Species diversity,niche metrics and species associations in harvested and undisturbed bogs

Abstract. Competition is considered an important force in structuring plant communities and in governing niche relations, but communities recovering from disturbance, may be less governed by species interactions and less orderly organized. To address this issue, we studied species richness, abundance and patterns of association between plant species at three spatial scales (1 m², 1/25 m², 1/625 m²) in two ombrotrophic mires in east-central Sweden. One was at a secondary successional stage following peat extraction 50 yr ago and the other was undisturbed. Peat extraction leads to a change in hydrology which is slowly restored by the formation of new peat. Niche breadth and niche overlap along the gradient of height above the water table were calculated for the five common Sphagnum species occurring at both mires in an attempt to better understand differences in species co-occurrence at each mire. Species cover differed between the mires, and the number of species per plot was higher in the undisturbed community at all scales, suggesting that the degree of species intermingling was greater than at the harvested site. At all scales, the number of non-random associations was higher, and niche overlap lower among ecologically similar species (e.g. hollow Sphagnum species) in the undisturbed mire. These differences indicate that random events are important in colonization, and that biotic interactions between neighbours later result in a higher degree of non-randomness. In addition, we surveyed a number of abandoned peat pit sites to test the effect of disturbance for species composition at a regional scale. Ombrotrophic peat pits contained several Sphagnum species normally associated with minerotrophic mires, and species of wooded mires occurred frequently in peat pits, making them more species-rich than undisturbed bogs. There were also Sphagnum species new to, or rare in, this part of Sweden which indicates effective long-distance dispersal. Even 50 yr after peat extraction had ceased, the vegetation had not recovered to its original composition.     

Recognition of peat-forming plant communities from their peat deposits in two south Swedish bog complexes

Peat samples, 3 015 from 103 boring points, on two mires (Åkhult mire, Store Mosse mire) south Sweden, have been subjected to macrofossil analysis. Based on plant remains, 9 peat groups were distinguished in the field. A further classification using phytosociological methods revealed 29 peat types. The affinities between the peat types were determined from TABORD classification and a Reciprocal Averaging ordination. The primary floristic differentiation is correlated with a gradient from treeless-to wooded stands, which coincides largely with the mire expanse-mire margin gradient. The poor-rich gradient seems to parallel the treeless-wooded gradient as well and may reflect the natural conditions in this mire before it was affected by man. The hummock-mud-bottom gradient is easy to distinguish in peat, formed by bog communities, but is not distinct in peat formed by fen communities and impossible to detect in peat dominated by wood remains. The amount of identifiable remains depends on the decomposition, which is determined by (1) the period of time the plant litter stays in the acrotelm, and (2) the nutrient status. The decomposition is greatest in fen-peat with abundant wood remains. This probably depends on a good supply of oxygen caused by greater horizontal water movements and better nutrient status.     

Ecological gradients in some Sphagnum mires in the south-eastern Alps (Italy)

Abstract. Vegetation was sampled at 170 plots in five mires in the Italian Alps to characterize the distribution of plant species along the main environmental gradients. Water-table depth, water pH, electrical conductivity, Ca-, Mg-, K- and Na-concen-trations in soil pore water, shade intensity, and distance from the mire margin were determined at each plot. At mire margins the water table was deeper than in the mire expanse, and K-, Ca- and Mg-concentrations, and conductivity were higher. The first and by far most important axis of a DCCA-ordination was interpreted as a gradient of water-table depth, in relation to the distance from the mire margin, whereas the second axis was mainly associated with water pH.     

Environmental factors explaining the vegetation patterns in a temperate peatland

Although ombrotrophic temperate peatlands are important ecosystems for maintaining biodiversity in eastern North America, the environmental factors influencing their flora are only partly understood. The relationships between plant species distribution and environmental factors were thus studied within the oldest temperate peatland of Québec. Plant assemblages were identified by cluster analysis while CCA was used to related vegetation gradients to environmental factors. Five assemblages were identified; three typical of open bog and two characterized by more minerotrophic vegetation. Thicker peat deposit was encounter underlying the bog assemblages while higher water table level and percentage of free surface water distinguished the minerotrophic assemblages. Overall, the floristic patterns observed were spatially structured along the margins and the expanse. The most important environmental factors explaining this spatial gradient were the percentage of free surface water and the highest water-table level. To cite this article: S. Pellerin et al., C. R. Biologies 332 (2009).     

