Swift recovery of <Emphasis Type="Italic">Sphagnum</Emphasis> nutrient concentrations after excess supply

Although numerous studies have addressed the effects of increased N deposition on nutrient-poor environments such as raised bogs, few studies have dealt with to what extent, and on what time-scale, reductions in atmospheric N supply would lead to recovery of the ecosystems in question. Since a considerable part of the negative effects of elevated N deposition on raised bogs can be related to an imbalance in tissue nutrient concentrations of the dominant peat-former Sphagnum, changes in Sphagnum nutrient concentration after excess N supply may be used as an early indicator of ecosystem response. This study focuses on the N and P concentrations of Sphagnum magellanicum and Sphagnum fallax before, during and after a factorial fertilization experiment with N and P in two small peatlands subject to a background bulk deposition of 2 g N m(-2) year(-1). Three years of adding N (4.0 g N m(-2) year(-1)) increased the N concentration, and adding P (0.3 g P m(-2) year(-1)) increased the P concentration in Sphagnum relative to the control treatment at both sites. Fifteen months after the nutrient additions had ceased, N concentrations were similar to the control whereas P concentrations, although strongly reduced, were still slightly elevated. The changes in the N and P concentrations were accompanied by changes in the distribution of nutrients over the capitulum and the stem and were congruent with changes in translocation. Adding N reduced the stem P concentration, whereas adding P reduced the stem N concentration in favor of the capitulum. Sphagnum nutrient concentrations quickly respond to reductions in excess nutrient supply, indicating that a management policy aimed at reducing atmospheric nutrient input to bogs can yield results within a few years.     

Comparative desiccation tolerance of two Sphagnum mosses

Summary Sphagnum fallax (Klinggr.) Klinggr., a moss growing in hollows close to the water table, is more desiccation tolerant than S. nemoreum Scop., a hummock former distributed high above the hollows. Sphagnum fallax recovered to a greater proportion of its predesiccation photosynthetic rate after one and five days of tissue dryness. Further, a greater percentage of S. fallax plants survived five and ten day periods at low tissue water contents. Longer desiccated periods and lower water contents during these periods decreased both photosynthetic recovery and survival.Water contents measured in Bloomingdale Bog (Adirondack Mountains, NY, USA) showed that S. fallax probably dries more frequently and for longer periods than S. nemoreum. These results support previous findings that the greater ability of S. nemoreum to remain moist in the field is the most important character in its success as a hummock former. Greater tolerance of desiccation helps S. fallax to compensate for its greater tendency to become dry, and is a key physiological feature enabling it to dominate hollows.     

Effects of water chemistry,nutrient supply and interspecific interactions on the replacement of Sphagnum subnitens by S. fallax in fens

Abstract

In Dutch rich fens, mid-successional stages dominated by Sphagnum subnitens are rapidly being replaced by other vegetation, dominated by S. fallax.

In both species growth was strongly reduced in more mineral-rich ground water and simulated river water, compared to growth in rain water.

Both species were positively affected by added phosphorus supply, whereas added supply of nitrogen did not alter growth rates.

Nutrient supply seemed to compensate for the intolerance to high mineral levels in S. fallax: nutrient addition stimulated growth in length in ground water and simulated river water. Such compensation was not found in S. subnitens.

Growth in mixed culture positively affected Sphagnum subnitens and negatively affected its successor S. fallax.

The success of Sphagnum fallax in early stages of succession in more eutrophic fens may to some extent be explained by its tolerance to high mineral levels with increased nutrient supply.     

Sphagnum growth under N saturation: interactive effects of water level and P or K fertilization

