Minna Väliranta

Bacterial communities are vital to northern peatlands' carbon and nutrient cycling, one of the Earth's largest terrestrial carbon stores. However, their diversity and ecological roles at broad geographic scales remain partially understood, limiting our ability to predict their response to global change. Here, we combine a trans‐Holarctic survey across 171 Sphagnum ‐dominated peatlands (SDPs) with a continental‐scale reciprocal transplantation experiment to quantify how bacterial diversity shapes carbon and nutrient cycling across bioclimatic gradients. We show that bacterial diversity and composition differ markedly among peatland bioclimatic regions and are primarily structured by the universal abiotic drivers of northern ecosystems: minimum temperature, snow cover, and soil water content. Ecological models further revealed that deterministic processes accounted for approximately 80% of bacterial community assembly, highlighting the strong influence of environmental filtering. Biodiversity‐Ecosystem Function analyses identified bacterial richness as a key driver of carbon and nutrient cycling. Using moving‐window structural equation models, we identified a critical bacterial richness threshold, below which extracellular enzyme activities increased by ~60%. This transition coincided with stronger environmental filtering and shifts towards bacterial communities with a greater predicted potential for extracellular nutrient acquisition and heterotrophic metabolism. Together, these findings demonstrate that bacterial richness underpins peatland biogeochemical functioning and suggest that biodiversity loss may accelerate carbon turnover, weakening the capacity of northern peatlands to retain carbon. As climate change threatens this unique microbiome, safeguarding bacterial diversity will be critical for maintaining ecosystem resilience and the stability of this globally important carbon sink.

Boreal peatlands store vast amounts of carbon and regulate regional hydrology, making their stability critical under accelerating climate change. This stability depends on community-level resilience and resistance shaped by the functional traits of organisms. Although climate-induced drying is already reshaping peatland communities, the trait-based mechanisms supporting stability in soil biota remain poorly resolved. We assessed stability in testate amoebae (TA) – a key group of the soil food web – by comparing functional trait patterns in ambient control plots with plots subjected to two decades of experimental water level drawdown (WLD) across three peatland types (rich fen, poor fen, and bog). In ambient conditions, null models revealed strengthened environmental filtering along the fertility gradient; nutrient-rich fen showed highest diversity and functional redundancy. Long-term WLD, however, intensified environmental filtering by reshaping the communities across all peatland types. Functional beta diversity revealed contrasting stability mechanisms: rich fen maintained functional stability through resilience, with major species turnover but minimal functional change, whereas bog communities retained function through resistance, relying on drought-adapted traits and showing minimal species turnover. Contrastingly, the poor fen lost functional stability, as low redundancy combined with dominance of wet-adapted traits led to both species and functional turnover. Under moderate drying, bogs are most likely to maintain functional stability, whereas fens, particularly poor fens, are more vulnerable. These trait-mediated differences indicate peatland type-specific functional thresholds with implications for predicting stability and carbon dynamics under future climates. Overall, continued warming increasingly compromises peatland soil biota and the ecosystem functions they mediate.

Peatlands are among Earth’s largest terrestrial carbon stores and are crucial for climate regulation, biodiversity conservation, and water security. Yet peatlands worldwide are deteriorating under pressures from climate change and human disturbance. Strategic, globally coordinated research is urgently needed to protect, restore and manage peatlands so they can continue to deliver essential ecosystem services. To meet this challenge, here we present a global research prioritisation for peatland science, based on a two-stage online survey and expert voting exercise involving 467 participants from 54 countries. We identify 50 priority research questions spanning carbon dynamics, climate impacts, restoration and management, technological innovation, and community and policy engagement. These questions provide a community-informed agenda to guide peatland research over the next decade. Addressing them will help close critical knowledge gaps, strengthen evidence-based decision making, and support the role of peatlands in achieving global climate and biodiversity goals.

Palsa mires are located at the southern border of the permafrost region. Rising temperatures have led to widespread permafrost thaw and rapid palsa and peat plateau degradation. Palsa mires store significant amounts of carbon (C), which is released when permafrost thaws, potentially changing the peatlands from long-term C sinks to temporary C sources. To improve predictions of how palsa mires will respond to ongoing climate change, a better understanding of their past dynamics is vital. Here, we reconstruct the permafrost history of a peat plateau in Karlebotn, northern Norway, using a multiproxy approach. A peat profile was dated by accelerator mass spectrometry radiocarbon method, and analyzed for oribatid mite communities, peat type and peat properties. Oribatid mite analysis is a relatively novel method suggested as a proxy for permafrost initiation. Here, we compared it to more established approaches based on peat characteristics and vegetation changes. Our results suggest that gradual permafrost aggradation at the site began after ca. 1000 calibrated years Before Present (cal year BP; present = 1950 AD) with stable permafrost conditions established by ca. 400 cal year BP. Three peatland succession phases were identified: the first wet fen phase, a transitional phase with wet and dry condition taxa and the last phase dominated by species typical of dry and permafrost-associated conditions. Our results show that integrating oribatid mite analysis into paleoecological studies offers a more robust reconstruction of past permafrost dynamics, thereby enhancing our understanding of the past, present and future development of palsa mires.

