The significance of large old trees and tree cavities for forest carbon estimates

•The 1% largest trees of a stand were postulated to represent 50% of stand biomass.•We challenge this conclusion analyzing oak forests with large-diameter trees.•Sonic tomography was used to study internal stem decay and its effect on biomass.•Allometric biomass functions neglecting wood decay overestimated biomass by only 1%.•11% of trees (diameter ≥ 7 cm) represented 50% of aboveground stand biomass.