential value used in several studies [1, 16, 17].  
iWUE* is defined as the product of GPP and vapor pressure deficit (VPD) per ET unit [18]. Because of the strong relationship between VPD and stomatal conductance [19, 20], iWUE* appears to be more relevant than the water use efficiency to describe the biochemical functions of plants at ecosystem level [18]. The dependence of iWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment.  
African ecosystems, especially the northern savannah, experienced a highly variable climate during the last decades, which directly affects pan-tropical climate and ecosystem water and carbon fluxes [21, 22, 23, 24, 25, 26]. Previous studies reported that high climate variability in terms of solar incidence (through cloudiness), rainfall, temperature and VPD impacts LUE and iWUE* [27, 28, 29]. Atmospheric CO\({}_{2}\) and land use change also alter ecosystem physiology and structure, and consequently LUE and iWUE* trends [30, 31, 32, 33, 34].  
Because of the scarcity and heterogeneity spatial distribution of existing _in situ_ measurements in Africa [35, 36], remote sensing products are valuable to constrain phenology and carbon fluxes over Africa, and were used in several previous studies [37, 38, 39]. Two satellite-based long-term records are used in this study to assess and analyze iWUE* and LUE simulated by different versions of the ORCHIDEE (ORganizing Carbon and Hydrology In Dynamic EcosystEms) land surface model. The 1982-2010 fAPAR3g third generation satellite dataset is used to generate the LUE data product [40]. 1982-2010 GPP and ET data products were generated from an empirical model calibrated from _in situ_ measurements at FLUXNET sites and are used in LUE and iWUE* calculation [41]. The VPD and PAR (photosynthetic active radiation), needed for the calculation of iWUE* and LUE respectively, are computed using the WATCH-Forcing-Data-ERA-Interim (WFDEI) climate reanalysis.  
The main goal of this present study is to assess the LUE and iWUE* trends during 1982-2010 using the remote sensing product and to evaluate the ability of ORCHIDEE to better reproduce these trends of vegetation resource use. Furthermore, we separate the CO\({}_{2}\) and climate effects on LUE and iWUE* trends by using factorial simulations: (1) a simulation with a constant CO\({}_{2}\) and (2) a simulation with an alternative climate forcing.