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Peatland restoration is seen as a key nature‐based solution to tackle climate change and biodiversity loss. In Europe, nearly 50% of peatlands have been drained during the last decades, which have shifted their soils to carbon dioxide (CO2) sources. Soils of forestry‐drained peatlands are known to vary from CO2 sources to small sinks depending on their fertility and wetness. When peatlands are restored, it can be expected that rates of CO2 and methane exchange will vary depending on site fertility and wetness. We generated seven restoration pathways with different starting and end points and assessed the climate impacts of them. The GHG emission coefficients were compiled from literature, and radiative forcing was calculated for a 500‐year time period since restoration. All seven restoration pathways improved carbon sink capacity; however, the climate impact differed from cooling to warming. The highest cooling impact occurred in a pathway leading from nutrient‐rich drained peatlands toward tree‐covered spruce or pine mires. Warming impacts occurred in a pathway leading from nutrient‐poor drained peatlands toward open peatlands. The results of this study can be used to help identify peatland sites and restoration targets to maximize climate change mitigation from restoration. In practice, however, restoration has to fulfill other targets, such as biodiversity safeguarding, improvement of hydrological conditions, and socio‐economic aspects. Fulfilling all targets simultaneously requires compromises on all targets. [ABSTRACT FROM AUTHOR]

Previous studies have shown that atmospheric nitrogen (N) deposition is detrimental to sphagna, which are a group of mosses that are important for carbon cycling in northern peatlands. Little is known about species interactions, such as relative responses of tall moss Polytrichum strictum Menzies ex Brid. and sphagna. We studied the effects of N deposition on growth, abundance, and C[O.sub.2] exchange of the moss species Sphagnum capillifolium (Ehrh.) Hedw. and Polytrichum strictum in an experiment at a temperate bog. Sphagnum growth and cover decreased significantly with high-dose N treatment (6.4 g N x [m.sup.-2] x [year.sup.-1]) in years 4 and 5 of treatment, whereas the same parameters increased for Polytrichum compared with the control. Net C[O.sub.2] exchange, gross photosynthesis ([P.sub.g]), and dark respiration (R) in the intact moss cores, which were measured in year 5 of treatment, were elevated in the cores that had been treated with the high-dose of N, compared with the control, and this was associated with increased abundance of Polytrichum. The moss cores where Polytrichum was removed, however, had increased mass-based R with the high-dose N treatment. Our results showed that S. capillifolium at Mer Bleue may be close to N saturation, as 5 years of high-dose N loading (6.4 g N x [m.sup.-2] x [year.sup.-1] + background) was harmful to this species, possibly as a result of increased respiratory cost. Polytrichum strictum had a competitive advantage, at least in the short-term, through allocating excess N to growth. This change in moss layer composition deserves further attention, as a shift to more easily decomposable litter, without corresponding increases in plant production, could reduce the carbon sequestration of the bog. Keywords: peatland, photosynthesis, chlorophyll fluorescence, respiration, vegetation change, moss. Des etudes precedentes ont montre que le depot de N atmospherique peut etre dommageable a la sphaigne, un genre important au recyclage de C dans les tourbieres du nord. On sait peu de choses des interactions entre les especes, telles les reponses relatives de la mousse Polytricum strictum Menzies ex Brid. et de la sphaigne. Les auteurs ont etudie les effets du depot de N sur la croissance en hauteur, l'abondance et l'echange de C[O.sub.2] des especes de mousses Sphaghum capillifolium (Ehrh.) Hedw. et Polytricum strictum lors d'une experience en tourbiere temperee. La croissance en hauteur de Sphaghum et son couvert diminuaient significativement en presence d'une forte concentration de N (6,4 g N x [m.sup.-2] x [an.sup.-1]) a la quatrieme et cinquieme annees de traitement, alors que ceux de Polytricum augmentaient, relativement au controle. L'echange net de C[O.sub.2], la photosynthese brute ([P.sub.b]) et la respiration mitochondriale (R) des mousses intactes, mesures a la cinquieme annee de traitement, etaient accrus par le traitement riche en N relativement aux controles, a cause de l'abondance accrue de Polytricum. Les mousses dont on avait retire Polytrichum presentaient cependant une R en fonction de la masse accrue par le traitement riche en N. Les resultats obtenus par les auteurs montrent que S. capillifolium de la tourbiere de Mer Bleue peut etre presqu'a saturation de N, car 5 ans de charge elevee en N (6,4 g N x [m.sup.-2] x [an.sup.-1] + niveau de base) ont ete dommageables a cette espece, possiblement a cause d'un cout respiratoire accru. Polytrichum strictum possedait un avantage competitif, du moins a court terme, reservant l'exces de N a la croissance. Ce changement dans la composition du couvert de mousse merite une plus grande attention car un deplacement vers une litiere plus facilement decomposable, sans augmentations correspondantes de la production vegetale, peut diminuer la sequestration du C par la tourbiere. [Traduit par la Redaction] Mots-cles: tourbiere, photosynthese, fluorescence de la chlorophylle, respiration, changement de la vegetations, mousse., Introduction The deposition of atmospheric nitrogen (N) drastically increased N availability in the industrialized world during the Anthropocene (e.g., Galloway et al. 2008). Excess N from deposition threatens many natural [...]

