Stand carbon density drivers and changes under future climate scenarios across global forests

Climate and soil factors drive forest carbon (C) density. However, their impacts on the aboveground carbon density (ACD) and belowground carbon density (BCD) of global forest types have not been quantified in previous studies. In this study, we compiled global forest biomass data that included 8800 plots to better quantify the changes in total carbon density (TCD) of five forest types under current climate and future climate change scenarios (e.g., RCP2.6, RCP4.5 and RCP8.5) using the boosted regression tree method and TCD-stand age-climate models. Tropical forests have the largest amount of C density (130.0 +/- 3.2 Mg Cha(-1); 173.60 +/- 17.24 Mg Cha(-1)), while boreal forests have the lowest TCDs (59.4 +/- 0.9 Mg Cha(-1); 62.34 +/- 15.10 Mg Cha(-1)) in the current and future. For the current forest scenarios, the dryness index (DI) for tropical forests exhibited a negative mean change rate for the ACD (4.5 Mg Cha(-1) /0.1 DI) and BCD (0.9 Mg Cha(-1) /0.1 DI). The mean annual precipitation (MAP) had a positive mean change rate for the ACD (7.1 Mg Cha(-1)/100 mm) and BCD (1.3 Mg Cha(-1)/100 mm) when the MAP was 5 1100 mm in the temperate forests. The MAP was negative for the ACD (1.9 Mg Cha(-1)/ 100 mm) and BCD (0.4 Mg Cha(-1)/100 mm) when the MAP was > 1100 mm. The mean annual temperature (MAT) in boreal forests had a positive mean change rate for the ACD (1.78 Mg Cha(-1)/1 degrees C) and BCD (0.4 Mg C ha(-1)/1 degrees C). We found that in contrast to 2050, there were positive impacts of the DI on the TCD change in tropical forests in 2070, and the DI was the most important driver for tropical forests. There were strong positive MAP impacts on the TCD change in temperate forests in both 2050 and 2070, where the MAP was the most important driver for temperate forests. There were negative MAT impacts on the TCD change in boreal forests under RCP4.5 (-0.22 +/- 10.53 Mg Cha(-1)) and RCP8.5 (- 4.75 +/- 10.80 Mg Cha(-1)) in 2070, and the MAT was the most important driver in boreal forests. Our results could be used to improve the accuracy of C density estimates and assess climate change impacts on the C budgets of global forests.