• Sphagnum biomass is a promising material that could be used as a substitute for peat in growing media and can be sustainably produced by converting existing drainage‐based peatland agriculture into wet, climate‐friendly agriculture (paludiculture). Our study focuses on yield maximization of Sphagnum as a crop.
• We tested the effects of three water level regimes and of phosphorus or potassium fertilization on the growth of four Sphagnum species (S. papillosum, S. palustre, S. fimbriatum, S. fallax). To simulate field conditions in Central and Western Europe we carried out a glasshouse experiment under nitrogen‐saturated conditions.
• A constant high water table (remaining at 2 cm below capitulum during growth) led to highest productivity for all tested species. Water table fluctuations between 2 and 9 cm below capitulum during growth and a water level 2 cm below capitulum at the start but falling relatively during plant growth led to significantly lower productivity. Fertilization had no effect on Sphagnum growth under conditions with high atmospheric deposition such as in NW Germany (38 kg N, 0.3 kg P, 7.6 kg K·ha^?1·year^?1).
• Large‐scale maximization of Sphagnum yields requires precise water management, with water tables just below the capitula and rising with Sphagnum growth. The nutrient load in large areas of Central and Western Europe from atmospheric deposition and irrigation water is high but, with an optimal water supply, does not hamper Sphagnum growth, at least not of regional provenances of Sphagnum.

     

Field Simulation of Global Change: Transplanting Northern Bog Mesocosms Southward

A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In these bogs, peat mosses (Sphagnum) and vascular plants occur in an apparent stable equilibrium, thereby sustaining the carbon sink function of the bog ecosystem. How global warming and increased nitrogen (N) deposition will affect the species composition in bog vegetation is still unclear. We performed a transplantation experiment in which mesocosms with intact vegetation were transplanted southward from north Sweden to north-east Germany along a transect of four bog sites, in which both temperature and N deposition increased. In addition, we monitored undisturbed vegetation in control plots at the four sites of the latitudinal gradient. Four growing seasons after transplantation, ericaceous dwarf shrubs had become much more abundant when transplanted to the warmest site which also had highest N deposition. As a result ericoid aboveground biomass in the transplanted mesocosms increased most at the southernmost site, this site also had highest ericoid biomass in the undisturbed vegetation. The two dominant Sphagnum species showed opposing responses when transplanted southward; Sphagnum balticum height increment decreased, whereas S. fuscum height increment increased when transplanted southward. Sphagnum production did not differ significantly among the transplanted mesocosms, but was lowest in the southernmost control plots. The dwarf shrub expansion and increased N concentrations in plant tissues we observed, point in the direction of a positive feedback toward vascular plant-dominance suppressing peat-forming Sphagnum in the long term. However, our data also indicate that precipitation and phosphorus availability influence the competitive balance between Sphagnum, dwarf shrubs and graminoids.     

Nutrient concentration in shape Sphagna at increased N-deposition rates and raised atmospheric CO2 concentrations

Sphagnum fuscum, S. magellanicum, S. angustifolium and S. warnstorfii were treated with N deposition rates (0, 10, 30 and 100 kg ha^-1 a^-1) and with four atmospheric CO₂ concentrations (350, 700, 1000 and 2000 ppm) in greenhouse for 71–120 days. Thereafter, concentrations of total N, P, K, Ca and Mg in the capitulae of the Sphagna were determined. The response of each species to N deposition was related to ecological differences. With increasing N deposition treatments, moss N concentrations increased and higher N:P-ratios were found, the increase being especially clear at the highest N load. Sphagnum fuscum, which occupies ombrotrophic habitats, was the most affected by the increased nitrogen load and as a consequence the other elements were decreased. Oligotrophic S. magellanicum, wide nutrient status tolerant S. angustifolium and meso-eutrophic S. warnstorfii tolerated better increased N deposition, though there were increased concentrations of Ca and Mg in S. warnstorfii and Mg in S. magellanicum. Nitrogen and P concentrations decreased with raised CO₂ concentrations, except for S. magellanicum. This seems to be the first time this kind of response in nutrient concentrations to enhanced CO₂ concentration has been shown to exist in bryophytes. The concentration of K clearly decreased in S. fuscum as did the concentration of Mg in the other Sphagna with increasing CO₂. Sphagnum angustifolium and S. magellanicum, which are the less specialized species, were the least affected by the CO₂ treatments.     

Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) Warnst

Sphagnum bogs play an important role when considering the impacts of global change on global carbon and nitrogen cycles. Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) was grown at 360 (ambient) and 700 μL L^?1 (elevated) atmospheric [CO₂] in combination with different nitrogen deposition rates (6, 15, 23 g N m^?2 y^?1), in a short‐ and long‐term growth chamber experiment. After 6 months, elevated atmospheric [CO₂] in combination with the lowest nitrogen deposition rate, increased plant dry mass by 17%. In combination with a high nitrogen deposition rate, biomass production was not significantly stimulated. At the start of the experiment, photosynthesis was stimulated by elevated atmospheric [CO₂], but it was downregulated to control levels after three days of exposure. Elevated [CO₂] substantially reduced dark respiration, which resulted in a continuous increase in soluble sugar content in capitula. Differences in growth response among different nitrogen and CO₂ treatments could not be related to measured carbon exchange rates, which was mainly due to interference of microbial respiration. Doubling atmospheric [CO₂] reduced total nitrogen content in capitula but not in stems at all nitrogen deposition rates. Reduction in total nitrogen content coincided with a decrease in amino acids, but soluble protein levels remained unaffected. Thus, elevated [CO₂] induced a substantial shift in the partitioning of nitrogen compounds in capitula. Soluble sugar concentration was negatively correlated with total nitrogen content, which implies that the reduction in amino acid content in capitula, exposed to elevated [CO₂], might be caused by the accumulation of soluble sugars. Growth was not stimulated by increased nitrogen deposition. High nitrogen deposition, resulting in a capitulum nitrogen content in excess of 15 mg g^?1 dw, was detrimental to photosynthesis, reduced water content and induced necrosis. We propose a capitulum nitrogen content of 15 mg g^?1 dw as a possible bioindicator for the detection of nitrogen pollution stress in oligo‐mesotrophic peat bog ecosystems. At the lowest nitrogen deposition level, nitrogen recovery was higher than 100%, which indicates substantial dry deposition and/or gaseous nitrogen fixation by bacteria, associated with Sphagnum. Increasing nitrogen deposition rates decreased nitrogen recovery percentages, which indicates reduced efficiency of nitrogen fixation.     

Microsite and regional variation in the potential decay rate of Sphagnum magellanicum in south Swedish raised bogs

In southern Sweden there are regional gradients in the rate of atmospheric deposition of nitrogen, and the rate of N deposition has increased in recent decades This may have caused a shift in the growth-limiting nutrient of Sphagnum growth from nitrogen to phosphorus In this study, the influence of N and P concentrations on the decay of litter peat formed by Sphagnum magellanicum was examined A total of 90 litter peat samples formed by this species was collected from 15 raised bogs (3 sites per bog, 2 microsites per site) Total N and P of samples were determined and the rate of decomposition (C0₂ release) was measured under aerated, laboratory conditions at 18°C Differences in decomposition rates, N and P concentrations were most pronounced among microsites within sites, whereas no significant differences were observed among bogs The results indicate that decomposition of 5 magellanicum litter peat is influenced more by P than by N Thus, it appears that the recent increase in atmospheric N deposition has not had a large direct effect on peat decomposition rates It is suggested that the efficient uptake of N and P by the Sphagnum plant may lead to a positive feedback mechanism, whereby more slowly growing Sphagnum produces more nutrient-enriched litter peat with a more rapid decay Such a mechanism could promote the development of microtopography (hummocks and hollows) on bogs     

Phenotypic variation in the Sphagnum recurvum complex: a cultivation experiment

Abstract

Three morphs in the Sphagnum recurvum complex sampled from two mire localities in Central Norway were cultivated under different water levels and nutrient conditions in order to assess the amount and direction of phenotypic plasticity in several morphological variables. The factor ‘morph’ accounted for much of the variation. However, many of the shoot, branch, leaf and cell variables responded to the experimentally altered conditions, and water level was particularly important. Only the spreading:hanging branch length ratio and various stem leaf variables distinguished the Sphagnum angustifolium moiph from the S. fallax and S. isoviitae morphs regardless of environmental conditions and sampling locality. The results indicated that there is a continuum of genotypes shared by S. fallax and S. isoviitae.     

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.     

