Sphagnum distribution patterns along environmental gradients in Bulgaria

Abstract

The distribution patterns of 18 Sphagnum species along base-richness and altitudinal gradients were studied in Bulgarian treeless wetlands which are noteworthy because of the edge-of-range occurrence of many mire species including Sphagnum. Of 483 spring and mire sites studied, 202 samples contained some Sphagnum species. The most common species were S. subsecundum (n=85), S. platyphyllum (46), S. contortum (41), S. teres (40) and S. capillifolium (26). The significance of Sphagnum responses to environmental gradients was tested by comparing generalized additive models against the null model. Many Sphagnum species displayed a significant response to the altitudinal gradient. Several species were clearly linked to low or to high altitudes, but the realized niche of other species was wide with respect to altitude. Most species significantly responded to water pH, both above and below the timberline. The same result was obtained for water conductivity below the timberline, whereas only a few species had a significant response to conductivity above the timberline. The highest water conductivity under which Sphagnum species occurred was 280 μS cm^?1. Sphagnum contortum was the species occupying the mires with the highest mineral content. On the contrary, Sphagnum warnstorfii, one of the most calcitolerant species in many regions of Europe, often occurred in extremely mineral-poor mires above the timberline. Some other Sphagnum species growing in mineral-rich mires below the timberline, also inhabited extremely mineral-poor mires above the timberline. This could be explained by adaptation to local conditions during long-term isolation on mineral-poor bedrock or by changed competition pressure.     

Dwarf-shrub-<Emphasis Type="Italic">Sphagnum</Emphasis> mires in taiga zone: Comparison of phytomass structure and productivity

The study compared phytomass structure (percentages of phytomass fractions) and annual primary production of mesotrophic dwarf-shrub-Sphagnum mires—the most widespread mires in Karelia, West Siberia, and Priamur’e. The analysis showed similarities and differences in phytomass structure among the regions. Fire-disturbed and undisturbed mires of Priamur’e were compared. The study revealed high productivity of underground phytomass of vascular plants and its close relation to water regime of mesotrophic mires in general and those of Priamur’e in particular.     

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.     

Rockin' in rhythm,growth dynamics of the peat moss Sphagnum

The genus Sphagnum is an essential component in the formation and maintenance of high latitude peatlands, bogs and mires. The species grows in dense, extended mats of agglomerated shoots that allow it to retain water necessary for its growth. These mats are partly responsible for maintaining the right conditions for other species in these wetlands to thrive. In this issue of Physiologia Plantarum, Mironov et al. (2020) monitored the growth of Sphagnum riparium for a period of 4 years and revealed three distinct growth rhythms: a seasonal temperature dependent, a circalunar and a third one, synchronized with the circalunar. This synchronised nature of Sphagnum growth could contribute to its position as a key species in the maintenance of peatlands.     

Vegetation-Environment Relationships in Peatlands Dominated by Sphagnum fallax in Western Poland

We investigated species composition and relative abundance of Sphagnum fallax dominated peatlands in relation to measured environmental variables on the basis of 26 sites in the Wielkopolska region. Most studied plots were characterized by soft waters, poor in Ca²⁺ but rich in nutrients, especially N-NH₄ ⁺ and P-PO₄ ^3-, with high electrolytic conductivity and high DOC (dissolved organic carbon) concentration. Six of the 19 measured variables of surface water chemistry (DOC, pH, SO₄ ^2-, P-PO₄ ^3-, Na⁺ and Ca²⁺) significantly explained 23% of the variation in floristic composition. In 65 vegetation plots, 107 species were observed. Cluster analysis revealed four types of vegetation in the studied mires. Sphagnum fallax was the most abundant species and formed plant communities in a wide range of habitats: in floating mats, with the plants usually adjoining the mineral basin edge (e.g. E. vaginatum, Andromeda polifolia and Ledum palustre) as well as it occupied central parts of Sphagnum lawn (e.g. Eriophorum angustifolium) and rich fen habitats (e.g. Carex rostrata or Phragmites australis). In Wielkopolska terrestrializating peatlands, four variables determine the poor-rich gradient: conductivity, DOC, SiO₂ dissolved, Ca²⁺ and alkalinity. This study provides new data on the ecology and typology of Sphagnum peatlands in western Poland.     

