Wood density and carbon concentration of decomposing stumps and coarse roots of European beech, Norway spruce and Scots pine

Abstract Root biomass contributes a significant proportion of total biomass and carbon storage in forest ecosystems. Deadwood (including dead roots) is an essential component of the ecosystem carbon (C) balance. Currently there are some shortcomings in the reporting of deadwood regarding the completeness and the level of detail. So far it is not possible to report about dead coarse roots since there is no existing data. Therefore, we assessed wood density and C concentration of excavated stumps and dead coarse roots (≥ 2 cm) of European beech ( Fagus sylvatica L.), Norway spruce ( Picea abies (L.) Karst .) and Scots pine ( Pinus sylvestris L.) in different stages of decay in a case study in Northeastern Germany. In total, 15 rootstocks – three for pine, four for spruce and eight for beech—could be excavated and analysed. The decay class (DC) belowground was found to be less advanced than the one aboveground in 60% of all cases where the DC above- and belowground differed. Significantly decreasing wood densities to a similar level in advanced DCs, i.e. DC 3 and 4, were observed with increasing DC for all tree species. DC has been found to be the main predictor of all variables tested; already explaining 83% of the variation in wood density as a single predictor. C concentration was highest in DC 4 for spruce and pine, but lowest for beech in the same DC. C concentration in beech was generally lower than in spruce and pine. Significantly increasing nitrogen (N) concentrations and decreasing CN ratios were observed with increasing DC for all tree species. Our results indicate that decomposing rootstocks i.e. coarse roots and stumps can be a significant sink (and source) of C. Based on this study—although it needs to be extended to more repetitions and sites in the future—a first estimation of the belowground deadwood C pool as part of the German greenhouse gas reporting can be conducted.