Fen-bog Transition Research Articles

Comparison of carbon and nitrogen accumulation rate between bog and fen phases in a pristine peatland with the fen-bog transition.

Long-term carbon and nitrogen dynamics in peatlands are affected by both vegetation production and decomposition processes. Here, we examined the carbon accumulation rate (CAR), nitrogen accumulation rate (NAR) and δ13 C, δ15 N of plant residuals in a peat core dated back to ~8500 cal year BP in a temperate peatland in Northeast China. Impacted by the tephra during 1160 and 789 cal year BP and climate change, the peatland changed from a fen dominated by vascular plants to a bog dominated by Sphagnum mosses. We used the Clymo model to quantify peat addition rate and decay constant for acrotelm and catotelm layers during both bog and fen phases. Our studied peatland was dominated by Sphagnum fuscum during the bog phase (789 to -59 cal year BP) and lower accumulation rates in the acrotelm layer was found during this phase, suggesting the dominant role of volcanic eruption in the CAR of the peat core. Both mean CAR and NAR were higher during the bog phase than during the fen phase in our study, consistent with the results of the only one similar study in the literature. Because the input rate of organic matter was considered to be lower during the bog phase, the decomposition process must have been much lower during the bog phase than during the fen phase and potentially controlled CAR and NAR. During the fen phase, CAR was also lower under higher temperature and summer insolation, conditions beneficial for decomposition. δ15 N of Sphagnum hinted that nitrogen fixation had a positive effect on nitrogen accumulation, particular in recent decades. Our study suggested that decomposition is more important for carbon and nitrogen sequestration than production in peatlands in most conditions and if future climate changes or human disturbance increase decomposition rate, carbon sequestration in peatlands will be jeopardized.

Inference of future bog succession trajectory from spatial chronosequence of changing aapa mires.

Climate change-driven vegetation changes can alter the ecosystem functions of northern peatlands. Several case studies have documented fen-to-bog transition (FBT) over recent decades, which can have major implications, as increased bog growth would likely cause cooling feedback. However, studies beyond individual cases are missing to infer if a common trajectory or many alternatives of FBT are in progress. We explored plant community and hydrology patterns during FBT of 23 boreal aapa mire complexes in Finland. We focused on mires where comparisons of historical (1940-1970) and new (2017-2019) aerial photographs indicated an expansion of Sphagnum-dominated zones. Vegetation plot and water chemistry data were collected from string-flark fens, transition zones with indications of Sphagnum increase, and bog zones; thus, in a chronosequence with a decadal time span. We ask, is there a common trajectory or many alternatives of FBT in progress, and what are the main characteristics (species and traits) of transitional plant communities? We found a pattern of fen-bog transitions via an increase in Sphagnum sect. Cuspidata (mainly S. majus and S. balticum), indicating a consistently high water table. Indicators only of transitional communities were scarce (Sphagnum lindbergii), but FBT had apparently facilitated shallow-rooted aerenchymatous vascular plants, especially Scheuchzeria palustris. Water pH consistently reflected the chronosequence with averages of 4.2, 3.9, and 3.8, from fen to transition and bog zones. Due to weak minerotrophy of string-flark fens, species richness increased towards bogs, but succession led to reduced beta diversity and homogenization among bog sites. Decadal chronosequence suggested a future fen-bog transition through a wet phase, instead of a drying trend. Transitional poor-fen vegetation was characterized by the abundance of Sphagnum lindbergii, S. majus, and Scheuchzeria palustris. Sphagnum mosses likely benefit from longer growing seasons and consistently wet and acidic conditions of aapa mires.

Climate-driven Holocene ecohydrological and carbon dynamics from maritime peatlands of the Gulf of St. Lawrence, eastern Canada

