A comparative study of carbon pools and sequestration in forests of the northern German lowlands

The historic large-scale forest conversion in the northern German lowlands resulted in a man-made dominance of Scots pine, in a landscape that would naturally be dominated by forests of European beech. Since drawbacks of pure pine forests such as their susceptibility to calamities have become clear, re-conversion to mixed and broadleaf stands has been promoted. Consequently, the share of pine is progressively declining in German forests. Nevertheless, planting pine is still a popular option from a silvicultural perspective, due to its rapid growth especially at young age, its ability to grow on nutrient-poor and dry sites, and the high demand for its wood. In the face of accelerating climate change, the ability of forests to store and sequester carbon (C) has become a focus in science, politics and forestry. The aboveground biomass represents the largest biomass fraction in the forest and can be modulated directly through management. Fine roots represent only a few percent of the tree´s biomass, but due to their fast turnover as well as through root exudation to the surrounding soil, they are the main source for soil organic carbon. The presented study therefore compared the C pools and sequestration in the above- and belowground (fine root) biomass in naturally developing, mostly European beech forests (ND) and regularly thinned Scots pine forests (YP), respectively representing the dominant natural and the dominant current forest type of the northern German lowlands. Aboveground biomass C stocks were further determined in pine forests in transition to (mixed) broadleaf stands (OP). The study was conducted in a network of 48 forests at 16 sites, distributed throughout the northern German lowlands, covering a climate continentality gradient from west to east. Aboveground biomass calculations were based on stand structural data and species-specific allometric regressions (live trees, saplings) or volume calculations and species-specific wood density (deadwood). Aboveground net primary production (ANPP) was measured in three consecutive years using permanently attached dendrometer tapes for wood increment, and litter traps for litter production. Two repetitive fine root inventories were conducted, measuring fine root bio- and necromass in the organic layer and the top 20 cm of the mineral soil. Fine root productivity was determined with the ingrowth-core approach in 0–20 cm soil depth, including the organic layer. Above- and belowground biomass C stocks were significantly higher in beech than in pine forests. A linear mixed-effects model revealed that the tree species was the most important factor in explaining aboveground biomass C stocks. Variation in stand age, with a range of roughly 100 years for both species, was surprisingly not influential. ANPP was higher in beech than in pine forests as well, which was mostly a result of higher litter production, while wood production was similar in the two forest types. Fine root productivity was also higher in beech than in pine forests, but the difference was only significant in 10–20 cm depth. The naturally dominant European beech forests thus have a considerably higher climate change mitigation potential than the Scots pine forests replacing them, although the high share of beech wood used for the production of bioenergy impairs their potential. By estimating the extent of forest conversion in the northern German lowlands, the significant loss in the C storage and sequestration potential on the landscape-scale was demonstrated. The climatic gradient of the region had only little influence in this study, but performances of beech and pine under future climatic conditions will certainly affect the functioning of the investigated forests. Evidence exists that both tree species will suffer from climate change in the study region, especially from more frequent climatic extremes. In combination with additional negative effects of pine on groundwater recharge, microclimate and soil acidity, the results of this study strongly suggest that Scots pine is not a suitable option in a silviculture focusing on the mitigation of, and the resilience against climate change.2022-06-1