Ion flux from precipitation to peat soil in spruce forest–<Emphasis Type="Italic">Sphagnum</Emphasis> bog communities in the Ochiishi district,eastern Hokkaido,Japan

To evaluate the contribution of proton flux from precipitation on peat acidification in mire ecosystems, we estimated ion fluxes to peat soils from bulk deposition in Sphagnum-dominated bogs and from throughfall plus stem flow in spruce forests in three cool-temperate ombrogenous mires in the Ochiishi district, northeastern Japan. We tested the hypothesis that proton fluxes from the atmosphere to peat soils are affected by vegetation types, leading to the consequent difference in soil acidity. The proton flux in bulk deposition was higher than that in throughfall plus stem flow, but the concentration of H⁺ in the peat surface water in Sphagnum bogs was lower than that in spruce forests. The inverse relationship between proton flux and soil water acidity means that the soil water acidity could not be explained quantitatively by proton flux from the atmosphere to the peat surface. The ion fluxes of sea-salt components were dependent on the distance from the coast to the mires. This means that the sea-salt accumulation in the peat surface soil can be directly attributed to the high flux of sea-salt from precipitation. The flux of sea-salts deposited on the mires positively correlated with the H⁺ concentration of the peat surface water in each community, implying that the acidity of peat surface water depends on the cation fluxes from precipitation.     

Ecological processes and trophic status of two low-alpine patterned mires,south-central South Island,New Zealand

Abstract Vegetation and environmental patterns, and associated ecological processes, were quantified from 42 sites on several transects in each of two extensive (5 and 220 ha) low-alpine patterned mires in the same region of south-central New Zealand. Plant communities, as derived from multivariate analyses, were correlated with 15 physical and chemical environmental factors. Various measures of water availability and chemistry were consistently the most significant factors in relation to vegetation patterns in both mires. In the smaller mire, plant cover adjacent to pools, which were partly or completely drained through underground tunnels, dominated the overall correlations. The nutrient status of surface water had a consistent negative relationship with water availability. No consistent spatial or temporal patterns were found in the concentrations of Ca, Mg, K or Na, nor pH or conductivity in pool water. Evaporative enrichment of cations on the surface of both mires was noted, with levels consistently higher in surface than in adjacent pool water. The somewhat higher nutrient status in the smaller mire may be a result of the size and/or the amounts of run-off from the surrounding slopes onto the mire surface or through the underground pipe system. Hydrogen (D) and oxygen (¹⁸O) isotopic compositions in water from pools, the mire surface and below ground from the smaller mire, suggested that there was negligible mixing of evaporation-enriched surface water with groundwater. Differences in overall nutrient levels in the two mires were relatively small and indicative of mesotrophic or marginally ombrotrophic status for these mires. Although of international significance, the wetland complex currently has inadequate formal protection. Possible options are assessed.     

The effect of light conditions on herbs, bryophytes and seedlings of temperate mixed forests in ?rség, Western Hungary

A vegetation survey was carried out in a relatively intact Atlantic blanket bog in Southwest Ireland to study the vegetation patterns in relation to environmental variation, and to quantify the effect of artificial and natural borders on compositional variation. The data were analysed using canonical correspondence analysis. In terms of both vegetation and water chemistry, the study site can be categorized as typical of Atlantic blanket bogs in the maritime regions of North-western Europe. The distribution of plant species was explained mainly by depth of the water table. The distribution of bryophytes was secondarily explained by the pH of the bog water, while the distribution of vascular plants was secondarily explained by concentrations of ammonia. The vegetation distribution exhibited little variation between the central sector of the peatland and its disturbed edges (hill-grazing and restoration areas), but a substantial variation was observed between the area along a natural edge (stream) and the areas close to the other peatland borders or centre. Similarly, the internal variation within each sector (centre, hill-grazing edge and restoration area edge) was small, but substantial vegetation variation was observed within the area located along the stream. The area along the stream was associated with relatively deep water table, shallow peat depth, high water colour, pH and NH₄ ⁺ concentrations, and low Cl⁻ concentrations in the bog water. Our results suggest the existence of strong centre-natural margin gradients, as in raised bogs, and indicate that human or animal disturbance do not give rise to the marked transition zones that often characterize natural margins of mire systems. This indicates that even small areas and remnants of Atlantic blanket bogs are worthy of conservation and that their conservation value would benefit from the inclusion of sectors close to the natural peatland borders, which would increase the plant biodiversity of the conserved area.     

