Internal stem decay as a potentially underestimated source of error in biomass estimates

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Forest ecosystems are important terrestrial carbon sinks sequestering large amounts of CO2 from the atmosphere. The sequestered carbon is allocated both in the tree biomass and the soil. Estimates of the organic carbon stocks in the tree biomass are commonly based on allometric biomass functions, which use stem diameter and tree height data for the calculation of tree biomass. Hence, these biomass functions completely rely on the out-er shape of the trees and ignore internal stem decay and tree cavities. This procedure is certainly justified for many tree individuals, if they lack large cavities or sections with re-duced wood density due to internal stem decay. However, as pointed out in a recent study of 2018 by Lutz and co-workers, old large-diameter trees contribute to a strongly dispropor-tionate degree to the total biomass of forests. On a global scale, the largest 1 % of trees were concluded to contain approximately 50 % of the total above-ground tree biomass. Against this background, internal stem decay and tree cavities in large old trees matter, since they might lead to an overestimation of the total carbon stock of forests.In the present project, we challenge the conclusion of Lutz and co-workers, because for the specific case of the oldest and largest trees of a forest stand, neglecting internal stem de-cay and tree cavities in the biomass estimates may result in a significant error, as internal stem decay can be assumed to be much more common in this subset of trees than in most other forest trees. Therefore, we doubt that merely relying on the outer shape of the trees for assessing their biomass is justified, as long as of all trees the individuals with the great-est probability for having large reduced wood density and cavities due to internal stem de-cay are regarded to be most significant for determining the organic carbon storage of the whole forest. In our project we will, therefore, scrutinize the effect of internal stem decay for tree biomass in trees of different size classes.Using a combined approach of sonic tomography, electric resistivity tomography and wood-core sampling, we want to integrate the mass loss by internal stem decay in the biomass estimates by subtracting the assumed mass loss from the tree biomass, which was deter-mined with conventional biomass functions. These investigations will be carried out in se-lected temperate and boreal forest ecosystems for testing the hypothesis that the above-ground biomass of the largest 1 % of trees is reduced to significantly less than 50 % of the stand above-ground biomass, if the mass loss by internal stem decay and tree cavities is integrated in the biomass estimates.