Anticosti Island, located in the Gulf of St. Lawrence (GSL; Quebec, Canada) is covered by ~25% peatland ecosystems, and conditions supporting their development remain poorly documented. We present the first reconstructions of the ecohydrological conditions (vegetation successions and water table variations) and related carbon accumulation of two maritime peatlands (Pluvier; PLU and Pointe de l’Est; PTE) located on the eastern part of Anticosti Island. These ombrotrophic peatlands developed under the influence of the GSL sea-surface conditions in interaction with atmospheric circulation during the Holocene. Long-term apparent Rate of Carbon Accumulation (LORCA, g C m−2 a−1) at PLU (13.3) and PTE (16.8) are lower than the mean global value for northern peatlands (22.9), likely due to high wind exposure. Results show that despite the earlier peat inception at PLU (9440 cal a BP) before PTE (6250 cal a BP), the fen-bog transition of both peatlands occurred almost synchronously (4540 and 5070 cal a BP respectively), suggesting favourable hydroclimatic conditions for peat growth. A slowdown in peat accumulation combined with dry and variable hydrological conditions after ca. 3300 cal a BP in the two studied peatlands coincided with the transition from the mid-Holocene to the Neoglacial cooling. At ca. 800 cal a BP, the shift from wet Sphagnum to dry ligneous peat corresponds to the transition from the Medieval Climate Anomaly to the Little Ice Age. Cooling periods, characterized by a decrease in incident solar radiation and the incursion of cold and dry Arctic air masses into the GSL, are marked by a decrease in peat accumulation, especially at PLU, due to its higher wind exposure, which influenced reduced snow cover thickness as well as frost penetration and duration through the peat.

Recent Lateral Expansion of Sphagnum Bogs Over Central Fen Areas of Boreal Aapa Mire Complexes

We investigated recent changes in spatial patterning of fen and bog zones in five boreal aapa mire complexes (mixed peatlands with patterned fen and bog parts) in a multiproxy study. Comparison of old (1940–1970s) and new aerial images revealed decrease of flarks (wet hollows) in patterned fens by 33–63% in middle boreal and 16–42% in northern boreal sites, as lawns of bog Sphagnum mosses expanded over fens. Peat core transects across transformed areas were used to verify the remote sensing inference with stratigraphic analyses of macrofossils, hyperspectral imaging, and age-depth profiles derived from 14C AMS dating and pine pollen density. The transect data revealed that the changes observed by remote sensing during past decades originated already from the end of the Little Ice Age (LIA) between 1700–1850 CE in bog zones and later in the flarks of fen zones. The average lateral expansion rate of bogs over fen zones was 0.77 m y−1 (range 0.19–1.66) as estimated by remote sensing, and 0.71 m y−1 (range 0.13–1.76) based on peat transects. The contemporary plant communities conformed to the macrofossil communities, and distinct vegetation zones were recognized as representing recently changed areas. The fen-bog transition increased the apparent carbon accumulation, but it can potentially threaten fen species and habitats. These observations indicate that rapid lateral bog expansion over aapa mires may be in progress, but more research is needed to reveal if ongoing fen-bog transitions are a commonplace phenomenon in northern mires.

Ongoing Fen–Bog Transition in a Boreal Aapa Mire Inferred from Repeated Field Sampling, Aerial Images, and Landsat Data

Northern aapa mire complexes are characterized by patterned fens with flarks (wet fen surfaces) and bog zone margins with Sphagnum moss cover. Evidence exists of a recent increase in Sphagnum over fens that can alter ecosystem functions. Contrast between flarks and Sphagnum moss cover may enable remote sensing of these changes with satellite proxies. We explored recent changes in hydro-morphological patterns and vegetation in a south-boreal aapa mire in Finland and tested the performance of Landsat bands and indices in detecting Sphagnum increase in aapa mires. We combined aerial image analysis and vegetation survey, repeated after 60 years, to support Landsat satellite image analysis. Aerial image analysis revealed a decrease in flark area by 46% between 1947 and 2019. Repeated survey showed increase in Sphagnum mosses (S. pulchrum, S. papillosum) and deep-rooted vascular plants (Menyanthes trifoliata, Carex rostrata). A supervised classification of high-resolution UAV image recognized the legacy of infilled flarks in the patterning of Sphagnum carpets. Among Landsat variables, all separate spectral bands, the Green Difference Vegetation Index (GDVI), and the Automated Water Extraction Index (AWEI) correlated with the flark area. Between 1985 and 2020, near-infrared (NIR) and GDVI increased in the central flark area, and AWEI decreased throughout the mire area. In aapa mire complexes, flark fen and Sphagnum bog zones have contrasting Landsat NIR reflectance, and NIR band is suggested for monitoring changes in flarks. The observed increase in Sphagnum mosses supports the interpretation of ongoing fen–bog transitions in Northern European aapa mires, indicating significant ecosystem-scale changes.

