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Climate warming and human impacts are thought to be causing peatlands to dry, potentially converting them from sinks to sources of carbon. However, it is unclear whether the hydrological status of peatlands has moved beyond their natural envelope. Here we show that European peatlands have undergone substantial, widespread drying during the last ~300 years. We analyse testate amoeba-derived hydrological reconstructions from 31 peatlands across Britain, Ireland, Scandinavia and Continental Europe to examine changes in peatland surface wetness during the last 2,000 years. We find that 60% of our study sites were drier during the period 1800–2000 ce than they have been for the last 600 years, 40% of sites were drier than they have been for 1,000 years and 24% of sites were drier than they have been for 2,000 years. This marked recent transition in the hydrology of European peatlands is concurrent with compound pressures including climatic drying, warming and direct human impacts on peatlands, although these factors vary among regions and individual sites. Our results suggest that the wetness of many European peatlands may now be moving away from natural baselines. Our findings highlight the need for effective management and restoration of European peatlands.

In rich fens, unlike bogs, the key drivers structuring testate amoeba communities are related to nutrient status, suggesting the potential for transfer functions to quantitatively reconstruct changing nutrient status from palaeoecological records. Such records could be useful tools to investigate the long-term impacts of pollution and landscape change. Here, we derive and test transfer functions for pH, water-table depth, conductivity and Ca and Mg concentrations using a data set from Polish fens. Results show that transfer functions for Ca and conductivity have apparent predictive power for surface samples; these models will require further validation and testing with palaeoecological data. Testate amoeba transfer functions for fen nutrient status may be a valuable addition to the peatland palaeoecologist’s tool-kit, although further work will be required to demonstrate their usefulness in practice.

Plant community composition is recognized more and more for playing an important role in ecosystem processes, such as C cycling. In particular, plant functional type (PFT) composition seems to have a key regulatory role, yet the underlying mechanisms in the interaction between PFTs and ecosystem processes are still to be identified. Here, we assess the link between PFTs and dominant microbial consumers along a calcareous poor to extremely rich fen gradient in western Poland. We particularly focussed on dominant microbial consumers (testate amoebae), which can exert large effects on the functioning of peatlands. Using moving‐window analyses and path‐relation networks subjected to structural equation modelling (SEM), we investigated linkages among abiotic factors, PFTs and testate amoebae. We show that along the poor to extremely rich fen gradient, the dependence of testate amoebae to PFTs is higher than their dependence to abiotic factors. We also found that the link between testate amoebae and PFTs differs between size assemblages of testate amoebae. While large testate amoeba species (i.e. high trophic level) were highly linked to Sphagnum mosses cover, small species (i.e. low trophic level) were linked to brown mosses. Distinction between shallow‐rooted and deep‐rooted vascular plants also showed that shallow‐rooted plants play a role on testate amoeba community structure at the ‘poor’ side of the gradient. Our results further show a dominant role for calcium content and the structure of the bryophyte community on testate amoeba size assemblages at the poor to extremely rich fen scale, both for diversity and abundance of testate amoebae. Synthesis. Variations in plant functional type composition drive niche‐size‐structure of testate amoebae along the (calcareous) poor to extremely rich fen gradient. Furthermore, strong relationships between moss types and testate amoeba size‐structure suggest that mosses specifically influence testate amoeba development through autogenic effects. Therefore, moss cover composition is key to microbial consumers and may be the driving factor determining microbial network structure and associated ecosystem processes, such as carbon cycling.

This paper describes a fossil record from northeastern Poland of Sphagnum obtusum Warnst., a plant that has rarely been found in Holocene sediments. This species occurred circa 1710–1570 cal. BP on Mechacz Wielki peatland and was present in the transitional phase between a fen that developed on lake sediments and a bog. Sphagnum obtusum was a dominant species and was accompanied by such vascular plants such as Scheuchzeria palustris, Carex rostrata, Comarum palustre and Oxycoccus palustris. Using testate amoebae, we reconstructed the past habitat (water table and pH), with S. obtusum occurring in a moderately wet habitat (mean depth to the water table or DWT  =  9.5, SD  =  0.5) and under slightly acid conditions (mean pH  =  5, SD  =  0.1). A clear correlation was found between the distributions of the subfossil S. obtusum and those of the indicator testate amoebae Hyalosphenia papilio and Cyclopyxis arcelloides. The disappearance of S. obtusum is assumed to have been caused by a trophic shift and succession of a more acidophilic species, Sphagnum angustifolium.

