## 3 Results
### CARAIB Simulations for the Future
#### 3.2.4 Changes in Biomass  
The future expansion of the species beyond their current distribution being hypothetical due to dispersal limitation, we restricted our assessment of change in biomass stock to current distribution. Our results display the uncertainty in the evolution of the tropical forest biomass linked to plant response to CO\({}_{2}\) (Figures 7 and 8). In absence of CO\({}_{2}\) fertilization, the species mean biomass is projected to decrease, whatever the climatic scenario (Supplementary S4). If we consider the climate model ensemble (Figure 7), the biomass change projected for the end of the century is significant for all the species. On average, species might lose 21.2% and 34.5% in biomass under RCP4.5 and RCP8.5, with at least \(-17\)% (_Elaeus guineensis_) under RCP4.5 to \(-46\)% (_Lophira alata_) and \(-47\)% (_Pericopsis elata_) under RCP8.5. For these latter two species, biomass reductions even reach respectively \(-76\)% and \(-79\)% under RCP8.5 CanESM2 scenario but are less dramatic (only \(-11\)%) under RCP4.5 CESM1-BGC and CNRM-CM5 (Supplementary S4). Assuming a CO\({}_{2}\) fertilization effect, biomasses of the species ensemble increase by 10.1% and 12.2% under RCP4.5 and RCP8.5 (up to 28% for _Elaeus guineensis_ with the climate ensemble and more than 40% with RCP8.5 INMCM4). The exceptions are _Guibourtia demeusei_ and _Pericopsis elata_ whose biomass remains unchanged or slightly decreases in the CO\({}_{2}\) configuration including a downregulation. It is important to note that, despite the CO\({}_{2}\) stimulation of growth, biomass drops (more than 40% with RCP8.5 CanESM2 and 20% with RCP4.5 MPI-ESM-LR) are still projected if we look at individual climatic scenarios. The analysis of the species biomass changes over time (Figure 8) shows that, despite a CO\({}_{2}\) stimulation, the biomass of most species might stop increasing or even decrease after 2070, especially under RCP8.5 scenario. Under this scenario, the model projects very contrasted biomass changes between the species (as already noticed in Figure 7). Figure 7: Species biomass change on current distribution between the 2080–2099 period and the reference 1980–1999 period. The values correspond to the ensemble mean of the results obtained with the seven climate projections under the RCP4.5 (squares) and RCP8.5 (circles) scenarios, and three CO\({}_{2}\) configurations assuming respectively a CO\({}_{2}\) constant [330] (white), a CO\({}_{2}\) variable downregulated [down] (grey) and a CO\({}_{2}\) variable [var] (black), and species-specific morpho-physiological traits. We also examined the spatial distribution of species biomass changes (examples in Figure 9, maps of all species in Supplementary S5). Changes projected for the 2080-2099 period are evaluated in the light of the temporal variability (standard deviation, \(\sigma\)) of species biomass over the 1980-1999 period. The magnitude of change is expressed as a multiple of the mean biomass standard deviation (average \(\sigma\) over the species distribution; [48]). Unsurprisingly, contraction and expansion of species range go together respectively with biomass collapse (especially for _Ceiba pentendra_, _Loplira alata_ and _Pericopsis elata_) and gain. Species disappearance is only projected on the distribution edges. Expansions appear in the mountainous areas in East Africa, around the African Great Lakes and the Ethiopian mountains, where climate conditions become suitable for germination. In absence of fertilization effect, our results show that climatic change might lead to an important forest dieback for most species, even in the preserved part of their distribution, with decrease in biomass two to three times the standard deviation for the reference period. When assuming the existence of a fertilization effect, species biomass substantially increases over most of the current distribution. Some biomass declines are however projected at the outer limits as well as in the core of the distribution.