Spring fen vegetation and water chemistry in the Western Carpathian flysch zone

In the western part of the Carpathian flysch zone, aquifers host several springwater chemistry types. Four vegetation types, distinguished along the poor-rich gradient (tufa-forming and peat forming brown moss fens, moderately rich and poorSphagnum fens), have been compared with respect to the main habitat factors. Water calcium and magnesium concentrations, pH and conductivity as well as the soil organic carbon content were the properties measured that showed the strongest correlation with the main vegetation gradient (the poor-rich gradient). Further, significant differences in iron, sodium, potassium, sulphate and phosphate concentrations were also found between pairs of related vegetation types. The range of calcium concentrations is wide (2–300 mg/l). The calcium concentration in tufa-forming springs is higher than values usually reported from northern and western Europe. Tufa formation is influenced not only by high calcium concentrations, but also by the total chemical composition of springwater and both climatic and topographic conditions. There is a great excess of cations over Cl⁻ and SO ₄ ²⁻ , balanced by HCO ₃ ⁻ and CO ₃ ²⁻ in springs with the most intense tufa precipitation. Unusually high calcium concentrations combined with high iron concentrations were found in peat-forming brown moss fens. RichSphagnum-fens with calcitolerantSphagnum species are distinctively low in phosphates. The Western Carpathian poor fens dominated bySphagnum flexuosum have water and soil calcium concentrations comparable to those reported from rich fens of some other areas. The springwater of these fens are rich in iron, phosphates and sulphates. The poorest spring fens withSphagnum fallax, S. magellanicum, S. papillosum andS. auriculatum are not only poor in calcium, but also in iron, sodium and potassium.     

Higher acidification rates in fens with phosphorus enrichment

Question: Why is bryophyte succession in eutrophicated fens faster than in natural fens? Location: Mineral‐rich fens in The Netherlands and NW Europe. Methods: Literature review on the ecology of four bryophyte species in various successional types as observed in Dutch fens. Results: Bryophyte succession in eutrophicated fens from the brown moss Calliergonella cuspidata to Sphagnum squarrosum is much faster than in natural fens with species shifts from Scorpidium scorpioides to Sphagnum subnitens. Under P‐poor conditions, the brown moss stage is stabilized as long as mineral‐rich water is supplied. This is because S. scorpioides is tolerant of rainwater, is a strong competitor and can counteract acidification to some extent while S. subnitens is intolerant to groundwater and has low growth rates and low acidification capacity. In contrast, the Sphagnum stage is stable after rapid succession from rich‐fen mosses under P‐rich conditions. Calliergonella cuspidata has suboptimal growth in rainwater, possibly due to ammonium toxicity, while the high growth rates of S. squarrosum in nutrient‐rich and highly acidic groundwater allow early establishment and rapid expansion. Conclusions: If measures to improve fen base status occur in environments of increased nutrient (P) availability, the management may not lead to the desired restoration of brown moss stages, but instead to rapid acidification by S. squarrosum.     

Diversity of wetland vegetation in the Bulgarian high mountains, main gradients and context-dependence of the pH role

We fill a gap in understanding wetland vegetation diversity and relationship with environmental determinants in Bulgarian high mountains. A total of 615 phytosociological samples were taken from springs, mires, wet meadows and tall-forb habitats throughout Bulgaria, of which 234 relevés are from mire and spring vegetation above timberline. The vegetation was classified by TWINSPAN and the resulting vegetation types were reproduced by the formal definitions using the combination of Cocktail species groups based on phi-coefficient of joint co-occurrence of the species. Nine vegetation types of springs and fens have been clearly delimited above the timberline. All vegetation types include Balkan endemic species, the representation of which varies. Fens generally harbour more Balkan endemics than do springs, with the exception of species-poor high-altitude Drepanocladetum exannulati. The gradient structure of the vegetation was revealed by DCA and by CCA with forward selection of environmental factors. The major determinants of vegetation variation strongly differ above and below the timberline and likewise between springs and fens. The base-richness gradient controls the floristic variation of Bulgarian submontane fens, whereas the complete data set including both submontane and subalpine fens is governed by the altitude gradient from lowland and basin fens to subalpine fens rich in Balkan endemics. When focusing on sites above the timberline only, the first DCA axis separates fens from springs without organic matter. The major species turnover in springs follows the variation in water pH and mineral content in water, whereas fen vegetation variation is primarily controlled by succession gradient of peat accumulation. Altitude remains an important factor in all cases. Weak correlation between water pH and conductivity was found. This correlation was even statistically insignificant in fens above the timberline. Water pH is not influenced by mineral richness in Bulgarian high mountains, since it is buffered by decomposition of organic matter in fens. In springs, pH reaches maximum values due to strong aeration caused by water flow. The plant species richness decreases significantly with increasing altitude. The increase of species richness towards circumneutral pH, often found in mires, was not confirmed in Bulgarian high mountains. The correlation between species richness and pH was significant only when arctic-alpine species and allied European high-mountain species were considered separately. The richness of boreal species was independent on pH. Some of them had their optima shifted to more acidic fens as compared to regions below the timberline. Our results suggest that subalpine spring and fen vegetation should be analysed separately with respect to vegetation-environment correlations. Separate analysis of fens below and above timberline is quite appropriate.     