Climate warming is leading to permafrost thaw in northern peatlands, and current predictions suggest that thawing will drive greater surface wetness and an increase in methane emissions. Hydrology largely drives peatland vegetation composition, which is a key element in peatland functioning and thus in carbon dynamics. These processes are expected to change. Peatland carbon accumulation is determined by the balance between plant production and peat decomposition. But both processes are expected to accelerate in northern peatlands due to warming, leading to uncertainty in future peatland carbon budgets. Here, we compile a dataset of vegetation changes and apparent carbon accumulation data reconstructed from 33 peat cores collected from 16 sub‐arctic peatlands in Fennoscandia and European Russia. The data cover the past two millennia that has undergone prominent changes in climate and a notable increase in annual temperatures toward present times. We show a pattern where European sub‐Arctic peatland microhabitats have undergone a habitat change where currently drier habitats dominated by Sphagnum mosses replaced wetter sedge‐dominated vegetation and these new habitats have remained relatively stable over the recent decades. Our results suggest an alternative future pathway where sub‐arctic peatlands may at least partly sustain dry vegetation and enhance the carbon sink capacity of northern peatlands. [ABSTRACT FROM AUTHOR]

We compared the amount and composition of different aboveground biomass (BM) fractions of four mires with their net ecosystem C[O.sub.2] exchange (NEE) measured by eddy covariance. We found clear differences in response of green biomass (GBM) of plant functional types (PFTs) to water table (WT), which resulted in larger spatial variation in GBM within a mire than variation between mires. GBM varied between mires from 126 ± 7 to 336 ± 16 g x [m.sup.-2] (mean ± SE), while within mire variation at largest was from 157 ± 17 to 488 ± 20 g x [m.sup.-2] (mean ± SE). GBM of dominant PFTs appeared to be better in explaining the peak growing season NEE than the total BM or GBM of a mire. The differences in photosynthetic capacity between PTFs had a major role, and thus a smaller GBM with different species composition could result in higher NEE than larger GBM. Vascular plant GBM, especially that of sedges, appeared to have a high impact on NEE. Eleven PFTs, defined here, appeared to capture well the internal variation within mires, and the differences in GBM between communities were explained by the water table response of PFTs. Our results suggest the use of photosynthesizing BM, separated into PFTs, in modelling ecosystem carbon exchange instead of using just total BM. Key words: bog, fen, microtopography, net ecosystem exchange, peatland, plant functional type, water table. L'auteure a compare la quantite et la composition des differentes fractions de la biomasse (BM) epigee dans quatre tourbieres, incluant la mesure de l'echange net de C[O.sub.2] par l'ecosysteme (NEE), a l'aide de la covariance de fluctuation. Elle a observe des differences nettes de reaction de la biomasse verte (BMV) des types fonctionnels de plantes (TFP) a la nappe phreatique, lesquelles conduisent a une variation plus importante de la BMV a l'interieur d'une tourbiere qu'entre les tourbieres. La BMV varie entre les tourbieres de 126 ± 7 a 336 ± 16 g x [m.sup.-2] (moyenne ± SE), alors qu'al'interieur des tourbieres la plus grande va de 157 ± 17 a 488 ± 20 g x [m.sup.-2] (moyenne ± SE). La BMV du TPF dominant semble la meilleure pour expliquer le pic de le NEE de la saison de croissance, que la BM ou la BMV d'une tourbiere. Les differences de capacite photosynthetique entre les TFP jouent un role majeur, et ainsi une plus petite BMV avec differentes especes peut conduire a des resultats de NEE plus eleves qu'une BMV plus grande. La BMV des plantes vasculaires, surtout pour bien capter la variation interne dans une tourbiere, et les differences de BMV entre les communautes s'expliquent par la reaction de la nappe phreatique des TPF. Les resultats suggerent d'utiliser la BM photosynthetique, distribuee selon les TPF, pour la modelisation de l' echange de carbone par les ecosystemes, plutot que la seule BM. Mots-cles : tourbiere ombrotrophe, tourbiere minerotrophe, microtopographie, echange ecosystemique net, tourbiere, type fonctionnel de plantes, nappe phreatique. [Traduit par la Redaction], Introduction Mires have a dual effect on atmospheric greenhouse gas concentrations. They assimilate atmospheric carbon dioxide (C[O.sub.2]) in photosynthesis and release C[O.sub.2] and methane (C[H.sub.4]) in respiration. Mires have stored [...]

