## 3 Results  
### CARAIB Simulations for the Present  
#### 3.1.1 Refinement of Morpho-Physiological Traits  
Substitution of PFT morpho-physiological traits by species-specific ones has an important impact on species NPP with significant differences (\(p<0.01\)) in mean simulated NPP (average over the distribution) for eight species (Figure 1). Both traits linked to water (height, root depth) and carbon (C:N ratio, SLA) uptake were adjusted. When only the tree height is adapted and set higher than the PFT one (_Lophira alata_, _Naucela diderichii_, _Pericopsis elata_, _Uapaca guineensis_), change in species productivity is not significant, except for _Lophira alata_ with a specific height much higher than the PFT value of 20 m. Importantly, increase of tree height leads to a decrease of the mean simulated NPP of at least 50 g C m\({}^{-2}\) yr\({}^{-1}\). Turbulence intensity, increasing with tree height, induces an increase in the evapotranspiration rate, which results in an increase of plant water stress and in a decrease of plant NPP, despite a concomitant strengthened assimilation (see Section 2.3). A reduction of root depth has little effect on plant productivity (_Elaies guineensis_). The most pronounced mean NPP decrease is observed for _Ceiba pentendra_ whose decrease of 150 g C m\({}^{-2}\) yr\({}^{-1}\) is a result of a much higher height together with a lower root depth and a SLA increase. The most important increases (_Cola acuminata_, _Musanga cercopioides_) correspond to a large decrease of foliage C:N when there is a rather limited increase of tree height. The negative impact of increasing SLA is supported by the trait adjustment of _Musanga cercopioides_ and _Pycanthus angolensis_. With same slight height increase and big C:N ratio decrease, the NPP increase is less important for _Pycanthus angolensis_ with a SLA of 0.036 than for _Musanga cercopioides_ with a SLA of 0.026. The large impact of modifying SLA and foliage C:N parameters is a logical consequence of their direct link with autotrophic respiration and V\({}_{\text{cmax}}\).  
Trait adjustment impacts the frequency distribution of NPP values over the species spatial distribution. In the case of _Pycanthus angolensis_ (Figure 2), the frequency of pixels with a NPP below the threshold of species presence (100 g C m\({}^{-2}\) yr\({}^{-1}\)) decreases. Above 100 g C m\({}^{-2}\) yr\({}^{-1}\), the number of grid cells with low NPP values rises consequently but the frequency in high NPP classes also increases. To deepen this analysis, the impact of parameter adjustment on water use efficiency has been examined (Figure 2). It shows that the NPP increase is a direct result of enhanced water use efficiency due to reduced transpiration water loss. However, as already discussed above, trait refinement does not always lead to an increase in species productivity.  
To quantify the uncertainty linked to intraspecific trait variations, two additional simulations were performed replacing respectively the mean species foliage C:N ratio and SLA values by an extreme value (mean \(\pm\) standard deviation). It results in differences in the mean simulated NPP over the  
Figure 1: Bar chart of the mean simulated net primary productivity (NPP, g C m\({}^{-2}\) yr\({}^{-1}\)) over the observed points of species occurrence, obtained with CRU (Climatic Research Unit) historical climatic data for the reference 1980–1999 period, using respectively FFT- (hatched bars) and species-specific (white bars) morpho-physiological traits (see Table 2). An asterisk behind species name indicates a significant difference (\(p\)-value of the Wilcoxon paired \(t\)-test < 0.01) between NPP values. Species name between brackets indicates that only height trait has been adjusted.  
observed species occurrences, sometimes significant (up to 90 g C m\({}^{-2}\) yr\({}^{-1}\) for _Pycanthus angolensis_ with an extreme C:N ratio and up to 170 g C m\({}^{-2}\) yr\({}^{-1}\) for _Pterocarpus soyauxii_ with an extreme SLA).