## 3 Results
### CARAIB Simulations for the Present
#### 3.1.2 Modelling Validation  
The model projects relatively well the distribution of the 12 tree species with sensitivity values higher than 90% (Table 4), regardless of adjustment of morpho-physiological parameters. Simulated NPP maps with superimposed species occurrences are available for all the species in the Supplementary S1 and the example of _Pycanthus angolensis_ is given in Figure 3. We note that all the simulated species have a rather similar pattern of distribution. For _Pycanthus angolensis_, we observe an extension of the species distribution when species-specific traits are used, encompassing some observed species occurrences (at the northernmost and southernmost latitudes) which were not considered in the species distribution using PFT traits. Yet more importantly, the increase in high NPP values occurs in regions where species presence is attested by observations. Simulated NPP and biomass values are in the range of the values for tropical trees [92]. According to [PERSON] [93], annual NPP varies between 800 g C m\({}^{-2}\) yr\({}^{-1}\) in the tropical seasonal forests and 1100 g C m\({}^{-2}\) yr\({}^{-1}\) in the tropical rain forests. In a seasonal tropical forest of Mexico, wood and root increments are as low as 285 g m\({}^{-2}\) yr\({}^{-1}\) of dry matter, i.e., 143 g C m\({}^{-2}\) yr\({}^{-1}\)[94]. Our GPP results obtained from a PFT-based simulation are also coherent with the ecosystem GPP presented in [PERSON] et al. [89]. For the studied region extended from 16\({}^{\circ}\) S to 16\({}^{\circ}\) N in latitude and from 17\({}^{\circ}\) W to 40\({}^{\circ}\) E in longitude, the correlation coefficient (r) is equal to 0.92 and the root-mean-square error (RMSE) is equal to 510.89 g C m\({}^{-2}\) yr\({}^{-1}\). Due to the lack of data for the individual species, we were unable to perform an evaluation of productivity at the species level. The fact that the projected species  
Figure 2: Net primary productivity and water use efficiency of the species _Pycanthus angolensis_. (**a**) Frequency distribution histogram of simulated NPP (g C m\({}^{-2}\) yr\({}^{-1}\)), using respectively PFT-(hatched bars) and species-specific (white bars) morpho-physiological traits (see Table 2). Dotted line indicates the 100 g C m\({}^{-2}\) yr\({}^{-1}\) NPP threshold. Annual mean water use efficiency (g C m\({}^{-2}\) mm\({}^{-1}\)), using (**b**) PFT- and (**c**) species-specific traits. Simulations were obtained with CRU historical climatic data for the reference 1980–1999 period. distributions do not cover all species occurrences could suggest that the model underestimates NPP values, but it could also be a consequence of the calibration of the germination and stress thresholds for the plant species. To give a perspective to the model projections for the future, we calculated the change in simulated carbon stock in African tropical forests over historic period for the three CO\({}_{2}\) configurations. The biomass change is calculated in terms of annual change between 1968 and 2007 with the idea of a comparison with field plot biomass studies [24]. A biome classification was derived from PFT distributions (same simulation as for GPP validation). Tropical rainforest and tropical deciduous forest (and woodland savanna) biomes were merged to consider entire tropical forests. Biomass change between 1968 and 2007 corresponds to a decrease of 8.1 g C m\({}^{-2}\) yr\({}^{-1}\) under the CO\({}_{2}\)[330] configuration, an increase of 48.8 g C m\({}^{-2}\) yr\({}^{-1}\) under the CO\({}_{2}\)[var] configuration and an increase of 46.8 g C m\({}^{-2}\) yr\({}^{-1}\) under the CO\({}_{2}\)[down] configuration. From long-term plot measurements extrapolated to the continent, [PERSON] et al. [24] estimated that carbon storage in the biomass of African tropical forests increased by 90.4 g C m\({}^{-2}\) yr\({}^{-1}\) from 1968 to 2007 (95% confidence interval: 38.7-116.2 g C m\({}^{-2}\) yr\({}^{-1}\)).