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

Maintaining appropriate peatland hydrology, notably through the regulation of the depth to water table (DWT), is crucial for peatland conservation, restoration, and the mitigation of greenhouse gas (GHG) emissions. We assess the long-term ecological impact of hydrological changes, primarily induced by drainage, in 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) models, we identified species-specific and community-level response thresholds to changes in reconstructed water table. This approach revealed two distinct change points: the first, at c. 7 cm DWT, corresponds to hydrological conditions favourable for moisture-dependent Sphagnum species. The second, at c. 22 cm DWT, is associated with more drought-adapted plant taxa and signals ecosystem degradation. The interval between these change points represents a transition zone between optimal and suboptimal conditions for peatland functioning. An additional TITAN analysis, designed to identify the timing of major ecological changes, indicates that peatland degradation has intensified over the past two centuries and accelerated in recent decades. Our findings further reveal that plant and testate amoebae communities often remain distinct from those of undisturbed peatlands, even after hydrological restoration. 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 the water table close to the surface, i.e., 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 GHG emissions and higher peat accumulation rates, reinforcing the role of ombrotrophic peatlands as long-term carbon sinks.