Grouping tree species to estimate afforestation-driven soil organic carbon sequestration

Aims: Any grouping of tree species concerned with SOC sequestration should include trees that are as homogeneous as possible in their carbon sequestration. We propose a classification of tree species into deciduous broadleaf, evergreen broadleaf, deciduous conifer and evergreen conifer and assess the validity of such classification. Methods: We conducted a quantitative review of 264 observations from 90 papers. One-way analysis of variance and hierarchical multiple regression were performed to test the homogeneity within the tree types and the extent to which the tree classifications could affect the SOC stock change, after controlling for temperature, precipitation, stand age, initial SOC stock and previous land use. Results: There is less variation in the SOC sequestration rates of tree types if they are grouped as deciduous broadleaf, evergreen broadleaf and evergreen conifer rather than evergreen and deciduous, or broadleaf and conifer. Evergreen broadleaf accumulates SOC at the fastest rate (0.73 Mg ha⁻¹ yr⁻¹), compared to deciduous broadleaf (0.42 Mg ha⁻¹ yr⁻¹) and evergreen conifer (0.48 Mg ha⁻¹ yr⁻¹). The proposed classification may help researchers to better estimate the changes in SOC stocks after afforestation with various tree species, when direct measurements are not available. Conclusions: Our proposed classification of tree species is more accurate than the more commonly used groups of deciduous and evergreen or broadleaf and conifers.