Integrating Decomposers, Methane-Cycling Microbes and Ecosystem Carbon Fluxes Along a Peatland Successional Gradient in a Land Uplift Region

Peatlands are carbon dioxide (CO2) sinks that, in parallel, release methane (CH4). The peatland carbon (C) balance depends on the interplay of decomposer and CH4-cycling microbes, vegetation, and environmental conditions. These interactions are susceptible to the changes that occur along a successional gradient from vascular plant-dominated systems to Sphagnum moss-dominated systems. Changes similar to this succession are predicted to occur from climate change. Here, we investigated how microbial and plant communities are interlinked with each other and with ecosystem C cycling along a successional gradient on a boreal land uplift coast. The gradient ranged from shoreline to meadows and fens, and further to bogs. Potential microbial activity (aerobic CO2 production; CH4 production and oxidation) and biomass were greatest in the early successional meadows, although their communities of aerobic decomposers (fungi, actinobacteria), methanogens, and methanotrophs did not differ from the older fens. Instead, the functional microbial communities shifted at the fen–bog transition concurrent with a sudden decrease in C fluxes. The successional patterns of decomposer versus CH4-cycling communities diverged at the bog stage, indicating strong but distinct microbial responses to Sphagnum dominance and acidity. We highlight young meadows as dynamic sites with the greatest microbial potential for C release. These hot spots of C turnover with dense sedge cover may represent a sensitive bottleneck in succession, which is necessary for eventual long-term peat accumulation. The distinctive microbes in bogs could serve as indicators of the C sink function in restoration measures that aim to stabilize the C in the peat.

Identification of peat type and humification by laboratory VNIR/SWIR hyperspectral imaging of peat profiles with focus on fen-bog transition in aapa mires

AimsHyperspectral imaging (HSI) has high potential for analysing peat cores, but methodologies are deficient. We aimed for robust peat type classification and humification estimation. We also explored other factors affecting peat spectral properties.MethodsWe used two laboratory setups: VNIR (visible to near-infrared) and SWIR (shortwave infrared) for high resolution imaging of intact peat profiles with fen-bog transitions. Peat types were classified with support vector machines, indices were developed for von Post estimation, and K-means clustering was used to analyse stratigraphic patterns in peat quality. With separate experiments, we studied spectral effects of drying and oxidation.ResultsDespite major effects, oxidation and water content did not impede robust HSI classification. The accuracy between Carex peat and Sphagnum peat in validation was 80% with VNIR and 81% with SWIR data. The spectral humification indices had accuracies of 82% with VNIR and 56%. Stratigraphic HSI patterns revealed that 36% of peat layer shifts were inclined by over 20 degrees. Spectral indices were used to extrapolate visualisations of element concentrations.ConclusionsHSI provided reliable information of basic peat quality and was useful in visual mapping, that can guide sampling for other analyses. HSI can manage large amounts of samples to widen the scope of detailed analysis beyond single profiles and it has wide potential in peat research beyond the exploratory scope of this paper. We were able to confirm the capacity of HSI to reveal shifts of peat quality, connected to ecosystem-scale change.

Abrupt Fen-Bog Transition Across Southern Patagonia: Timing, Causes, and Impacts on Carbon Sequestration

Fens and bogs are distinct in terms of their biogeochemistry, water table behavior, and net peat-accumulation regimes. While most peatlands start developing as fens, a large fraction of them eventually shift to bogs in a step-like ecosystem shift. This transition has traditionally been assumed to be primarily controlled by the ecosystem itself (autogenic control). Here we use 90 peat profiles from southernmost South America as a case study that illustrates a synchronous, regional-scale shift from fen to bog around 4200 years ago. In light of these results, we propose and discuss conceptual models that link environmental change (allogenic control) as a trigger to the fen-bog transition. In addition, our stratigraphic analyses show that Sphagnum deposits are associated with greater peat masses, larger soil-carbon stocks, and higher rates of peat-carbon accumulation than their non-Sphagnum counterparts, with Sphagnum bogs being characterized by soil-carbon densities over twice that of non-Sphagnum peatlands (medians = 141 vs. 56 kgC/m2). Since fens and bogs also behave differently in terms of their carbon exchanges with the atmosphere, a better appraisal of the processes involved in the fen-bog transition could help elucidate the role of this critical ecosystem shift in the past and future global carbon cycle.