Methane flux dynamics during mire succession

Vegetation, temperature and hydrology are major factors controlling wetland methane (CH₄) dynamics. In order to test their importance, we measured CH₄ emissions and environmental characteristics over 2 years from five mires representing a successional sequence, ranging in age from 178 to 2,520 years. We hypothesized CH₄ emissions to be higher from the sedge-dominated fens than from the older bog stage. The more constant hydrological conditions at later successional stages as a consequence of the thicker peat layer appeared to result in lower temporal variation in CH₄ emissions. Accordingly, the other controls, temperature and vegetation, had an effect on CH₄ emissions only when the water table was sufficiently high. The seasonal variation in CH₄ emissions was controlled by temperature only at the oldest study site, which had the lowest variation in water table. Within-season variation in emissions related to plant phenology was highest at the fen stage, which was dominated by aerenchymatous plants with a strong seasonal pattern, namely sedges and forbs. In contrast to our hypothesis, CH₄ emissions increased with mire age towards the bog stage. However, the trend did not emerge during a rainy growing season, due to a rise in CH₄ emissions at the younger stages. The results may imply two different mechanisms during mire succession: while old mires are able to avoid the perturbation associated with variation in the water table and maintain their function as CH₄ emitters, young mires are exposed to perturbation but are able to recover their function.     

Actinobacteria community structure in the peat profile of boreal bogs follows a variation in the microtopographical gradient similar to vegetation

Background and aims

Boreal mires are globally important carbon stores. In nutrient poor mires, i.e. bogs, vegetation is highly structured between hydrologically different microforms: hummocks, lawns and hollows. We studied whether the vegetation structure in four boreal bogs is reflected in the actinobacterial decomposer communities.

Methods

Vegetation was determined by projection cover of plant species. Actinobacteria were detected from peat profiles at depths of 0–20, 20–40 and 40–60 cm by molecular methods. The data was analysed by a hierarchical set of ordinations.

Results

The inter-bog variation in plant communities was insignificant, while the intra-bog differences (microforms) at each bog were highly significant. This variation was reflected in actinobacteria communities in the two upper peat layers. The deepest peat layer (40–60 cm) showed some inter-bog differences, possibly due to the different history of the bogs.

Conclusions

Our results show that the actinobacteria communities reflect the variation in bog vegetation, which in turn is tied to hydrological conditions. The possible alterations in the water level caused by changing climate are likely to cause predictable changes in vegetation and microbial communities in bog ecosystems.     

Present-day vegetation and the Holocene and recent development of Egelsee-Moor,Salzburg province,Austria

This paper describes the present-day vegetation, stratigraphy and developmental history of the mire of Egelsee-Moor (Salzburg, Austria; 45°45′N, 13°8.5′E, 700 m a.s.l., 15 ha in area) since the early Late Glacial on the basis of 4 transects with 14 trial borings across the peatland. We present a vegetation map of the mire, a longitudinal section through the peat body based on six cores showing the peat types, overview macrofossil diagrams of six cores showing the local mire development and two pollen diagrams covering the Late Glacial and Holocene. The chronology of the diagrams depends on biostratigraphic dating for the Late Glacial and early Holocene and radiocarbon dating for the remaining Holocene. The northern part of the mire originated through terrestrialisation of nutrient-rich, mostly inundated fen and the southern part through paludification of wet soils. The very small lake of today was a reservoir until recently for providing water-power for timber rafting (‘Holztrift’). The mire vegetation today is a complex of forested parts (mainly planted Pinus sylvestris and Thuja occidentalis, but also spontaneous Picea abies, Betula pubescens and Frangula alnus), reed-lands (Phragmites) and litter meadows (Molinietum, Schoenetum, etc.). The central part has hummock-hollow complexes with regionally rare species of transitional mires (Drosera anglica, D. intermedia, Lycopodiella inundata, Scorpidium scorpioides, Sphagnum platyphyllum, S. subnitens). The results indicate that some of the mid-Holocene sediments may have been removed by the timber-rafting practices, and that water extraction from the hydrological catchment since 1967 has resulted in a partial shift of transitional mire to ombrotrophic bog. The latter potentially endangers the regionally rare species and was used as an argument to stop further water extraction.     

Are nutrient availability and acidity‐alkalinity gradients related in Sphagnum‐dominated peatlands?