Testate Amoeba (Arcellinida,Euglyphida) Ecology along a Poor‐Rich Gradient in Fens of Western Poland

We investigated the relationships between testate amoebae (Arcellinida, Euglyphida), vegetation and water chemistry along environmental gradients in minerotrophic peatlands (fens) in western Poland. We hypothesized that: a) hydrochemistry significantly influences structure of testate amoeba communities, and b) testate amoeba communities are more closely correlated with the hydrochemical variables (environment) than with the vegetation data. Testate amoeba communities and vegetation from 71 sample plots were investigated together with the hydro‐chemistry and hydrology based on 16 environmental variables and vegetation composition. Testate amoeba communities revealed a distinctive poor‐rich gradient in analysed fens. Mineral‐rich habitats, which were dominated by brown mosses, were preferred by a higher number of taxa than acidic habitats, which were dominated by Sphagnum. We recorded a total of 107 testate amoebae taxa. The average species richness of testate amoebae for brown mosses was higher (20) than for Sphagnum (13). We found that testate amoebae communities were similarly correlated with vascular plants, mosses and environmental parameters. Results of direct ordination demonstrate that hydrology, pH, Mg²⁺ and sodium remain the most important environmental control for the entire data set. CCA showed that in case of brown mosses hydrology, sodium and oxygen affect testate amoeba communities significantly whereas in Sphagnum only sodium emerge as most significant determining testate amoeba assemblages. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diversity and pH changes after 50 years on the boreal mire Skattlösbergs Stormosse,Central Sweden

Abstract. The Skattlösbergs Stormosse mire was reinvestigated in 1995 after 50 years of natural development since the previous investigation. The undrained mire is situated in an area with low anthropogenic deposition. The distribution of 106 plant species was mapped in detail and pH was measured at 251 locations, providing a unique opportunity to quantify long‐term mire dynamics. The results show decreased pH in the richer (high‐pH) parts of the mire, but little or no change in the poor fen and ombrotrophic parts. 14 species had disappeared while two new species were recorded. Most species had a more restricted distribution in the mire area in 1995 than in 1945. Species richness in 10m x 10m plots had decreased, especially in plots with higher pH. Most Sphagnum species had unchanged distributions over the mire, while 73% of other bryophyte species and 38% of vascular plant species had decreased by more than 20% in plot frequency. There was a strong relationship between number of species and pH‐value. The mean and standard deviation of pH in plots where the species occurred have both decreased since 1945. We interpret the changes in species richness in the richer fens to be mainly caused by acidification. This could partly be an autogenic succession, but may be enhanced by increased atmospheric deposition. The mire represents an almost untouched site which can act as a reference for mires in more polluted areas.     

