The overlooked soil carbon under large, old trees

•We measured soil organic carbon that is typically ignored close to large trees.•90% of the total soil organic carbon was within ~2.6 m of the surface.•Under large tree trunks soil organic carbon was four times more concentrated.•The stand-level had ~7% [95% CI: 3–12%] more soil carbon than previously estimated.•There may be higher emissions from past forest disturbances than have been inferred.