Abstract. Gradients in acidity‐alkalinity and nutrient availability were studied in 2 Sphagnum‐dominated peatlands on the southeastern Italian Alps. Decreasing concentrations of most mineral elements (Ca²⁺, Mg²⁺, Mn²⁺, Al³⁺ and Si⁴⁺) in pore water indicated a progressively lower influx of mineral‐soil water from the slightly minerotrophic conditions in the peatland margins to ombrogenous conditions in the central part of the peatlands. This was paralleled by decreasing concentrations of ash, bulk density, Ca, Fe and, partly, Mn in the peat. The nutrient gradient, as defined by pore water concentrations of N and P, was largely independent of the acidity‐ alkalinity gradient: NO^3‐ and PO₄^3‐ had similar concentrations throughout the gradient, whereas NH₄⁺ concentrations increased with increasing pore‐water pH. In contrast, the peat nutrient gradient coincided with the acidity‐alkalinity gradient, with total concentrations of N and P decreasing from the margin to the centre. Bryophytes and vascular plants had different responses along the acidity‐alkalinity gradient and the nutrient gradient. Bryophyte distribution reflected the acidity‐alkalinity gradient both in pore water and in peat. Vascular plant distribution was mainly influenced by variations in nutrient availability.     

Quantifying patterns and controls of mire vegetation succession in a southern boreal bog in Finland using partial ordinations

Question: How do we distinguish between concurrent allogenic and autogenic forcings behind changing patterns in plant community structures during mire development? Location: Lakkasuo raised bog, southern Finland. Methods: Two radiometrically dated peat profiles were studied using high resolution plant macrofossil analysis. A combination of partial direct and indirect gradient analyses (CCA and DCA) was applied to quantify the role of different drivers of vegetation changes. Results: Autogenic hydroseral succession explained 16% of the compositional variation in the vegetation. Disturbance successions initiated by fire explained 15% of the variation in the hummock, but only 9% in the wetter lawn. The early post‐disturbance successional stages were characterized by Eriophorum vaginatum. After partialling out the effects of peat depth and time since fire, a moisture gradient explained 29% of variation in the hummock core and 26% in the lawn. The analyses also indicated alternation between species with a similar niche. This interaction gradient explained 26% and 31% of the compositional variation in the hummock and lawn, respectively. The similar order of species replacement from both cores supported the existence of general directional succession in mire vegetation, both during the mire development and after fire events. The autogenic succession was slow and gradual while the disturbance successions were episodic and fast. Conclusion: Our results support the paradigm of the complex nature of mire vegetation dynamics where several interlinked agents have simultaneous effects. The approach of combining partial ordinations developed here appeared to be a useful tool to assess the role of different environmental factors in controlling the vegetation succession.     

Effects of nutrient loadings from catchments on Asajino mire, a small coastal ombrotrophic mire in northernmost Japan

The relationship between water chemistry and vegetation was studied in a coastal ombrotrophic mire in northern Hokkaido, Japan. The distributions of Sphagnum and Phragmites communities were separated clearly by the pH and ion concentration of the peat surface-pore water. The drainage ditches along the road across the center of the mire had a high pH and ion concentration, as did the peat water in the western part of the mire. It was found that fields used for livestock farming on a hill to the west of the mire leached materials into the mire through drainage ditches, surface runoff, and probably also through ground water, and thus influenced the water chemistry of the mire. Management of the water, including that in the catchment of the mire, should be introduced before biological buffering capacity against excess nutrient loading caused by human activity is exceeded and the mire loses its ombrotrophic status.     

长白山区苔草泥炭沼泽植物物种多样性

本研究以长白山区典型苔草沼泽为对象,分析了密丛型苔草(瘤囊苔草、乌拉草)沼泽和疏丛型苔草(毛苔草)沼泽的植物物种多样性.结果 表明:3种苔草沼泽植物群落共有83个物种,隶属于36科59属.其中,乌拉草沼泽有71个物种,瘤囊苔草沼泽有61个物种,毛苔草沼泽有26个物种.密丛型苔草沼泽植物物种数和物种丰富度明显高于疏丛型苔...     