Current peatland models generally treat vegetation as static, although plant community structure is known to alter as a response to environmental change. Because the vegetation structure and ecosystem functioning are tightly linked, realistic projections of peatland response to climate change require the inclusion of vegetation dynamics in ecosystem models. In peatlands, Sphagnum mosses are key engineers. Moss community composition primarily follows habitat moisture conditions. The known species habitat preference along the prevailing moisture gradient might not directly serve as a reliable predictor for future species compositions, as water table fluctuation is likely to increase. Hence, modelling the mechanisms that control the habitat preference of Sphagna is a good first step for modelling community dynamics in peatlands. In this study, we developed the Peatland Moss Simulator (PMS), which simulates the community dynamics of the peatland moss layer. PMS is a process-based model that employs a stochastic, individual-based approach for simulating competition within the peatland moss layer based on species differences in functional traits. At the shoot-level, growth and competition were driven by net photosynthesis, which was regulated by hydrological processes via the capitulum water content. The model was tested by predicting the habitat preferences of Sphagnum magellanicum and Sphagnum fallax – two key species representing dry (hummock) and wet (lawn) habitats in a poor fen peatland (Lakkasuo, Finland). PMS successfully captured the habitat preferences of the two Sphagnum species based on observed variations in trait properties. Our model simulation further showed that the validity of PMS depended on the interspecific differences in the capitulum water content being correctly specified. Neglecting the water content differences led to the failure of PMS to predict the habitat preferences of the species in stochastic simulations. Our work highlights the importance of the capitulum water content with respect to the dynamics and carbon functioning of Sphagnum communities in peatland ecosystems. Thus, studies of peatland responses to changing environmental conditions need to include capitulum water processes as a control on moss community dynamics. Our PMS model could be used as an elemental design for the future development of dynamic vegetation models for peatland ecosystems. [ABSTRACT FROM AUTHOR]

Sphagnum is the major genus in northern peatlands that contributes to peat formation and carbon sequestration. Sphagnum growth in summer has been fairly well studied but the information about growth in autumn and winter is limited. Therefore, we studied how the growth of Sphagnum is seasonally distributed with a particular interest on possible winter growth. The linear increment and biomass production of three Sphagum species was measured in three Northern European bogs over a year. In all sites, our results indicate the highest annual linear increment in S. angustifolium (28 mm), followed by S. magellanicum (20 mm) and S. fuscum (13 mm), but the biomass production was fairly even among the species (189, 192 and 215 g m−2, respectively). Both linear increment and biomass production depended mostly on meteorological parameters rather than ecophysiological or microsite properties. The seasonal measurements revealed a significant linear increment and biomass production during the winter that accounted for ca. 10% and ca. 5% from the annual values, respectively. Moreover, the mean daily rates of linear increment in autumn often exceeded the increment in summer. Our results thus indicate the ability for year‐around growth of Sphagna if the conditions are favorable, including during boreal winter. [ABSTRACT FROM AUTHOR]

Current peatland models generally lack dynamic feedback between the plant community structure and the environment, although the vegetation dynamics and ecosystem functioning are tightly linked. Realistic projections of peatland response to climate change requires including vegetation dynamics in ecosystem models. In peatlands, Sphagnum mosses are key engineers. The species composition in a moss community varies primarily following habitat moisture conditions. Hence, modelling the mechanisms in controlling the habitat preference of Sphagna is a good first step for modelling the community dynamics in peatlands. In this study, we developed the Peatland Moss Simulator (PMS), a process-based model, for simulating community dynamics of the peatland moss layer that results in habitat preferences of Sphagnum species along moisture gradients. PMS employed an individual-based approach to describe the variation of functional traits among shoots and the stochastic base of competition. At the shoot-level, growth and competition were driven by net photosynthesis, which was regulated by hydrological processes via capitulum water retention. The model was tested by predicting the habitat preferences of S. magellanicum and S. fallax, two key species representing dry (hummock) and wet (lawn) habitats in a poor fen peatland (Lakkasuo, Finland). PMS successfully captured the habitat preferences of the two Sphagnum species, based on observed variations in trait properties. Our model simulation further showed that the validity of PMS depended on the interspecific differences in capitulum water retention being correctly specified. Neglecting the water-retention differences led to the failure of PMS to predict the habitat preferences of the species in stochastic simulations. Our work highlights the importance of capitulum water retention to the dynamics and carbon functioning of Sphagnum communities in peatland ecosystems. Studies of peatland responses to changing environmental conditions thus need to include capitulum water processes as a control on the vegetation dynamics. For that our PMS model could be used as an elemental design for the future development of dynamic vegetation models for peatland ecosystems. [ABSTRACT FROM AUTHOR]