Climate change-induced drying is projected for northern peatlands, but evidence on its extent is inconsistent. Testate amoebae (TA) are a water table depth proxy that also responds to vegetation succession and abiotic changes. However, it is not well-known how TA communities and functional traits differ between unaffected and drying areas in different peatland types. Thus, we compared TA communities and functional traits in undrained control areas and areas (initially similar in vegetation and WT) subjected to moderate water level drawdown (WLD) for ∼20 years. The experiment covers a rich fen, a poor fen, and a bog. Arboreal vegetation has been established in the fen WLD areas. Taxonomic composition and functional traits differed between the WLD and control, most notably in the rich fen and the least in the bog, mirroring the vegetation. Eleven taxa favored WLD or pristine conditions; six taxa had a site-specific preference. Small tests and apertures, heterotrophy, and siliceous tests were more common in the WLD than control areas, but in the bog, these differences were not significant. Overall, besides drying, the establishment of arboreal vegetation and changes in abiotic conditions affected TA community and trait compositions in the fens, showing that they deliver information not only about water table depth but also sensitivity/resistance of peatlands. When aiming for quantitative water table reconstructions, account should be taken that the secondary changes amplify the signal of WLD in fens, whereas the resistance of vegetation and testate amoeba communities may hinder it in bogs, possibly causing bias when the reconstruction runs through a fen-bog-transition.

Northern peatlands are an important carbon store in mid to high latitudes, but become increasingly discontinuous in the higher latitudes, associated with temperature and precipitation limits on plant growth. During the last four decades, mean annual temperatures in the Arctic have increased on average by ~3 °C. Warmer temperatures and longer growing seasons likely drive increases in plant productivity throughout northern latitudes, but it is not clear whether warming has resulted in lateral spread of Arctic peatlands. Using long time-series Landsat satellite data, coupled with information gathered from fieldwork in situ, we show that Arctic peatlands have likely undergone lateral expansion over the last 40 years. On 21 transects from the edges of 16 extant peatlands in the European and Canadian Arctic (both high and low Arctic locations from 62 to 79°N), over two thirds of the peatland edges we studied showed statistically significant peak-summer greening (as Normalised Difference Vegetation Index) in the last 15 to 20 years, compared to the period 1985–1995. Peak summer moisture (as normalised Difference Moisture Index) levels remained stable or increased at most study sites. The lateral expansion of Arctic peatlands suggests they are an increasingly important natural carbon sink, at least in the near term.

Peatlands are critically important ecosystems, yet they are facing significant risks due to anthropogenic pressures. In Europe, drying of pristine peat ecosystems during the recent decades have been observed due to both direct and indirect anthropogenic impacts. Historically, peatlands have also been extensively drained by human activity, but efforts to restore them to their natural state have recently increased. The combined effects of climate change and human activities are expected to influence the carbon dynamics of peatlands in the future. Changes in peatland microbial communities can be used to assess past changes of peatlands as well as the current soil health.Microscopical testate amoebae play a crucial role in peatland carbon cycling as important members of microbial communities, acting as predators and decomposers. Testate amoebae are sensitive to changes in moisture conditions and, therefore, they have traditionally been used to model past moisture conditions in peatlands. However, the response of peatland microbial communities to restoration needs further investigation, as it is essential for improving our understanding of the underlying processes affecting carbon cycling in these ecosystems.This study aims to examine changes in testate amoeba communities in forestry-drained peatlands in the boreal zone over time including time before, during, and after peatland drainage and restoration. The study seeks to expand the understanding of the community structure of testate amoebae in peatlands, particularly in forested peatlands/swamps, as there is a lack of research on testate amoeba community structure in these habitats.In this study, testate amoeba analyses have been conducted on three peatlands located in southern Finland: one serves as a natural reference site, while the other two have been drained and subsequently restored. Preliminary results suggest that the structure of testate amoeba communities varies significantly across different peat layers in the reference site. In the drained and restored sites, testate amoeba communities in some peat cores indicate the presence of species typical of moister habitats at shallower peat depths (restored sections), whereas the signal in other cores remains more unclear.

