Harmonization and variation of deadwood density and carbon concentration in different stages of decay of the most important Central European tree species

Coarse woody debris (CWD) is a major component of the ecosystem carbon (C) balance. The estimation of C storage in CWD is an important element of the German greenhouse gas (GHG) reporting of forests, which is mainly based on the German National Forest Inventory. The deadwood C stock is calculated based on deadwood volume and, according to deadwood density (DD) and carbon concentration (CC) for each decay class (DC). Yet, the data basis of DD and CC per DC for above-ground CWD is still insufficient since there are very few country-specific measurements. Values from literature provide a first approximation for national-level estimates. However, different DC systems often prevent the use of DD and CC of other countries. Therefore, we developed a conversion method for harmonization of these data with the German four-class system. Following this, we conducted a meta-analysis to calculate mean DD and CC values for the main Central European tree species and to assess their variation. Significantly lower DDs were observed with increasing DC, except for beech between DC 3 and 4. Compared to spruce and pine, DD of beech CWD was significantly higher, overall as well as in DC 1 and 2. Species became similar in DD in advanced decay stages. A maximum of 92% of the variation in DD could be explained mainly by DC, CWD type, tree species and their interaction. DD values were mostly higher than current values in GHG reporting. CC increased with increasing DC in spruce and pine and was higher than in beech CWD, where no variation was detected. About 86% of the variation in CC could be explained mainly by DC, tree species and their interactive effect. The default value of 50% employed by the Intergovernmental Panel on Climate Change might under- (spruce, pine) and/or overestimate (spruce, pine, beech) the real CC depending on DC by up to 3.4 (pine) and/or 4.2% (beech). Based on our calculated mean DD and CC values, the accuracy of C stock assessment in deadwood as part of the GHG reporting for Germany can be substantially improved.