Patterns and gradients of diversity in South Patagonian ombrotrophic peat bogs

Many north‐hemispherical mires seemingly untouched by drainage and cultivation are influenced by a diffuse sum of man‐made environmental changes, such as atmospherical nitrogen deposition that mask general patterns in species richness and functional group responses along resource gradients. To obtain insights into natural diversity‐environment relationships, we studied the vegetation and the peat chemistry of pristine bog ecosystems in southern Patagonia along a west–east transect across the Andes. The studied bog ecosystems covered a floristic gradient from hyperoceanic blanket bogs dominated by cushion building vascular plants via a transitional mixed type to Sphagnum‐dominated raised bogs east of the mountain range. To test the influence of resource availability on diversity patterns, species richness and functional groups were related to environmental variables by calculating general regression models and generalized additive models. Species richness showed strong linear correlations to peat chemical features and the general regression model resulted in three major environmental variables (water level, total nitrogen, NH₄Cl soluble calcium), altogether explaining 76% of variance. Functional group response illustrated a clear separation along environmental gradients. Mosses dominated at the low end of a nitrogen gradient, whereas cushion plants had their optimum at intermediate levels, and graminoids dominated at high nitrogen contents. Further shifts were related to NH₄Cl soluble calcium and water level. The models documented partly non‐linear relationships between functional group response and trophical peat properties. Within the three bog types, the calculated models differed remarkably illustrating the scale‐dependency of the explanatory factors. Our findings confirmed several general patterns of species richness and functional shifts along resource gradients in a surprisingly clear way and underpin the significance of undisturbed peatlands as reference systems for testing of ecological theory and for conservation and ecological restoration in landscapes with strong human impact.     

Effects of Increased Nitrogen Deposition on the Distribution of 15N-labeled Nitrogen between Sphagnum and Vascular Plants

To elucidate the sensitivity of bog ecosystems to high levels of nitrogen (N) deposition, we investigated the fate of ¹⁵N-labeled N deposition in bog vegetation in the Netherlands, both at ambient and increased N deposition. We doubled N deposition by adding 5 g N m^?2 y^?1 as dissolved NH₄NO₃ during three growing seasons to large peat monoliths (1.1 m diameter) with intact bog vegetation kept in large outdoor containers. A small amount of ¹⁵N tracer was applied at the start of the second growing season, and its distribution among Sphagnum, vascular plant species, and peat was determined at the end of the third growing season. The ¹⁵N tracer was also applied to additional plots at the untreated field site to check for initial distribution. One week after addition, 79% of the total amount of ¹⁵N retrieved was found in the living Sphagnum layer and less than 10% had been captured by vascular plants. Fifteen months later, 63% of the total amount of ¹⁵N retrieved was still present in the living Sphagnum layer at ambient N deposition. Increased N deposition significantly reduced the proportion of ¹⁵N in Sphagnum and increased the amount of ¹⁵N in vascular plants. Deep-rooting vascular plant species were significantly more ¹⁵N enriched, suggesting that at higher atmospheric inputs N penetrates deeper into the peat. Our results provide the first direct experimental evidence for that which has often been suggested: Increased atmospheric N deposition will lead to increased N availability for vascular plants in ombrotrophic mires.     

Bryophyte and vascular plant responses to base-richness and water level gradients in Western Carpathian<Emphasis Type="Italic">Sphagnum</Emphasis>-rich mires

We investigated the importance of water chemistry and water regime for vascular plant and bryophyte species distribution in Western Carpathian mires dominated bySphagnum. Seventy-seven small circle plots distributed across a wide geographical area, a wide range of mineral richness and all possible microtopographical features were sampled in terms of species composition, physical-chemical water properties and water regime during one growing season. Both water chemistry and water regime were found to be important factors for vegetation composition. Bryophytes reflected only one clear gradient, connected to base-richness (pH, conductivity) and maximal water-level, whereas three different environmental gradients determined the occurrence of vascular plants: water-level amplitude, base-richness and an indistinct gradient presumably connected to peat layer thickness. When the entire data set was subjected to DCA ordination, the first resulting axis was governed by the bryophyte subset, whereas the second one was governed by the vascular plant subset. The species density of vascular plants was positively correlated with pH and conductivity. On the contrary, bryophyte species density showed no relationship to environmental factors. We further compared the pH values measured in groundwater and in water squeezed from bryophytes from the same plot; these plots were distributed along the base-richness gradient. Only in the acidic mires did the use of squeezed-water chemistry in the analyses give results similar to the use of groundwater pH. Further, we found thatSphagnum species with a similar response to the base-richness gradient had differentiated niches with respect to the water level gradient and vice versa.Sphagnum contortum andS. warnstorfii exhibiting the same demands for groundwater pH were segregated along the gradient of maximum water level. An analogous pattern was detected for acidophilous speciesSphagnum magellanicum andS. papillosum.     

