Internal stem decay is not a major source of error for carbon stock estimates in managed temperate forests of Central Europe

Forest biomass and carbon stock estimates are critically important for the understanding of the global carbon cycle and the mitigation potential of forest ecosystems with respect to climate change. Forest biomass is routinely assessed using allometric biomass functions, which rely on stem diameter and tree height data and thus on the outer shape of the trees. Internal stem decay and tree cavities are neglected in these calculations. In a global survey, it has been postulated that the 1% largest trees contain 50% of the above‐ground tree biomass of a forest stand. This postulate assigns the largest trees a prominent role in the biomass of the entire stand. This also means that estimation errors in determining the biomass of these individual trees would have a significant effect on stand biomass. At the same time, the largest and oldest trees have an increased risk of internal wood decay, which in turn increases the probability of overestimating the stand biomass using allometric biomass functions. We studied above‐ground tree biomass and carbon stocks in temperate forests of Fagus sylvatica, Abies alba, Picea abies and Pinus sylvestris in SW Germany and selected 168 plots with 2359 trees with a disproportionally high proportion of large old trees. We calculated biomass based on allometric regression functions and corrected the results with data of internal stem decay obtained with sonic tomography. In the studied tree species, 18%–25% of the largest trees constituted 50% of the above‐ground biomass and thus far more than the 1% largest trees. Overestimation of above‐ground stand biomass and carbon stock density due to internal stem decay amounted to only 0.2%–0.3% on stand level and was thus negligible compared to other potential sources of error linked with biomass and carbon stock estimates. Allometric biomass functions can be used for biomass and carbon stock estimates in managed temperate forests of Central Europe, even in stands with high shares of large old trees. It should be tested whether this conclusion is transferable to forests with less or no intensive management in other regions and biomes.