Sphagnum growth and ecophysiology during mire succession

Sphagnum mosses are widespread in areas where mires exist and constitute a globally important carbon sink. Their ecophysiology is known to be related to the water level, but very little is currently known about the successional trend in Sphagnum. We hypothesized that moss species follow the known vascular plant growth strategy along the successional gradient (i.e., decrease in production and maximal photosynthesis while succession proceeds). To address this hypothesis, we studied links between the growth and related ecophysiological processes of Sphagnum mosses from a time-since-initiation chronosequence of five wetlands. We quantified the rates of increase in biomass and length of different Sphagnum species in relation to their CO₂ assimilation rates, their photosynthetic light reaction efficiencies, and their physiological states, as measured by the chlorophyll fluorescence method. In agreement with our hypothesis, increase in biomass and CO₂ exchange rate of Sphagnum mosses decreased along the successional gradient, following the tactics of more intensely studied vascular plants. Mosses at the young and old ends of the chronosequence showed indications of downregulation, measured as a low ratio between variable and maximum fluorescence (F _v/F _m). Our study divided the species into three groups; pioneer species, hollow species, and ombrotrophic hummock formers. The pioneer species S. fimbriatum is a ruderal plant that occurred at the first sites along the chronosequence, which were characterized by low stress but high disturbance. Hollow species are competitive plants that occurred at sites with low stress and low disturbance (i.e., in the wet depressions in the middle and at the old end of the chronosequence). Ombrotrophic hummock species are stress-tolerant plants that occurred at sites with high stress and low disturbance (i.e., at the old end of the chronosequence). The three groups along the mire successional gradient appeared to be somewhat analogous to the three primary strategies suggested by Grime.     

Nitrogen concentration and δ15N signature of ombrotrophic Sphagnum mosses at different N deposition levels in Europe

Alteration of the global nitrogen (N) cycle because of human‐enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. Because Sphagnum‐dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, and δ¹⁵N isotopic signature of Sphagnum plants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m^?2 yr^?1. Mean N concentration in Sphagnum capitula was about 6 mg g^?1 in less polluted mires and about 13 mg g^?1 in highly N‐polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum. Sphagnum plants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation in Sphagnum tissues. The latter probably is related to a shift from N‐limited conditions to limitation by other nutrients. The capacity of the Sphagnum layer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO₃^?+NH₄⁺). Sphagnum plants had δ¹⁵N signatures ranging from about ?8‰ to about ?3‰. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating that δ¹⁵N signature of Sphagnum plants can be used as an integrated measure of δ¹⁵N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH₃ (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower δ¹⁵N signature compared with mires located in areas dominated by NO_x emissions (i.e., mainly affected by industrial activities).     

Mineralization of N and P along a trophic gradient in a freshwater mire

Release of inorganic nitrogen and phosphorus in the soil of a peatland (fen) in The Netherlands was measured by means of an in situ incubation technique. Three sampling stations were chosen along a gradient in the plant productivity and water chemistry of the fen. The station with the highest biomass production was located near the ditch that supplied the fen with water in amounts matching water losses through evaporation and downward percolation to the groundwater. Water chemistry at this station strongly resembled that of the ditch water. The two stations remote from the ditch had much lower plant biomass, and significantly lower pH, conductivity, and calcium and bicarbonate concentrations. The vegetation at these two stations was characterized by a thick Sphagnum carpet.The release of inorganic N and P was much faster at the two stations remote from the ditch than at that located near the ditch. The differences in mineralization rate are probably due to the differences in water chemistry; phosphates are more soluble at low than at high pH. The fast N mineralization at stations with a thick Sphagnum carpet may be related to the chemical composition of Sphagnum litter. The difference in productivity is not explained by the N and P mineralization rates. Direct supply of N and P from the ditch are probably the main cause of the high productivity at the station bordering the ditch.     

Production and microtopography of bog bryophytes: response to warming and water-table manipulations

Boreal peatlands, which contain a large fraction of the world's soil organic carbon pool, may be significantly affected by changes in climate and land use, with attendant feedback to climate through changes in albedo, fluxes of energy or trace gases, and soil carbon storage. The response of peatlands to changing environmental conditions will probably be dictated in part by scale-dependent topographic heterogeneity, which is known to interact with hydrology, vegetation, nutrients, and emissions of trace gases. Because the bryophyte community can contribute the majority of aboveground production in bogs, we investigated how microscale topography affects the response of bryophyte species production and cover to warming (using overhead infrared lamps) and manipulations of water-table height within experimental mesocosms. We removed 27 intact peat monoliths (2.1-m² surface area, 0.5-0.7 m depth) from a bog in northern Minnesota, USA, and subjected them to three warming and three water-table treatments in a fully crossed factorial design. Between 1994 and 1998, we determined annual production of the four dominant bryophyte taxa within three microtopographic zones (low, medium, and high relative to the water table). We also estimated species cover and calculated changes in topography and roughness of the bryophyte surface through time. Total production of all bryophytes, and production of the individual taxa Polytrichum strictum, Sphagnum magellanicum, and Sphagnum Section Acutifolia, were about 100% greater in low microtopographic zones than in high zones, and about 50% greater in low than in medium zones. Production of bryophytes increased along the gradient of increasing water-table heights, but in most years, total production of bryophytes was negatively correlated with height above the set water table only for the wettest water-table treatment. Although bryophyte production was unaffected by the warming treatments, the bryophyte surface flattened in proportion to the degree of warming. These results indicate that production of bryophytes is driven most strongly by the absolute and relative height of the bryophyte surface above the water table. Predicted changes in water-table height commensurate with changes in surface temperature may thus affect both production and superficial topography of bryophyte communities.     

