## 5 Conclusions  
Previous studies have reported the sensitivity of light use efficiency (LUE) and inherent water use efficiency (iWUE*) to atmospheric CO\({}_{2}\) increase, climate trends and land use change [27, 28, 29, 30, 31, 32, 33, 34] with very few studies over Africa. These studies were mainly based on _in situ_ measurements and focused on specific locations. The main novelty of this study is the use of recently available satellite data to investigate trends of LUE and iWUE* over African biomes for the past 30 years. The capability of a process based land surface model to reproduce the LUE and iWUE* trends in Africa was also analyzed, which can serve as a basis for evaluation of other models. In addition, we have attempted to attribute the effect of rising CO\({}_{2}\) and the impact of two different gridded climate forcing datasets on LUE and iWUE* trends.  
We found from satellite and climate observations that iWUE* increased significantly over the last three decades over the northern savannas and central African forest biomes (10%-20% per decade), consistent with the long-term increase in WUE observed at forest sites elsewhere [32,64,74, 75, 76, 77, 78, 79, 80]. The ORCHIDEE model only succeeded to reproduce a significant and positive iWUE* trend in the central African forest and not in savannas. Between 30% and 50% of the simulated iWUE* trend is attributed to the increased atmospheric CO\({}_{2}\). We note however that diagnosing trends of iWUE* is subject to systematic errors related to the use of a specific climate forcing dataset. This source of error is very important over the central African forest. This result shows the importance of having accurate climatic historical reconstructions to investigate iWUE* and LUE trends in observations and in models as recently pointed out for drought trends as well [81,82].  
In opposite to iWUE* trend, the observation-derived LUE trend is not statistically significant over most part of Africa. In contrast to the observations, ORCHIDEE shows a 5% LUE increase per decade in central African forest, probably due to the high CO\({}_{2}\) fertilization effect of the model, particularly in forest ecosystems. The simulated LUE trend shows an opposite sign depending on whether rising CO\({}_{2}\) is included or not in the simulation. The effect on simulated LUE trend caused the use of a specific climate forcing is important, especially over Cameroon, for the observation-based product, whereas ORCHIDEE shows a sensitivity to climate forcing over the central African forest.  
The way forward to improve ORCHIDEE simulations of LUE and iWUE* is a better representation of phenology using e.g., _in situ_ observations for calibration. Because the trends of annual LUE and iWUE* can result from different responses during different seasons, evaluation of LUE and iWUE* seasonal trends and variability is a priority for future studies. Finally, it will be necessary to analyze land use change effects on LUE and iWUE* in regions where agricultural area and practice have changed significantly over the last decade.  
This work is supported by the ClimAfrica project funded by the European Commission under the 7 th Framework Programme (FP7). We are grateful to the GIMMS group for sharing the NDVI3g data (we thank [PERSON] and [PERSON]). Through [PERSON], we are thankful to the Department Biogeochemical Integration at the Max Planck Institute for Biogeochemistry for providing Monthly gross primary productivity and evapotranspiration deduced from FLUXNET data from the \"La-Thuile-2007\" synthesis effort.  
All authors contributed extensively to the work presented in this paper. [PERSON] and [PERSON] provided respectively fAPAR3g data and MTE-GPP/ET products. [PERSON], [PERSON] and [PERSON] designed the experiments and carrier out the interpretation of the results. [PERSON] and [PERSON] drafted the manuscript, which was revised by all authors. All authors read and approved the final manuscript.  
The authors declare no conflict of interest.