Photosynthetic performance in Sphagnum transplanted along a latitudinal nitrogen deposition gradient

Increased N deposition in Europe has affected mire ecosystems. However, knowledge on the physiological responses is poor. We measured photosynthetic responses to increasing N deposition in two peatmoss species (Sphagnum balticum and Sphagnum fuscum) from a 3-year, north–south transplant experiment in northern Europe, covering a latitudinal N deposition gradient ranging from 0.28 g N m⁻² year⁻¹ in the north, to 1.49 g N m⁻² year⁻¹ in the south. The maximum photosynthetic rate (NP_max) increased southwards, and was mainly explained by tissue N concentration, secondly by allocation of N to the photosynthesis, and to a lesser degree by modified photosystem II activity (variable fluorescence/maximum fluorescence yield). Although climatic factors may have contributed, these results were most likely attributable to an increase in N deposition southwards. For S. fuscum, photosynthetic rate continued to increase up to a deposition level of 1.49 g N m⁻² year⁻¹, but for S. balticum it seemed to level out at 1.14 g N m⁻² year⁻¹. The results for S. balticum suggested that transplants from different origin (with low or intermediate N deposition) respond differently to high N deposition. This indicates that Sphagnum species may be able to adapt or physiologically adjust to high N deposition. Our results also suggest that S. balticum might be more sensitive to N deposition than S. fuscum. Surprisingly, NP_max was not (S. balticum), or only weakly (S. fuscum) correlated with biomass production, indicating that production is to a great extent is governed by factors other than the photosynthetic capacity.     

Interspecific and geographical differences of plant tissue nutrient concentrations along an environmental gradient in Southern Patagonia, Chile

Foliar nutrient concentrations of South Patagonian ombrotrophic peatland species were analyzed along a gradient of increasing continentality. We paid particular attention to nutrient demands of Sphagnum magellanicum, the dominant constituent of continental raised bogs, and to those of cushion-forming vascular plants such as Astelia pumila, dominating in western hyperoceanic blanket bogs. With exception of one graminoid species, nutrient concentrations in leaf tissues were generally low, but showed considerable variation. Extremely low concentrations were measured in S. magellanicum reflecting the unpolluted, pristine character of the investigated bog ecosystems. Nutrient concentrations in cushion plants showed remarkable variation. While nutrient concentrations in Donatia fascicularis were almost as low as in S. magellanicum, they were significantly higher in A. pumila. Variation in foliar nutrient concentrations was predominantly due to differences between species. Nevertheless, there was a significant shift within species along the considered west-to-east continental gradient. The stronger minerotrophic character of the western cushion bogs was reflected by higher Ca contents. N concentrations increased and P concentrations decreased westward in most species. Consequently, the most noticeable shift was an increasing N:P ratio indicating a higher degree of P limitation towards western cushion dominated blanket bogs.     

Response of <Emphasis Type="Italic">Sphagnum</Emphasis> species mixtures to increased temperature and nitrogen availability

To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understand how Sphagnum vegetation in bogs will respond to global change. We performed a greenhouse experiment to study the effects of two temperature treatments (17.5 and 21.7°C) and two N addition treatments (0 and 4 g N m⁻² year⁻¹) on the growth of four Sphagnum species from three geographically interspersed regions: S. fuscum, S. balticum (northern and central Sweden), S. magellanicum and S. cuspidatum (southern Sweden). We studied the growth and cover change in four combinations of these Sphagnum species during two growing seasons. Sphagnum height increment and production were affected negatively by high temperature and high N addition. However, the northern species were more affected by temperature, while the southern species were more affected by N addition. High temperature depressed the cover of the ‘wet’ species, S. balticum and S. cuspidatum. Nitrogen concentrations increased with high N addition. N:P and N:K ratios indicated P-limited growth in all treatments and co-limitation of P and K in the high N treatments. In the second year of the experiment, several containers suffered from a severe fungal infection, particularly affecting the ‘wet’ species and the high N treatment. Our findings suggest that global change can have negative consequences for the production of Sphagnum species in bogs, with important implications for the carbon sequestration in these ecosystems.     

Does nitrogen addition to raised bogs influence peat phosphorus pools?