Mass balance and nitrogen accumulation in hummocks on a South Swedish bog during the late Holocene

Two peat cores from the Store Mosse mire in the central part of South Sweden have been analyzed for dry bulk density, carbon, and nitrogen They cover the development of the peat mound from the time of the conversion of the initial fen to an ombrotrophic bog at 5450 BP through three different bog stages, the Fuscum. the Rubellum-Fuscum and the Magellanicum bog stages, each one characterized by a specific macrofossil assemblage All N supplied to the bog surface is assumed to be contained in the organic matter At the beginning of the Magellanicum bog stage, 1000 BP, the nitrogen accumulation rate increased from an earlier value of ca 0.4 g m^–2 yr^–1 to 0.8 g m^–2 yr^–1 These accumulation rates for N have been used to establish time scales for the periods between the ¹⁴C-datings The estimated litter deposition rate in the hummocks is 120 g m^–2 yr^–1 in the two older bog stages and 270 g m^–2 yr^–1 in the Magellanicum bog stage The decay losses in the acrotelm increased, as a proportion of the addition, with time through each one of the bog stages The range of variation in the cores for the acrotelm decay losses was 25-80%. and the annual input of organic matter to the catotelm, 30-130 g m^–2 These ranges are greater than those found among recent bog hummocks in NW Europe and North America The decay losses during 5000 yr in the catotelm may not have exceeded 20% of the original input The over-all net rate of accumulation of C was highest, ca 40 g m^–2 yr^–1, at the beginning of the Fuscum bog stage The changes in the macrofossil assemblages are all associated with rapid increases in the peat accumulation rate, but decreases in accumulation rate are not At the conversion from fen to bog the increased input of organic matter to the catotelm depended on expansion of Sphagnum fuscum which formed a decay resistant litter Later increases depended on rapid rises of the mean water table, resulting in shorter residence times and smaller decay losses from the acrotelm The periods of decreases in input of organic matter to the catotelm depended on longer residence times in the acrotelm when the water table fell relative to the bog surface However, comparisons with recent conditions suggest that the variation in mean water level relative to the surface may not have exceeded 10-15 cm     

Secondary succession in a Swiss mire after a bog burst

Severe natural disturbances can lead to the recovery of the original vegetation or the shift to new vegetation types. While post-disturbance succession is well documented for regularly disturbed ecosystems, little is known about the pathways and rapidity of vegetation dynamics after rare events such as peat mass movements in bogs. We monitored the floristic changes in a mire subject to a bog burst in 1987 for two decades through the repeated sampling of permanent plots. The mean species number per plot increased continuously, while the evenness increased only in the first decade and then slightly decreased. Declining species were mostly mire species, while colonist species were mostly wet meadow species. Species turnover was higher in the first decade after the disturbance, and was also higher in the area of peat erosion than in the area of peat accumulation. Changes in plant species composition indicate a succession towards tall-forb vegetation (Filipendulion), acidic fen vegetation (Caricion fuscae) and swamp willow forest (Salicion). We conclude that the effects of the disturbance are still ongoing, and that the mire’s potential for recovery is therefore difficult to predict.     

Ecophysiological mechanisms characterising fen and bog species: focus on variations in nitrogen uptake traits under different soil–water pH

Although the productivity and nitrogen (N)-use traits of mire plants differ dramatically between fens and bogs, soil N richness does not necessarily differ, whereas the soil–water pH is distinctly lower in bogs than in fens. The ecophysiological mechanisms underlying these relations are unclear. To assess the relative availability of N forms in relation to soil–water pH, we focused on the net N uptake rate per unit root weight (NNUR), glutamine synthetase activity and nitrate reductase activity, and performed reciprocal transplant experiments with the seedlings of fen (Carex lyngbyei) and bog (C. middendorffii) sedge species in intact habitat sites. The soil–water pH was clearly lower at the bog site, but the NH₄ ⁺, NO₃ ⁻ or dissolved organic-N concentrations did not differ between the fen and bog sites. The activity of both enzymes for inorganic-N assimilation did not differ among the sites and species. However, the fen species grown at bog sites showed a drastic decrease in the NNUR, suggesting a suppression of organic-N uptake. The bog species showed no NNUR difference between the sites. These results indicate that inorganic-N availability does not differ between the two habitats, but organic-N availability is lowered in a low-pH bog, particularly in the case of fen species. Therefore, the relative availability of N forms shows species-specific variations that depend on the differences in the soil–water pH of root zone, even at similar N richness, which would play a key role in plant distribution strategies in relation to the fen-bog gradient.