The Hudson Bay Lowlands (HBL) of central Canada are characterized by expansive complexes of patterned forested and open bog, rich and poor fen, swamp, and marsh ecosystems. These systems have accumulated thick Holocene peat deposits, making them globally significant carbon (C) stocks. However, the magnitude of peatland C sinks, and their potential to be C sources through methane release, are sensitive to variability in temperature and moisture balance, creating uncertainties around responses to projected climate change. Periods of significant ice retreat pre-dating the Last Glacial Maximum (LGM), including interglacials and interstadials, provide opportunities to investigate wetland C dynamics under different boundary conditions including warmer and/or drier climatic conditions. Despite palynological and paleontological evidence that wetlands existed in the HBL region during Late Pleistocene periods of ice retreat, their C dynamics are poorly understood. This is in part because the available non-glacial records are mainly preserved in fluvial contexts, are often overlain by massive glacial deposits, contain both organic and mineral fractions, and are thus not readily comparable to in situ peatland records of Holocene age that are used to measure and model apparent rates of net C accumulation. Nevertheless, these non-glacial intervals preserve fossil pollen and macrofossil assemblages, and thus, have potential to contribute new insights into peatland C dynamics during ice-free intervals pre-dating the LGM. In this study, we compare fossil pollen assemblages and quantitative paleoclimate reconstructions from pre-LGM sections in the HBL, situated in the heart of glaciated North America to reconstruct paleoenvironments. We then present an approach to extract peatland C information from these deposits by identifying the presence of specific peatland types using combined modern and fossil pollen assemblages and a random forest algorithm. Using the modern analogs, assigned peatland types can then be linked to C accumulation and methane flux regimes. Ordinations of available samples indicate the presence of several distinct wetland types including Sphagnum-dominated bogs, wet fen environments in both open and forested contexts, riparian or stream bank environments with relatively better drainage, thicket swamps and marshes with Typha. These wetland types can then be used to make initial inferences on potential peat C dynamics during ice-free intervals pre-dating the LGM.

Palsa mires are located at the southern border of the permafrost zone making them highly sensitive to climate warming. When annual temperature and precipitation conditions are optimal, palsas form and collapse in a natural cycle. However, due to current climate warming, permafrost is widely thawing and palsas are degrading rapidly. In addition to being unique and valuable habitats for many species, palsas store significant amount of ancient carbon (C), which is released by reactivated decomposition processes when permafrost thaws, potentially changing the peatlands temporarily from C sinks to C sources. However, the age and extent of the C released remain uncertain. To predict how palsas will respond to ongoing climate change, understanding of the past dynamics is vital. In this study, we determined the permafrost aggradation date and reconstructed the past dynamics of a palsa in Karlebotn, northern Norway. A peat profile was radiocarbon (14C) dated, and oribatid mite analysis together with a peat type analysis and peat properties (C, nitrogen and bulk density) were performed. Oribatid mites are a diverse group of soil-living arthropods that have shown potential as environmental indicators. Recent findings suggest they can indicate the past initiation of permafrost.Our results suggest that gradual permafrost aggradation at the Karlebotn palsa began after 1500 calibrated years Before Present (cal yr BP; present = 1950 AD) and that at 700 cal yr BP the permafrost conditions had stabilized. Using multiproxy analysis, we identified three phases in the palsa history. The first phase was characterized by moist fen conditions, the second phase was a transition phase with wet and dry condition species occurring together and the last phase was dominated by species adapted to dry conditions, and which are typical in permafrost environments. Our data also indicate that during permafrost conditions, the C accumulation rate was lower than in the early non-permafrost fen stage. While permafrost thaw will temporarily increase C emissions, the C sink capacity may ultimately increase again as the peatland shifts back to a fen stage following ground subsidence.Only few palaeoecological studies exist from Fennoscandia where age of permafrost formation is determined. Most studies have used vegetation succession and peat properties to infer past permafrost presence, however, these methods are associated with uncertainties such as the absence of permafrost indicator plant species. This study provides additional data on historical palsa dynamics with a relatively robust chronology, based on multiple proxies, including oribatid mite community analysis. These findings contribute to our understanding of how palsas are responding to the ongoing and future climate change.

Palaeoecological studies reporting long-term development history of subarctic fens are scarce and overall permafrost-free peatlands located at the immediate vicinity of permafrost zone have deserved little attention in Fennoscandia. We use a multiproxy approach to study millennial-scale dynamics of two peatlands located in Finnish Lapland (Katsapuli and Maader). In addition to study autogenic succession and external forcing, we aimed in resolving the impact of surrounding landscape changes, potentially related to reindeer herding. The pollen data does not indicate major regional-scale vegetation changes but suggest an increase in the proportion of sedges towards modern times and decrease in tree pollen. These, together with an increase in regional fire events based on micro-charcoal and erosion rates measured as sediment mineral component coincide with both colder temperatures (Little Ice Age) and the emergence of reindeer-based pastoralism. The peat macrofossil data of the two profiles suggest a clear and relatively simultaneous local regime shift from sedge-dominated local habitat conditions to a Sphagnum community, where S. lindbergii becomes increasingly dominant towards the present day, suggesting prevalence of relatively moist conditions. In both sites, the regime shift is coeval with the onset of Medieval Climate Anomaly (MCA) climate conditions. Newly- established communities are sustained thereafter. Vegetation changes to Sphagnum mosses resulted in high peat growth rates, thus most of the peat stock is relatively new. Based on these data, we suggest that Sphagnum moss communities are resilient to climate fluctuations and might continue to act as effective carbon accumulation systems under warmer climates.