Forest Overstory Effect on Soil Organic Carbon Storage: A Meta‐analysis

A meta‐analysis using 77 studies from 28 countries was performed to assess the effect of hardwood vs. conifer overstory on soil organic C (SOC) storage in forest floor (FF), mineral soil, and whole soil (FF + mineral soil). Overall, FF stocks were 38% higher under conifers, mineral SOC stocks were similar, and whole soil SOC was 14% higher under conifers. An analysis with six of the seven most reported tree genera reaffirmed higher FF and whole soil C stocks under conifer stands. Analysis with all seven of the genera showed more pronounced variability in mineral SOC results compared with the overall results. Eucalyptus was the only hardwood that stored significantly (17%) more SOC in the mineral soil than adjacent conifers. Picea was the only conifer that stored significantly (7%) more SOC in the mineral soil than hardwoods. Differences in FF SOC stocks had a limited predictive power in explaining the variability of mineral SOC stock differences, suggesting that they are not very closely linked with regard to SOC storage. Only when comparing FF SOC stocks among genera did precipitation, age difference, soil texture, and previous land use moderate SOC storage differences between conifers and hardwoods. In other cases, neither climate nor soil variables could explain differences between SOC stocks. Our findings suggest that using plant‐trait‐driven vegetation categories may be a more descriptive way of detecting vegetation effects on SOC.