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

Large old trees have declined worldwide, even though they play a prominent role in biodiversity and also in carbon stocks of forests. We were interested in the contribution of large old trees to aboveground stand biomass and the role of wood decay inside the trunk and tree cavities for biomass and thus organic carbon stocks. We investigated this question using the example of old-growth oak forests, since oak is the tree species with the highest proportion of large old trees in Germany. Although we studied oak stands with a strongly above-average proportion of large old trees (with an aboveground biomass of 563 Mg ha−1), internal stem decay and tree cavities resulted in only about 1% overestimation of stand biomass. This was determined by subtracting biomass losses from internal stem decay from conventional biomass estimates derived from allometric regression. The extent of internal stem decay was measured using sonic tomography. Internal stem decay is thus negligible for stock density, but should be considered for continent-wide or global analyses where even small percentages add to large absolute errors. If trees of a stem diameter ≥ 7 cm were included in the analysis, the 11% largest trees of the stands accounted for 50% of the aboveground biomass. This result differs from recently published values of a global analysis suggesting that the 1% largest trees represent 50% of the stand biomass, but this difference is influenced by the minimum diameter at which young trees are included in the analysis. We suggest excluding young growth with diameters so small that their biomass does not significantly affect stand biomass from such analyses.