Use of testate amoebae to infer paleohydrology during fen and fen-bog transition stages of ombrotrophic mire development

We evaluated the feasibility of using testate amoebae to infer the quantitative paleohydrology of ombrotrophic mires during their early stages (fen and fen-bog transition) of development. Two transfer functions, one derived from ombrotrophic and the other from minerotrophic mires, were applied to a peat core from an ombrotrophic mire in a taiga region of west Siberia. An ombrotrophic transfer function was applied to the bog stage of mire development. In contrast, ombrotrophic and minerotrophic transfer functions were applied independently to infer water table depth in the fen and fen-bog transition stages. Results of the two approaches for calculating water table depth during the fen and fen-bog transition stages differed by as much as 38 cm for the same peat sample. The main reason for this discrepancy is presence of testate amoeba taxa (e.g. Centropyxis aculeata, Cyclopyxis eurystoma, Cyclopyxis eurystoma v. parvula) in the peat that inhabit both modern ombrotrophic and minerotrophic mires, but differ substantially, in cases by > 20 cm, in terms of their water table depth optima in the ombrotrophic and minerotrophic mire calibration data sets. This difference in inferred water table depth also stems, to a lesser degree, from the fact that the ombrotrophic mire model does not include taxa that inhabited exclusively minerotrophic mires in the fen and fen-bog transition stages. Given these findings, we propose that different models be used for different stages of development, to reconstruct past water table depth in ombrotrophic mires. We recommend use of a model based solely on the ombrotrophic mire data set for the bog stage, and application of a second model based on the minerotrophic mire data set, for the fen and fen-bog transition stages. Application of an ombrotrophic model to the early stages of bog development can yield erroneous paleohydrological reconstructions.

Dendrochronologically dated pine stumps document phase-wise bog expansion at a northwest German site between ca. 6700 and ca. 3400 BC

Abstract. The investigated northwest German mire site at “Totes Moor” is densely covered with subfossil pine stumps (Pinus sylvestris L.) from the fen–bog transition. This facilitates the spatio-temporal reconstruction of mire development, which is based on 212 in situ tree stumps in the case study presented here. Six dendrochronologically dated site chronologies together cover 2345 years between 6703 and 3403 BC. The gaps in between are 6 to 550 years long. Additionally, a floating chronology of 309 years, containing 30 trees, was radiocarbon-dated to the beginning of the 7th millennium cal BC. Peat-stratigraphical survey was carried out additionally, and elevations a.s.l. were determined at several locations. Tree dying-off phases, which indicate water level rise at the site, mostly in context of the local fen–bog transition, are evident for ca. 6600–6450, ca. 6350–5750, ca. 5300–4900, ca. 4700–4550, ca. 3900–3850, ca. 3700–3600, ca. 3500–3450 and ca. 3400 BC. The spatial distribution of the dated in situ trees illustrates the phase-wise expansion of raised bog over fen peat at the site. The documented bog expansion pulses likely correspond to climatic wet sifts.

Peatlands in a eutrophic world – Assessing the state of a poor fen-bog transition in southern Ontario, Canada, after long term nutrient input and altered hydrological conditions

Excessive nutrient supply may threaten the carbon storage function of nutrient limited peatlands. We conducted a detailed study in a bog ecosystem (Wylde Lake peatland, Canada), which was once ombrotrophic and since AD 1954 borders a water reservoir, which is enriched with nutrients. Our objective was to elucidate whether the inner peatland parts maintain typical characteristics of a pristine bog. To achieve this goal, along a transect of study sites, we dated peat cores, determined nutrient concentrations and N input and mapped the vegetation. The peatland's central part showed large N input rates of ∼4.3 g N•m−2•y−1, but even greater rates of 5.90 ± 0.10 g N•m−2•y−1, were found in the periphery. Elements essential for plant growth, such as N, P, S, Ca, Mg, Mn, Fe, Cu and Zn were increased in concentration upwards in the profile of all peat cores, especially near the reservoir, presumably due to supply by the reservoir water. Also, a more graminoid dominated vegetation near the reservoir indicated a transformation of the once ombrotrophic bog into a poor fen. To our surprise and in contrast to previous studies the peatland did not seem to decay after long-term excessive nutrient load, instead it accelerated peat accumulation, leading to maximum growth rates of up to 500 g C•m−2•y−1 immediately after flooding of the reservoir. Peatland functioning in terms of carbon storage appeared maintained.