A chronosequence approach for detecting revegetation patterns after Sphagnum-peat mining, northern Japan

The aim of this study is to detect how vegetation development proceeds after Sphagnum-peat mining and how physical and chemical factors in groundwater are related to the revegetation patterns in Sarobetsu mire, Hokkaido, Japan. A total of 189 plots on peat-mining sites were set in a chronosequence and 18 plots were set on unmined control sites. A vegetation survey was conducted, and seasonal changes in groundwater levels and chemistry (pH, electrical conductance, total nitrogen, total phosphorus, anions, and cations) were monitored. Species richness and plot cover tended to increase with increasing age, but were significantly lower in mined sites than in unmined sites dominated by Sphagnum spp. The trends in vegetation change were (1) bare ground, (2) grasslands dominated by grasses and sedges, e.g., Rhyncohospora alba, Phragmites communis, and Moliniopsis japonica and (3) Sphagnum-dominated vegetation. The characteristics of groundwater level during the plant-growth period mostly determined vegetation recovery, i.e., Sphagnum establishment was promoted when groundwater declined greatly in early summer. The patterns of temporal vegetation changes affected by groundwater characteristics were detected by chronological sequence, and hydrological factors in groundwater were more important for revegetation than chemical factors. The original vegetation has not returned after three decades.     

Bryophyte communities in mesotrophic fens in the Netherlands

Seven bryophyte and three vascular plant community types were described from eight small fens in the Vechtplassen area (prov. of Utrecht, the Netherlands). Clear relationships between the species composition of the bryophyte and vascular plant layer were found. The bryophyte species composition shows a gradual change from quaking fens dominated by Calliergon cordifolium to fens dominated by several species of Sphagnum . Fens dominated by C. cordifolium have relatively high pH and conductivity values and are strongly quaking, whereas fens dominated by Sphagnum have relatively low pH and conductivity values and the peat layer is continuous down to the bottom sediment. A comparison of the present-day vegetation with an earlier successional scheme for this vegetation type (Segal 1966) indicates a raise in nutrient levels and a decrease in the amount of seepage water welling up at the sites, both due to human activities. We suggest that in early successional phases bryophytes react faster to these changes, whereas in later phases vascular plants react faster.     

Release of inorganic N,P and K in peat soils; effect of temperature,water chemistry and water level

In the Netherlands, fens that are fed by polluted river water are often eutrophic, whereas fens fed by calcium-rich groundwater often are mesotrophic. Differences in trophic status can not always be attributed to differences in the nutrient load of the water. In this paper we try to determine if the inflow of river water in fens, in fact, accelerates the soil nutrient release, thereby creating more eutrophic conditions (‘internal eutrophication’). For this purpose, we compared nutrient release rates (N, P and K) in soil cores fromSphagnum peat andCarex peat saturated with different media, that were artificially created to mimic the three basic water sources: polluted river water, unpolluted calcium-rich groundwater and rainwater. In addition, we studied the effect of temperature and water level on nutrient release rates. The experiments proved thatSphagnum peat released much more P and ammonium thanCarex peat. The strong site effect proved consistent throughout the water chemistry treatments, which indicates that soil quality may be the most important agent determining nutrient release rates. Nevertheless, it was established that water chemistry and water level are of significant influence on nutrient release rates in peat soils. In particular, river water stimulated P release by the peat, most notably in theSphagnum peat. P-release in both soils was only minor when the soils were incubated in clean Ca-rich groundwater. It is suggested that P release is strongly associated with soil chemical processes, and that high P release rates after incubation in river water are due to the high sulphate content of the water. The net release from the soil of ammonium, potassium and phosphate increased with increasing temperature. A freezing treatment significantly increased nutrient availability. The results of the experiments are examined in the context of hydrologic management strategies for the conservation of fens in agricultural landscapes.     

