Bioclimatic envelope models predict a decrease in tropical forest carbon stocks with climate change in Madagascar

1.Recent studies have underlined the importance of climatic variables in determining tree heightand biomass in tropical forests. Nonetheless, the effects of climate on tropical forest carbon stocksremain uncertain. In particular, the application of process-based dynamic global vegetation modelshas led to contrasting conclusions regarding the potential impact of climate change on tropical forestcarbon storage.
2.Using a correlative approach based on a bioclimatic envelope model and data from 1771 forestplots inventoried during the period 1996–2013 in Madagascar over a large climatic gradient, weshow that temperature seasonality, annual precipitation and mean annual temperature are key vari-ables in determining forest above-ground carbon density.
3.Taking into account the explicative climate variables, we obtained an accurate (R2= 70% andRMSE = 40 Mg ha�1) forest carbon map for Madagascar at 250 m resolution for the year 2010.This national map was more accurate than previously published global carbon maps (R2≤26% andRMSE≥63 Mg ha�1).
4.Combining our model with the climatic projections for Madagascar from 7 IPCC CMIP5 globalclimate models following the RCP 8.5, we forecast an average forest carbon stock loss of 17%(range: 7–24%) by the year 2080. For comparison, a spatially homogeneous deforestation of 0.5%per year on the same period would lead to a loss of 30% of the forest carbon stock.
5.Synthesis.Our study shows that climate change is likely to induce a decrease in tropical forestcarbon stocks. This loss could be due to a decrease in the average tree size and to shifts in tree spe-cies distribution, with the selection of small-statured species. In Madagascar, climate-induced carbonemissions might be, at least, of the same order of magnitude as emissions associated with anthro-pogenic deforestation.