Non-Pollen Palynomorphs from Mid-Holocene Peat of the Raised Bog Borsteler Moor (Lower Saxony, Germany)

Abstract In order to reconstruct regional vegetation changes and local conditions during the fen-bog transition in the Borsteler Moor (northwestern Germany), a sediment core covering the period be tween 7.1 and 4.5 cal kyrs BP was palynologically investigated. The pollen diagram demonstrates the dominance of oak forests and a gradual replacement of trees by raised bog vegetation with the wetter conditions in the Late Atlantic. At ~ 6 cal kyrs BP, the non-pollen palynomorphs (NPP) demonstrate the succession from mesotrophic conditions, clearly indicated by a number of fun gal spore types, to oligotrophic conditions, indicated by Sphagnum spores, Bryophytomyces sphagni, and testate amoebae Amphitrema, Assulina and Arcella, etc. Four relatively dry phases during the transition from fen to bog are clearly indicated by the dominance of Calluna and associated fungi as well as by the in crease of microcharcoal. Several new NPP types are described and known NPP types are identified. All NPP are discussed in the context of their palaeoecological indicator values.

Plant macrofossil and biomarker evidence of fen–bog transition and associated changes in vegetation in two Finnish peatlands

Past vegetation assemblages, preserved in peat layers, are one of the key proxies when reconstructing historical peatland dynamics. Northern peatlands can be divided into two main types: fens and bogs. Compared with bog peat, the fen peat is usually more decomposed because of different eco-hydrological conditions and effective humification processes. A high level of decomposition hampers reliable identification of plant remains and constrains palaeoecological approaches. Biogeochemical studies on bog plants and bog peat have shown that plant group–specific biomarkers can be applied to identify fossil plants or plant groups from peat, given the identification of plant group–specific markers in living fen plants. In this study, we applied plant macrofossil, biomarkers and multivariate statistical analyses to two mid-boreal peat sequences to investigate whether biomarkers can be applied to distinguish fen and bog environments and whether plant-specific biomarkers can be identified from fen peat. Macrofossil analyses clearly separate dry bog hummocks, moist lawns and wet fen habitats apart. Corresponding division emerged when the biomarker data were combined with the macrofossil data. Moreover, we succeeded to separate bog and fen habitats apart by the changes in n-alkane and the n-alkane ratio distributions along the cores. The fen–bog transition zone was indicated by high sterol and triterpenoid concentrations and changes in degradation measures. However, it remains a challenging task to attain species-level information of past plant assemblages from highly humified fen peat layers based on biomarkers only.

Assessing the role of parameter interactions in the sensitivity analysis of a model of peatland dynamics

Identifying the processes controlling the carbon balance in northern peatlands is crucial for their integration in global climate models. As a first step, current models of peatlands dynamics need to be adequately evaluated to verify the consistency of processes before their integration in global models.We compared here the sensitivity of the total carbon mass of a peatland simulated by the Holocene Peat Model (HPM) derived from different sensitivity methods: a ‘classic’ local sensitivity analysis method and a global approach with the screening method.We observed that the conclusions drawn by the different methods are very different and, moreover, that it is not advisable to draw any general conclusion from a local one-at-a-time (OAT) experiment because the model space represented is very limited. The results also stressed that the representation of several processes, such as runoff or peat decomposition under different saturation conditions, lack sufficient empirical data and knowledge to be adequately represented in the model. In addition to the evaluation of the model per se, the exploration of its behaviour allowed us to observe simulations of two fen-bog transition patterns.This study showed that interactions between parameters should be taken into account when evaluating peatland dynamics models. The results of the screening method are useful for model improvement and simplification. Moreover, this method enables the exploration of model processes in detail, thus providing insight into peatland system dynamics.

