Tree stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and abiotic factors (mean annual temperature (MAT), mean annual precipitation (MAP) and biotic factors including gross primary production (GPP), net primary production (NPP), leaf area index (LAI), soil CO2 efflux (RS), root CO2 efflux (RA), stand age and tree stem diameter (DBH). We examined the relationships between annual ES and biotic and abiotic factors at multiple scales including individuals, biomes, and globally, and made an observation-based global gridded estimate of annual ES.
Positive linear relationships were found between ES and LAI, as well as GPP, NPP, wood NPP, RS and RA. A multiple linear regression model regression model using LAI, MAT and MAP as independent variables predicted ES in boreal, temperate and tropical forest biomes well (adjusted-R2 = 0.74). Stem CO2 efflux tended to decrease with stand age. We also found a relationship between ES and RA at biome and global scales. Modelled mean annual ES was 89 ± 53, 248 ± 127 and 506 ± 262 g C m-2 yr-1 for boreal, temperate, and tropical forests, respectively. We estimated that the global forest annual stem CO2 efflux was 6.7 Pg C yr-1 over the period of 2000-2012, equivalent to 84% of annual anthropogenic carbon emissions, and accounted for 13%-20% of global annual net primary production of forest ecosystems.