Soil carbon responses to forest restoration trajectories: Lessons learned from four regions facing spruce forest dieback

Norway spruce and Sitka spruce are economically important coniferous tree species in Europe and have been widely planted beyond their natural ranges. However, spruce monocultures show low resistance to climate change-enhanced disturbances, which leads to large-scale forest dieback events that impact ecosystem services such as carbon storage. Moreover, due to their recalcitrant litter, spruce monocultures can accelerate soil acidification, especially when compared to natural broadleaved and mixed forests in northern and central Europe, adversely impacting soil health. Therefore, active admixture of broadleaved trees is a prevalent restoration practice in planted spruce monocultures, particularly in regions where spruce was introduced outside its natural range. It is, however, unclear how current forest restoration practices alter carbon content along the soil profile. We present four case studies from the European SUPERB network: Queen Elizabeth Forest Park (United Kingdom, 18 stands), North Rhine-Westphalia (Germany, 18 stands), Vysočina (Czech Republic, 33 stands), and the Făgăraș Mountains (Romania, 36 stands). Across these demonstration regions, three restoration trajectories after spruce forest dieback were implemented: (1) a ‘business-as-usual’ trajectory, with reforestation through replanting of spruce, (2) a conversion to mixed forests, and (3) a conversion to broadleaved forests. Within these trajectories, different forest development stages were identified as space-for-time substitutes: clearcuts with 0–10 year-old stands and further development stages with stand ages of 11–40, 41–80, and > 80 years. Soil samples (to 80 cm depth) taken in 2022–2023 revealed region as the main determinant of forest soil carbon, followed by soil pH. Differences between restoration trajectories were most pronounced in the forest floor. Despite slightly higher soil pH in broadleaved stands (0–5 cm) compared to coniferous stands, no significant difference in total carbon stocks was found between restoration trajectories. Furthermore, the differences in C between restoration trajectories were even less pronounced in the mineral soil layers and appeared to be driven by mainly deep soil pH (40–80 cm). Surprisingly, stand age (years since dieback) had no significant effect on carbon content. • Broadleaves had smaller SOC stocks than conifers, with mixed stands intermediate. • Stand age had no significant effect on soil C. • pH of the 0–5 cm soil layer was higher in broadleaved than in conifer forests. • C - stocks: RO>DE∼UK∼ CZ linked to climate, soil, and legacy effects.