Wetland chronosequence as a model of peatland development: Vegetation succession, peat and carbon accumulation

Model validation experiments are fundamental to ensure that the peat growth models correspond with the diversity in nature. We evaluated the Holocene Peatland Model (HPM) simulation against the field observations from a chronosequence of peatlands and peat core data. The ongoing primary peatland formation on the isostatically rising coast of Finland offered us an exceptional opportunity to study the peatland succession along a spatial continuum and to compare it with the past succession revealed by vertical peat sequences. The current vegetation assemblages, from the seashore to a 3000 year old bog, formed a continuum from minerotrophic to ombrotrophic plant communities. A similar sequence of plant communities was found in the palaeovegetation. The distribution of plant functional types was related to peat thickness and water-table depth (WTD) supporting the assumptions in HPM, though there were some differences between the field data and HPM. Palaeobotanical evidence from the oldest site showed a rapid fen–bog transition, indicated by a coincidental decrease in minerotrophic plant functional types and an increase in ombrotrophic plant functional types. The long-term mean rate of carbon (C) accumulation varied from 2 to 34 g C/m2 per yr, being highest in the intermediate age cohorts. Mean nitrogen (N) accumulation varied from 0.1 to 3.9 g N/m2 per yr being highest in the youngest sites. WTD was the deepest in the oldest sites and its variation there was temporally the least but spatially the highest. Evaluation of the HPM simulations against the field observations indicated that HPM reasonably well simulates peatland development, except for very young peatlands.

Testate amoebae as indicators of hydroseral change: An 8500 year record from Mer Bleue Bog, eastern Ontario, Canada

Testate amoebae have been used widely as a proxy of hydrological change in ombrotrophic peatlands, although their response to abiotic controls in other types of mire and fenland palaeo-environments is less well understood. This paper examines the response of testate amoebae to hydroseral and other environmental changes at Mer Bleue Bog, Ontario, Canada, a large ombrotrophic peatland, which evolved from a brackish-water embayment in the early Holocene. Sediments, plant macrofossils and diatoms examined from a 5.99 m core collected from the dome of the bog record six stages of development: i) a quiet, brackish-water riverine phase (prior to ca. 8500 cal BP); ii) a shallow lake (ca. 8500–8200 cal BP); iii) fen (8200–7600 cal BP); iv) transitional mire (7600–6900 cal BP); v) pioneer raised mire (6900–4450 cal BP); and vi) ombrotrophic bog (4450 cal BP-present).Testate amoebae, notably small (<25 μm diameter) specimens of Centropyxis aculeata type, first appear in low abundances in sediments ascribed to the lacustrine phase. Diatoms from the same horizons record a shallowing in water depth, a decline in salinity and the development of emergent macrophytic vegetation, which may have provided favourable conditions for testate amoeba colonization. The testate amoeba communities of the inferred fen phase are more diverse and include centropyxids, cyclopyxids, Arcellidae and Hyalospheniidae, although the assemblages show some differences to those recently reported in modern European fen environments. The Fen–Bog Transition (FBT) is also dominated by C. aculeata type. The change in testate amoeba communities around this key transition is apparent in the results of Detrended Correspondence Analysis (DCA), and appears to reflect a latent nutrient gradient and a secondary moisture gradient. DCA analyses of plant macrofossil remains around the FBT show a similar trend, although the sensitivity of the two proxies to the inferred environmental changes differs. Comparisons with other regional mid-Holocene peatland records confirm the important influence of reduced effective precipitation on the testate amoeba communities during the initiation and development of Sphagnum-dominated, raised bog communities.

Can testate amoeba‐based palaeohydrology be extended to fens?

AbstractNumerous palaeoecological studies have used testate amoeba analysis to reconstruct Holocene hydrological change in peatlands, and thereby past climatic change. Current studies have been almost exclusively restricted to ombrotrophic bogs and the period since the fen–bog transition. Although the critical link between peatland surface wetness and climate is less direct in minerotrophic peatlands, such records may still be of value where there are few others, particularly if multiple records can be derived and inter‐compared. Expanding the temporal and spatial scope of testate amoeba‐based palaeohydrology to minerotrophic peatlands requires studies to establish the primacy of hydrology and the efficacy of transfer functions across a range of sites. This study analyses testate amoeba data from wetlands spanning the trophic gradient in the eastern Mediterranean region. Results demonstrate that different types of wetlands have distinctly different amoeba communities, but hydrology remains the most important environmental control (despite water table depth being measured at different times for different sites). Interestingly, Zn and Fe emerge as significant environmental variables in a subset of sites with geochemical data. Testate amoeba–hydrology transfer functions perform well in cross‐validation but frequently perform poorly when applied to other sites, particularly with sites of a different nutrient status. It may be valid to use testate amoebae to reconstruct hydrological change from minerotrophic peatlands with an applicable transfer function; however, it may not be appropriate to use testate amoebae to reconstruct hydrological change through periods of ecosystem evolution, particularly the fen–bog transition. In practice, the preservation of amoeba shells is likely to be a key problem for palaeoecological reconstruction from fens. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Is Empodisma minus the ecosystem engineer of the FBT (fen–bog transition zone) in New Zealand?