Vegetation and hydrology are important controlling factors in peatland methane dynamics. This study aimed at investigating the role of vegetation components - sedges, dwarf-shrubs, and Sphagnum mosses - in methane fluxes of a boreal fen under natural and experimental water level drawdown conditions. We measured the fluxes during four growing seasons using static chamber technique in a field experiment where the role of the ecosystem components was assessed via plant removal treatments. The first year was a calibration year after which the water level drawdown and vegetation removal treatments were applied. Under natural water level conditions, plant-mediated fluxes comprised 68-78% of the mean growing season flux (1.95 ± 0.21 g CH4 m-2 month-1 from June to September), of which Sphagnum mosses and sedges accounted for 1/4 and 3/4, respectively. The presence of dwarf shrubs, on the other hand, had a slightly attenuating effect on the fluxes. In water level drawdown conditions, the mean flux was close to zero (0.03 ± 0.03 g CH4 m-2 month-1) and the presence/absence of the plant groups had a negligible effect. In conclusion, water level acted as a switch; only in high water level conditions vegetation regulated the net fluxes. The results are relevant for assessing the response of peatland fluxes in changing climatic conditions, as water level drawdown and the consequent vegetation succession are the major projected impacts of climate change on northern peatlands. [ABSTRACT FROM AUTHOR]

Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub‐arctic sedge fen carbon dioxide (CO2) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO2 exchange is more strongly affected by drying than warming. Experimental water level draw‐down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO2 exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO2 uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO2 exchange. [ABSTRACT FROM AUTHOR]

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Growing public interest in conserving peatlands has created a need for restoration and rapid indicators of progress in peat formation. Vegetation and hydrological indicators are commonly assessed, but changes in mineralization and decomposition rates might better indicate when peat formation is underway in restored peatlands. In Finland, we investigated differences in mineralization and decomposition in the upper peat layer of five undrained and eight drained Pinus-dominated fens from 2006 to 2009. Forestry-drained fens were restored in 2007 by harvesting either whole trees or only stems, and by damming and filling ditches. Before restoration, net N mineralization rate was slightly higher in the drained than in undrained fens, whereas soil pH and Betula leaf litter decomposition rate were lower. After restoration, net N mineralization rate was similar for the undrained and restored fens, except near ditches after stem harvest. Also, soil pH and decomposition rate of Betula leaf litter became similar for undrained and restored fens. We conclude that whole tree harvest is a more suitable method for peatland restoration than stem harvest and that mineralization and decomposition rates are suitable indicators for peat formation after restoration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We compared the amount and composition of different aboveground biomass (BM) fractions of four mires with their net ecosystem CO2 exchange (NEE) measured by eddy covariance. We found clear differences in response of green biomass (GBM) of plant functional types (PFTs) to water table (WT), which resulted in larger spatial variation in GBM within a mire than variation between mires. GBM varied between mires from 126 ± 7 to 336 ± 16 g·m-2 (mean ± SE), while within mire variation at largest was from 157 ± 17 to 488 ± 20 g·m-2 (mean ± SE). GBM of dominant PFTs appeared to be better in explaining the peak growing season NEE than the total BM or GBM of a mire. The differences in photosynthetic capacity between PTFs had a major role, and thus a smaller GBM with different species composition could result in higher NEE than larger GBM. Vascular plant GBM, especially that of sedges, appeared to have a high impact on NEE. Eleven PFTs, defined here, appeared to capture well the internal variation within mires, and the differences in GBM between communities were explained by the water table response of PFTs. Our results suggest the use of photosynthesizing BM, separated into PFTs, in modelling ecosystem carbon exchange instead of using just total BM. [ABSTRACT FROM AUTHOR]

Questions: What impact do a wet and a dry growing season have on CO2 dynamics of mire plant communities along a primary succession gradient from the initiation stage to the bog stage? Location: Mires on a land uplift coast, Finland. Methods: We measured CO2 dynamics and vascular plant green area development on five mires that form a sequence of mire succession. TWINSPAN was used to define successional mire plant communities and regression analyses were used to explore the temporal variation in CO2 dynamics of the communities. Results: CO2 dynamics of successional plant communities reacted differently to a wet and a dry growing season. The net CO2 uptake rate of the earlier successional communities decreased in the dry growing season due to a decrease in photosynthesizing leaf area. Concurrently, CO2 uptake of the later successional communities moderately increased or did not change. Generally, the difference in net ecosystem exchange (NEE) between a dry and a wet year resulted from the altered rate of gross photosynthesis (PG) rather than ecosystem respiration (RE). Conclusions: Critical factors for the more stable carbon (C) gas dynamics in the later stages of mire succession were (1) higher autogenic control of the physical environment and (2) an increase in the number of factors regulating the PG rate. These factors may buffer mire ecosystems (in terms of the C sink function) from extreme and unfavourable variations in environmental conditions. [ABSTRACT FROM AUTHOR]