The effect of increased temperature and nitrogen deposition on decomposition in bogs

Despite their low primary production, ombrotrophic peatlands have a considerable potential to store atmospheric carbon as a result of their extremely low litter decomposition rates. Projected changes in temperature and nitrogen (N) deposition may increase decomposition rates by their positive effects on microbial activity and litter quality, which can be expected to result in enhanced mass loss and N release from Sphagnum and vascular plant litter. This is the first study that examines the combined effects of increased temperature and N deposition on decomposition in bogs. We investigated mass loss and N release at four bog sites along a gradient from north Sweden to northeast Germany in which both temperature and N deposition increased from north to south. We performed two litterbag experiments: one reciprocal experiment with Eriophorum vaginatum litter and one experiment using recalcitrant (Sphagnum fuscum) and more degradable (Sphagnum balticum) Sphagnum litter collected from the most northern site. We measured mass loss and N release during two (Sphagnum) and three (E. vaginatum) years. The N concentration and decomposability of the E. vaginatum litter did not differ between the sites. Mass loss from E. vaginatum litter increased over the gradient from north to south, but there was no such effect on Sphagnum litter. N loss of all litter types was affected by collection site, incubation site and time and all interactions between these factors. N release in Sphagnum was positively related to N concentration. We conclude that decomposition of vascular plants and Sphagnum litter is influenced by different environmental drivers, with enhanced temperatures stimulating mass loss of vascular plant litter, but not of Sphagnum. Enhanced N deposition increases Sphagnum litter N loss. As long‐term consequences of climate change will presumably entail a higher vascular plant production, overall litter decomposition rates are likely to increase, especially in combination with increased temperature.     

Potassium limits potential growth of bog vegetation under elevated atmospheric CO2 and N deposition

The free air carbon dioxide enrichment (FACE) and N deposition experiments on four ombrotrophic bogs in Finland, Sweden, the Netherlands and Switzerland, revealed that after three years of treatment: (1) elevated atmospheric CO₂ concentration had no significant effect on the biomass growth of Sphagnum and vascular species; and (2) increased N deposition reduced Sphagnum growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum, while vascular plant biomass growth was not affected. This paper focuses on water chemistry, plant nutrient content, and litter decomposition rates. Potassium limitation, or low supply of K and P, may have prevented a significant increase of Sphagnum growth under elevated CO₂ and N deposition. Vascular plant growth under elevated CO₂ and N deposition was also limited by K, or by K in combination with P or N (N in CO₂ experiment). Elevated CO₂ and N deposition had no effect on decomposition rates of Sphagnum and vascular plant litter. Aside from a possible effect of N deposition on light competition between species, we expect that elevated atmospheric CO₂ and N deposition concentrations will not affect Sphagnum and vascular plant growth in bogs of north‐west Europe due to K‐, or K in combination with N‐ or P‐, limited growth. For the same reason we expect no effect of elevated CO₂ and N deposition on litter decomposition. Net primary production of raised ombrotrophic bogs that are at or close to steady state, is regulated by input of nutrients through atmospheric deposition. Therefore, we hypothesize that the expected increase of plant growth under elevated CO₂ and N deposition is diminished by current levels of K (and to some extent P and N) in atmospheric deposition.     

Responses of Sphagnum species to atmospheric nitrogen and sulphur deposition

The responses of Sphagnum species to atmospheric pollution by sulphur and nitrogen oxides are reviewed. An explanation of the tolerance of sulphur dioxide in Sphagnum cuspidatum is provided. This involves an exclusion mechanism by the oxidation of bisulphite ions to less toxic sulphate ions catalysed by the presence of iron and possibly other metals on the cell wall cation-exchange sites. Tolerance is influenced by changing the iron content of the plants. A similar mechanism may account for sulphur dioxide tolerance in Sphagnum recurvum.
Evidence for a close coupling of nitrate assimilation with the atmospheric nitrogen supply in ombrotrophic Sphagnum species in unpolluted regions is reviewed, and this coupling is shown to be lost in polluted environments with high atmospheric nitrogen supply. The potential effects of a supra-optimal atmospheric nitrogen supply to Sphagnum species on the ecology of mires is discussed.
The effects of nitrogen and sulphur pollutants on Sphagnum species are used to make broad generalizations as to how other bryophyte species might respond to these pollutants and to explain known effects of atmospheric pollution on bryophyte communities.