We studied the vegetation, testate amoebae and abiotic variables (depth of the water table, pH, electrical conductivity, Ca and Mg concentrations of water extracted from mosses) along the bog to extremely rich fen gradient in sub-alpine peatlands of the Upper Engadine (Swiss Alps). Testate amoeba diversity was correlated to that of mosses but not of vascular plants. Diversity peaked in rich fen for testate amoebae and in extremely rich fen for mosses, while for testate amoebae and mosses it was lowest in bog but for vascular plants in extremely rich fen. Multiple factor and redundancy analyses (RDA) revealed a stronger correlation of testate amoebae than of vegetation to water table and hydrochemical variables and relatively strong correlation between testate amoeba and moss community data. In RDA, hydrochemical variables explained a higher proportion of the testate amoeba and moss data than water table depth. Abiotic variables explained a higher percentage of the species data for testate amoebae (30.3% or 19.5% for binary data) than for mosses (13.4%) and vascular plants (10%). These results show that (1) vascular plant, moss and testate amoeba communities respond differently to ecological gradients in peatlands and (2) testate amoebae are more strongly related than vascular plants to the abiotic factors at the mire surface. These differences are related to vertical trophic gradients and associated niche differentiation.

Transfer functions are widely used in palaeoecology to infer past environmental conditions from fossil remains of many groups of organisms. In contrast to traditional training-set design with one observation per site, some training-sets, including those for peatland testate amoeba-hydrology transfer functions, have a clustered structure with many observations from each site. Here we show that this clustered design causes standard performance statistics to be overly optimistic. Model performance when applied to independent data sets is considerably weaker than suggested by statistical cross-validation. We discuss the reasons for these problems and describe leave-one-site-out cross-validation and the cluster bootstrap as appropriate methods for clustered training-sets. Using these methods we show that the performance of most testate amoeba-hydrology transfer functions is worse than previously assumed and reconstructions are more uncertain.

In Chlebowo mire (Wielkopolska region), we investigated testate amoebae in relation to 10 environmental parameters in the semi-natural floating vegetation of flooded peat workings. The measured parameters included: depth to water table (DWT), ground-water pH, color, conductivity, PO4, NO3, NH4, SO4, Ca, and Mg. Detrended correspondence analysis and canonical correspondence analysis (CCA) were used to analyze relations between the composition of testate amoebae communities and those variables. In canonical correspondence analysis, DWT, pH, and Mg remained after forward-selection as the main factors characterizing the changes in testate amoebae communities along the moisture and nutrient gradients. Characteristic species of testate amoebae for the various stages of floating-mat development in the flooded peat workings were distinguished. Communities of testate amoebae along with present-day vegetation reflect the process of progressive acidification, driven mainly by Sphagnum fallax. We consider this as secondary succession, since preliminary investigations of peat stratigraphy revealed sedge peat below no more than 5–40 cm of Sphagnum peat in undisturbed parts of the mire. This study increases our understanding of relationships between testate amoebae and their habitat, which is valuable for palaeoenvironmental studies. A local transfer function was developed with the use of six models: partial least squares, maximum likelihood, modern analogue technique, weighted averaging, tolerance down-weighted averaging, and weighted averaging-partial least squares. The weighted averaging model performed the best for depth to water table (root mean square error of prediction RMSEP  =  6.99) and pH (RMSEP  =  0.8). Results will be used as part of a regional training set to improve palaeoenvironmental reconstructions of Sphagnum peatlands.

Peatlands are globally-important carbon sinks at risk of degradation from climate change and direct human impacts, including drainage and burning. Peat accumulates when there is a positive mass balance between plant productivity inputs and litter/peat decomposition losses. However, the factors influencing the rate of peat accumulation over time are still poorly understood. We examine apparent peat accumulation rates (aPAR) during the last two millennia from 28 well-dated, intact European peatlands and find a range of between 0.005 and 0.448 cm yr-1 (mean = 0.118 cm yr-1). Our work provides important context for the commonplace assertion that European peatlands accumulate at ~0.1 cm per year. The highest aPAR values are found in the Scandinavian and Baltic regions, in contrast to Britain, Ireland, and Continental Europe. We find that summer temperature is a significant climatic control on aPAR across our European sites. Furthermore, a significant relationship is observed between aPAR and water-table depth (reconstructed from testate-amoeba subfossils), suggesting that higher aPAR levels are often associated with wetter conditions. We also note that the highest values of aPAR are found when the water table is within 5-10 cm of the peatland surface. aPAR is generally low when water table depths are < 0 cm (standing water) or > 25 cm, which may relate to a decrease in plant productivity and increased decomposition losses, respectively. Model fitting indicates that the optimal water table depth (WTD) for maximum aPAR is ~10 cm. Our study suggests that, in some European peatlands, higher summer temperatures may enhance growth rates, but only if a sufficiently high water table is maintained. In addition, our findings corroborate contemporary observational and experimental studies that have suggested an average water-table depth of ~10 cm is optimal to enable rapid peat growth and therefore carbon sequestration in the long term. This has important implications for peatland restoration and rewetting strategies, in global efforts to mitigate climate change.