The role of ecosystem engineers (EE) in the formation of ombrotrophic mires (bogs) from fens, called the fen–bog transition (FBT), can be best understood through categorization of the autogenic and allogenic processes causing bog initiation. Here we review these pathways, discuss the drivers of change in both cases, and tabulate an approach for distinguishing between them. We then compare the engineering ability of acknowledged and putative engineers against a number of characters which plants require to cross the FBT, and to stabilize occupancy on the bog side. While some Sphagnum spp. are accepted as the EE of the fen–bog transition in northern hemisphere bogs, they appear unimportant in New Zealand. Instead their role appears to be occupied by a restiad, Empodisma minus, a plant with leafless, wiry stems and capillaroid roots. Empodisma minus appears capable of engineering autogenic transitions from fen to bog across New Zealand, even more efficiently than Sphagnum.

Contrasting pathways to ombrotrophy in three raised bogs from Ireland and Cumbria, England

The plant macrofossil records from three raised bogs from western and central Ireland and from Cumbria were compared to investigate variations in the character and timing of the fen-bog transition (FBT). Two modes of raised or ombrotrophic peat formation were recognized. In the first mode the transition develops through a sedge-fen stage directly to raised-bog lawn, maintaining a near surface water table throughout the FBT (Abbeyknockmoy Bog). The second mode is characterized by a switch from fen or reedswamp to a dry Eriophorum-dominated mire type, suggestive of low or unstable water tables (Mongan Bog and Bolton Fell Moss). Comparisons with the Holocene effective precipitation record suggest that the type of transition and its timing may be affected by the prevailing climate regime. All that is required for ombrotrophy is a separation of the growing peat surface from the influence of ground and surface waters. This separation may be achieved by rapid peat accumulation in a moist climate, the accumulation of decay-resistant Eriophorum vaginatum hummocks or by a lowering of the water table, leading to a ‘perched peat bed’. In the latter two cases, a ‘pseudo-raised bog’ community often persists, laying down highly humified peat, until increased oceanicity enables Sphagnum to dominate.

Mire development across the fen–bog transition on the Teifi floodplain at Tregaron Bog, Ceredigion, Wales, and a comparison with 13 other raised bogs

Summary A literature comparison of 14 radiocarbon‐dated macrofossil records of raised peat bog initiation indicates that there is a relationship between the prevailing climate and the character of the first ombrotrophic vegetation communities at peatland sites in Britain and Ireland. All that is required for ombrotrophy is the separation of the mire growing surface from the influence of surface and subsurface waters. This could occur via vertical accumulation of the peat mass or a lowering of the water table. The establishment of bog species can be rapid once isolation occurs. Peatlands may become ombrotrophic in a variety of water table conditions and climatic regimes. There are at least two distinctive routes to ombrotrophy, via a ‘dry‐pioneer oligotrophic community’ or via a ‘wet‐pioneer oligotrophic community’. Tregaron South‐east Bog does not fit the pattern suggested by the literature comparison. The Fen–Bog Transition (FBT) occurred in a period of increased effective precipitation but the first ombrotrophic community was indicative of relatively dry, ‘hummocky’ bog and a deep or unstable water table. The transitional poor fen communities at Tregaron South‐east Bog were short‐lived. Sphagnum palustre mire lasted for 90 years compared to 300 years at Bolton Fell Moss in Cumbria, and the FBT was synchronous across much of the bog. The Tregaron peatland complex has a long history of water table fluctuations as shown by the stratigraphy of the marginal peats. Channel incision in the River Teifi could have contributed to the development of ‘dry hummock’ pioneer bog in the humid climatic conditions at 7300 cal. BP, by reducing the level of lagg streams.