Two Sphagnum moss species occupying hummock areas (Sphagnum capillifolium) and wetter hollows (Sphagnum recurvum) on a raised bog in north east Scotland were treated every two weeks with NH₄NO₃ solutions to supply 3g N m^–2 yr^–1. Although S. recurvum moss contained a greater concentration of total P than S. capillifolium the amounts and N:P ratios were similar in both species. Larger amounts of total dissolved P (TDP) and molybdate reactive P (MRP) were extracted from beneath S. recurvum to 25 cm below the moss. Additions of N both increased and decreased the amounts of TDP at different times, and decreased MRP. The MRP fraction accounted for about 20 per cent of TDP and the difference was assumed to be in organic forms (DOP). Nitrogen addition had no effect on the amounts of DOP, but C:P ratios of this fraction changed with species, depth and N addition. Microbial P accounted for as much as 70 per cent of total P and showed seasonal variations, but no differences between the two moss species and N addition.     

Sphagnum mosses harbour highly specific bacterial diversity during their whole lifecycle

Knowledge about Sphagnum-associated microbial communities, their structure and their origin is important to understand and maintain climate-relevant Sphagnum-dominated bog ecosystems. We studied bacterial communities of two cosmopolitan Sphagnum species, which are well adapted to different abiotic parameters (Sphagnum magellanicum, which are strongly acidic and ombrotrophic, and Sphagnum fallax, which are weakly acidic and mesotrophic), in three Alpine bogs in Austria by a multifaceted approach. Great differences between bacterial fingerprints of both Sphagna were found independently from the site. This remarkable specificity was confirmed by a cloning and a deep sequencing approach. Besides the common Alphaproteobacteria, we found a discriminative spectrum of bacteria; although Gammaproteobacteria dominated S. magellanicum, S. fallax was mainly colonised by Verrucomicrobia and Planctomycetes. Using this information for fluorescent in situ hybridisation analyses, corresponding colonisation patterns for Alphaproteobacteria and Planctomycetes were detected. Bacterial colonies were found in high abundances inside the dead big hyalocytes, but they were always connected with the living chlorocytes. Using multivariate statistical analysis, the abiotic factors nutrient richness and pH were identified to modulate the composition of Sphagnum-specific bacterial communities. Interestingly, we found that the immense bacterial diversity was transferred via the sporophyte to the gametophyte, which can explain the high specificity of Sphagnum-associated bacteria over long distances. In contrast to higher plants, which acquire their bacteria mainly from the environment, mosses as the phylogenetically oldest land plants maintain their bacterial diversity within the whole lifecycle.     

Efficient regeneration of <Emphasis Type="Italic">Sphagnum fallax</Emphasis> from isolated protoplasts

Summary While the in vitro clonal propagation of peat mosses (Sphagnaceae) in bioreactors has been established since the late 1980s, it has never been possible to regenerate Sphagnum species from isolated protoplasts, which is a key step towards the production of closely defined genetically modified clones. The present study describes an efficient protocol for protoplast isolation and regeneration of Sphagnum fallax. Protoplast survival rates of over 50% and regeneration rates of up to 20% were achieved by using excised capitulum buds as starting material and by co-cultivating Sphagnum protoplasts with protoplasts from a chlorophyll-deficient Solanum hybrid clone. Besides the effects of nutrient components and differential osmotic readjustment of the regenerant cell clusters, the interference of unique Sphagnum phenolics, sphagnum acid and hydroxybutenolide, with protoplast isolation efficiency is demonstrated.     

Foliar <Superscript>15</Superscript>N natural abundance indicates phosphorus limitation of bog species

Foliar δ¹⁵N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant δ¹⁵N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the bogs, with foliar δ¹⁵N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where ¹⁵N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ¹⁵N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ¹⁵N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in δ¹⁵N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation.     

