Long-Term Ecological Baselines and Critical Thresholds in Ombrotrophic Peatlands of Europe: Implications for Restoration Strategies

Maintaining appropriate peatland hydration, particularly through the regulation of the depth to the water table (DWT), is crucial for peatland conservation, restoration, and the mitigation of greenhouse gas emissions. In this study, we assess the long-term ecological impact of hydrological changes, primarily induced by drainage, on ombrotrophic peatlands across Europe. Our analysis is based on novel palaeoecological data from seven peat cores collected from sites that have experienced varying degrees of anthropogenic disturbance. We reconstructed historical DWT fluctuations using plant macrofossil and testate amoeba analyses at high resolution. By applying Threshold Indicator Taxa Analysis (TITAN), we identified species-specific and community-level response thresholds to changes in water level. This approach revealed two distinct change points: the first, at approximately 7 cm DWT, corresponds to hydrological conditions favourable for moisture-dependent Sphagnum species, while the second, near 22 cm DWT, is associated with more drought-adapted taxa and signals ecosystem degradation. The interval between these points represents a transition zone between optimal and suboptimal conditions for peatland functioning. An additional TITAN analysis aimed at identifying the timing of the most significant ecological changes indicates that peatland degradation intensified over the past two centuries and has accelerated in recent decades. Our findings also show that even after hydrological restoration, microbial and plant communities often remain distinct from those in undisturbed peatlands. This underscores the importance of preserving sites that still retain near-natural conditions. Based on our results (and consistent with previous studies) we recommend maintaining a DWT of approximately 10 cm below the surface as an optimal target for both peatland conservation and restoration. Such conditions not only support ecological integrity but are also associated with reduced greenhouse gas emissions and higher peat accumulation rates, reinforcing the role of ombrotrophic peatlands as long-term carbon sinks.