Bodemgezondheid in Europese bossen onder herstel

Forests and forest soils provide a wide range of ecosystem services, including regulating, provisioning and cultural services. Among the regulating services, European forests act as a net carbon sink, sequestering CO₂ in living and dead biomass and in soils. Delivery of these services depends on healthy forests and soils, which are increasingly threatened by degradation, potentially even turning forests from carbon sinks into carbon sources. Declining soil health both reflects and amplifies forest degradation, highlighting the tight link between the aboveground and belowground compartment in forest ecosystems. Recognising this interdependence is essential for restoration strategies that rebuild not just tree cover, but also physical, chemical, and biological soil functions to ensure long-term ecosystem recovery. This study investigates the effects of forest restoration on soil health and is structured into chapters that address key aspects of soil health across different European regions, soil depths, and soil properties. Chapter 2 examines the effects of forest restoration on biological, physical, and chemical soil properties in the topsoil (0-5 cm) across 11 European regions. The results reveal substantial variability both between and within regions, with no consistent trends across restoration stages. Soil properties are presented for each region and evaluated against values from the literature, enabling comparison of soil health status across regions. The findings highlight the strong context-dependency of forest restoration outcomes, as key drivers differ among regions, and identify soil pH as a central variable shaped by both restoration and soil texture, with cascading effects on multiple soil properties. Chapter 3 focuses on microbial soil health in the forest floor and topsoil (0-5 and 5-15 cm) of European forests, using rapid indicators for assessment. We found that legacy effects, more than restoration, shape microbial activity. This highlights how past disturbances and management continue to influence soil functioning in the next generation of forests. In addition, soil depth, pH, and carbon content emerged as important contributors of microbial dynamics. Despite some limitations, Biolog EcoPlates™ and Microburst respiration were well correlated and offer cost-efficient, informative insights into microbial activity, making them suitable for large-scale monitoring. Building on Chapters 2 and 3, which demonstrate the strong context-dependency of forest restoration effects, Chapter 4 focuses on four regions with comparable legacy conditions (conifer dominated forests) experiencing similar disturbances from spruce dieback, and examines soil carbon dynamics down to 80 cm depth. The results reveal limited short-term effects of restoration on soil carbon stocks and content, challenging the assumption that converting conifer plantations to broadleaved forests increases soil carbon. Soil pH emerged as a key regulator of carbon decomposition and forest floor accumulation, contributing to lower total carbon stocks in broadleaved stands. However, increased mineral soil carbon in mixed stands indicates a promising pathway for long-term carbon sequestration without the carbon losses observed in fully broadleaved systems. Overall, legacy effects of former forest composition outweighed current restoration trajectories in shaping soil carbon dynamics. Chapter 5 is a valorisation chapter that integrates existing literature with field experience and soil sampling data to develop practical guidelines for improving forest restoration outcomes. The guidelines emphasize treating forests and soils as an integrated system that should be restored and monitored together. They define what to restore, how to restore, and how to monitor progress, and propose a set of scalable soil health indicators to assess restoration success. In addition, this chapter includes a synthesis table summarising the effects of common restoration techniques on soil properties. In conclusion, this research demonstrates that forest restoration outcomes are strongly context-dependent and shaped by legacy effects of past forest composition, management, and disturbance. It shows that forest restoration is a slow process, with soil properties responding over long timescales rather than rapidly following management interventions. Consequently, conserving existing forests is critical, as developing healthy forest soils comparable to those of long-established forests can take decades to centuries. The findings further highlight that multiple soil properties - spanning biological, chemical, and physical domains - must be considered to capture restoration outcomes comprehensively. Finally, the use of a limited set of scalable soil health indicators is essential to ensure that forest restoration success is consistently and feasibly monitored across regions.