Effects of snow-load and shading by vascular plants on the vertical growth of hummocks formed by Sphagnum papillosum in a mire of northern Japan

The growth of hummock Sphagnum species in bogs depends on hydrological and climatic conditions, with different hummock morphologies being found across geographical regions. We investigated how plant shade and winter snow-load regulate the growth pattern and height of Sphagnum papillosum hummocks in a cool-temperate bog in Japan. Hummocks were subjected to four treatments across 2?years (1 hummock per treatment per year): no treatment (control), snow-load-free (S), vascular plant trimming (T), and snow-load-free and vascular plant trimming in combination (S&T). Further, the effects of vascular plant cover and snow -load on the vertical growth and height of hummocks were examined. Annual growth rates of S. papillosum were higher for the control (34?C41?mm) than S&T treatment, and were intermediate for separate S and T treatments. In contrast to vascular plant-growing hummocks, vascular plant-trimmed hummocks showed a negative correlation between water-table depth (measured from the capitulum to the water-table) and Sphagnum growth. Hence, in summer, shading by vascular plants may prevent desiccation and facilitate the growth of Sphagnum. Snow-loaded hummocks were weighed down by 3?C11?cm. After snowmelt, the shoots continued to grow within the water-table depth range that allowed growth. Hence, heavy winter snow-loads may depress the surfaces of hummocks closer to the water-table, which stimulates Sphagnum growth, resulting in the recovery of hummock height. Thus, the water stress caused by summer desiccation is critical in regulating the upper limit of hummock height in bogs subject to dry summer conditions and heavy winter snowfall.     

Effects of climate warming on Sphagnum photosynthesis in peatlands depend on peat moisture and species‐specific anatomical traits

Climate change will influence plant photosynthesis by altering patterns of temperature and precipitation, including their variability and seasonality. Both effects may be important for peatlands as the carbon (C) sink potential of these ecosystems depends on the balance between plant C uptake through photosynthesis and microbial decomposition. Here, we show that the effect of climate warming on Sphagnum community photosynthesis toggles from positive to negative as the peatland goes from rainy to dry periods during summer. More particularly, we show that mechanisms of compensation among the dominant Sphagnum species (Sphagnum fallax and Sphagnum medium) stabilize the average photosynthesis and productivity of the Sphagnum community during summer despite rising temperatures and frequent droughts. While warming had a negligible effect on S. medium photosynthetic capacity (A_max) during rainy periods, A_max of S. fallax increased by 40%. On the opposite, warming exacerbated the negative effects of droughts on S. fallax with an even sharper decrease of its A_max while S. medium A_max remained unchanged. S. medium showed a remarkable resistance to droughts due to anatomical traits favouring its water holding capacity. Our results show that different phenotypic plasticity among dominant Sphagnum species allow the community to cope with rising temperatures and repeated droughts, maintaining similar photosynthesis and productivity over summer in warmed and control conditions. These results are important because they provide information on how soil water content may modulate the effects of climate warming on Sphagnum productivity in boreal peatlands. It further confirms the transitory nature of warming‐induced photosynthesis benefits in boreal systems and highlights the vulnerability of the ecosystem to excess warming and drying.     

Contrasting effects of ammonium enrichment on fen bryophytes

Abstract

In the second half of the 20th century, frequency and diversity of brown mosses strongly declined in Dutch fens while species of Sphagnum and Polytrichum increased markedly. We hypothesized that high ammonium (NH₄⁺) concentrations in Dutch precipitation have promoted these species changes. To test this hypothesis, we examined the effects of varying concentrations (1–1000 μM) of NH₄⁺ on a brown moss (Calliergonella cuspidata), two species of Sphagnum (S. contortum and S. squarrosum) and Polytrichum commune in a hydroponic experiment. A second goal was to identify possible interspecific differences in the mechanisms responsible for NH₄⁺ detoxification. Dry matter production and tissue cation concentrations indicated decreasing sensitivity to NH₄⁺ in the order C. cuspidata>S. contortum>S. squarrosum and P. commune. In S. squarrosum and P. commune, the highest NH₄⁺ treatments induced strong increases in tissue nitrogen (N) and amino acid concentrations. Sphagnum contortum showed a similar, but less marked response. In contrast, C. cuspidata showed a negative relationship between external NH₄⁺ concentrations and both tissue N and amino acid concentrations (except arginine). Our findings support the hypothesis that the present rates of NH₄⁺ deposition in The Netherlands are detrimental to brown mosses, but not to fast-growing Sphagnum and Polytrichum species. The efficiency of the NH₄⁺ assimilation apparatus, producing N-rich amino acids, probably plays a key role in determining sensitivity to increased NH₄⁺ deposition.