Page 1. The behaviour of the kleptoparasite, Pseudohypocera kerteszi (Diptera, Phoridae), in hive... more Page 1. The behaviour of the kleptoparasite, Pseudohypocera kerteszi (Diptera, Phoridae), in hives of stingless bees (Hymenoptera, Apidae) in Central America Bjorn JM Robroek1, Harriet de Jong2 & Marinus J. Sommeijer1 ...
Page 1. The development of Pseudohypocera kerteszi (Diptera, Phoridae), a kleptoparasite in nests... more Page 1. The development of Pseudohypocera kerteszi (Diptera, Phoridae), a kleptoparasite in nests of stingless bees (Hymenoptera, Apidae) in Central America Bjorn JM Robroek1, Harriet de Jong2, Henry Arce3 & Marinus J. Sommeijer1 ...
To evaluate the effects of changes in water level and temperatures on performance of four Sphagnu... more To evaluate the effects of changes in water level and temperatures on performance of four Sphagnum mosses, S. magellanicum, S. rubellum, S. imbricatum and S. fuscum were grown at two water levels, −5 cm and −15 cm, and at two temperatures, 15°C and 20°C. These species differ in their position along the microtopographical gradient and in their geographical distribution. Height increment, subcapitulum bulk density, biomass production, capitulum water content and cumulative evaporation were measured. Height increment and biomass production of S. magellanicum was lower at low water table than at high water table, whereas height increment and biomass production of S. rubellum, S. imbricatum and S. fuscum were unaffected. Height increment of S. magellanicum, S. rubellum and S. imbricatum was higher at high temperature than at low temperature. Biomass production of only S. magellanicum and S. rubellum was higher at high temperature than at low temperature, corresponding with their more southern distribution. Cumulative evaporation of S. magellanicum and S. rubellum was lower at low water table and could be explained by hampered water transport towards the capitula. We conclude that changes in water table and temperature may alter the Sphagnum composition on raised bogs, which may result in changes to important ecosystem processes. Therefore, it is important that species composition and changes therein are taken into account when evaluating global change effects on raised bog ecosystems.
Soaks (areas of mesotrophic/minerotrophic vegetation within acid bog) add to the overall heteroge... more Soaks (areas of mesotrophic/minerotrophic vegetation within acid bog) add to the overall heterogeneity and biodiversity of raised bog landscapes due to the presence of flora and fauna communities not typically associated with acid bog systems. A field experiment was set up to investigate the potential to restore the minerotrophic and aquatic communities that previously occurred within a soak of an oceanic raised bog in Ireland, which has recently undergone acidification with the expansion of acid bog type vegetation. Three different treatments, control (intact sphagnaceous raft), permeable (sphagnaceous raft removed), and enclosed (sphagnaceous raft removed and plots isolated from surrounding surface water influence) were applied to a total of six plots (each measuring 4 × 4 m), each treatment consisting of two replicates. Within 3 years a sphagnaceous raft with similar vegetation to the surroundings had developed in both permeable plots, while aquatic communities similar to those that occurred at the site in the past had established within the enclosed pots. Our results show that with manipulation of local hydrology it is possible to recreate conditions suitable for aquatic plant communities that once characterized the site. The results also give an insight into the likely processes responsible for the initial terrestrialization of the entire soak over the past century. Application of the results in relation to the site and the widespread practice of restoring bog vegetation on degraded peatlands are discussed.
Sphagnum cuspidatum, S. magellanicum and S. rubellum are three co-occurring peat mosses, which na... more Sphagnum cuspidatum, S. magellanicum and S. rubellum are three co-occurring peat mosses, which naturally have a different distribution along the microtopographical gradient of the surface of peatlands. We set out an experiment to assess the interactive effects of water table (low: −10 cm and high: −1 cm) and precipitation (present or absent) on the CO2 assimilation and evaporation of these species over a 23-day period. Additionally, we measured which sections of the moss layer were responsible for light absorption and bulk carbon uptake. Thereafter, we investigated how water content affected carbon uptake by the mosses. Our results show that at high water table, CO2 assimilation of all species gradually increased over time, irrespective of the precipitation. At low water table, net CO2 assimilation of all species declined over time, with the earliest onset and highest rate of decline for S. cuspidatum. Precipitation compensated for reduced water tables and positively affected the carbon uptake of all species. Almost all light absorption occurred in the first centimeter of the Sphagnum vegetation and so did net CO2 assimilation. CO2 assimilation rate showed species-specific relationships with capitulum water content, with narrow but contrasting optima for S. cuspidatum and S. rubellum. Assimilation by S. magellanicum was constant at a relatively low rate over a broad range of capitulum water contents. Our study indicates that prolonged drought may alter the competitive balance between species, favoring hummock species over hollow species. Moreover, this study shows that precipitation is at least equally important as water table drawdown and should be taken into account in predictions about the fate of peatlands with respect to climate change.
McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies ... more McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies to explore the disappearance of Sphagnum imbricatum from a peat bog in northern England, Wales and Ireland, respectively. McClymont et al. used their results to argue that a combination of rapid water-table rise and increased aeolian nutrient input from surrounding (agricultural) areas may have caused the
McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies ... more McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies to explore the disappearance of Sphagnum imbricatum from a peat bog in northern England, Wales and Ireland, respectively. McClymont et al. used their results to argue that a combination of rapid water-table rise and increased aeolian nutrient input from surrounding (agricultural) areas may have caused the
<div><i>Sphagnum</i> mosses are key-stone species in European raised bogs, and ... more <div><i>Sphagnum</i> mosses are key-stone species in European raised bogs, and as such play an important role in the functioning of the ecosystem and its response to environmental change. Within <i>Sphagnum</i>, species may vary widely in their characteristics, making the identity of the dominant <i>Sphagnum</i> species an important factor explaining ecosystem response. We conducted several glasshouse and field studies, aimed at investigating how the main <i>Sphagnum</i> species in European raised bog respond to changes in climatic variables such as temperature, water table and precipitation with special focus on their effect on interspecific competition. </div> <div>Our results showed that <i>Sphagnum</i> species differ in their response to water table, although the effect of water table was not as straightforward as hitherto expected. Despite differences in the potential to transport water from the water table towards their apical parts, co-occurring <i>Sphagnum</i> species possessed similar water contents. Nevertheless, we found species which naturally occur closest to the water table (hollow) to be ousted by species that naturally occur further from the water table (hummock), indicating that competition is related to interspecific (physiological) differences in the response to capitulum water content. We show that carbon assimilation of hollow species has a narrow optimum at relatively high capitulum water contents, whereas carbon assimilation of hummock species had its optimum under dryer conditions. </div> <div>Additionally, our results show that precipitation plays an important role in determining the water content of <i>Sphagnum</i> mosses. Especially at low water tables, precipitation was found to be an important driver for carbon assimilation. Particularly, the competitive ability of hollow species may largely be determined by precipitation. </div> <div>A considerable part of the competitive performance of <i>Sphagnum</i> mosses may be explained by the size of anisotropic peat moss vegetation. Our results show that the persistence of <i>Sphagnum</i> species which are transplanted generally increases with increasing patch size. Our results indicate that with increasing patch size the ability of <i>Sphagnum</i> mosses to create their own optimal micro-hydrology increases. Especially, under drier conditions, this ability may increase the competitive strength of species.</div> <div>Summarizing, competition between <i>Sphagnum</i> species is influenced by the photosynthetic capacity at distinct capitulum water content. In turn, the capitulum water content of <i>Sphagnum </i>species<i> </i>depends on its capillary system, its water holding capacity (i.e. size of hyaline cells) and on the surrounding vegetation. The results presented in this thesis show that if the availability of water decreases, due to decreasing water tables or prolonged dry periods, hummock species will benefit at the expense of hollow species. Additionally, vascular plants like ericoids will increase. Since <i>Sphagnum </i>species differ in their production rates, changes in the species composition in raised bogs may result in changes in the production of the vegetation, and ultimately change the functioning of these ecosystems. It is important that peatland scientist continue to monitor the effects of environmental changes on changes in the species composition of peatlands. Especially long term measurement may provide a lot of information, which will help us to understand the mechanisms of competition between <i>Sphagnum</i> species to a higher degree.</div>
This report contains opening remarks, kickoff address and keynote address, and 89 papers presente... more This report contains opening remarks, kickoff address and keynote address, and 89 papers presented at the First International Symposium on Explosive Detection Technology held November 13-15, 1991, in Atlantic City, New Jersey. The general papers deal with the outlook of civil ...
A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In the... more 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.
1Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately th... more 1Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately the functioning of the whole ecosystem. We investigated the effect of different water tables on the competition between six Sphagnum species in the glasshouse. The amount of precipitation (234 mm year−1) and precipitation frequency (every 2 weeks) were kept low to encourage water-table effects. Relevant species combinations and monocultures were grown at different water tables for a 16-month period. We studied changes in cover, height increment and capitulum water content (WCcap) in order to understand competitive responses.2Species naturally occurring further above the water table generally showed higher competitive strength than species naturally occurring closer to the water table. Surprisingly, this effect was irrespective of the water table, indicating a minor role for capillary water transport. Cover change seemed to be related to differences in length growth, but not to water table or WCcap.3The WCcap of species within a mixture did not differ, but was lower than the WCcap of the individual species growing in monoculture, indicating differences in ability to supply water to the capitula between mono- and mixed cultures. Subcapitulum bulk densities between mono- and mixed cultures did not differ, or were even lower in monocultures, but did differ between species within mixed cultures.4Our results indicate that structural heterogeneity of the peat in mixed cultures has a negative effect on WCcap of both species. Furthermore, we show that sustained periods of drought cause species that naturally occur further above the water table to oust species that naturally occur closer to the water table, even if the water table remains high. Ultimately, the Sphagnum vegetation in raised bogs may shift from hollow to hummock species, evening out the natural microtopography of raised bogs.Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately the functioning of the whole ecosystem. We investigated the effect of different water tables on the competition between six Sphagnum species in the glasshouse. The amount of precipitation (234 mm year−1) and precipitation frequency (every 2 weeks) were kept low to encourage water-table effects. Relevant species combinations and monocultures were grown at different water tables for a 16-month period. We studied changes in cover, height increment and capitulum water content (WCcap) in order to understand competitive responses.Species naturally occurring further above the water table generally showed higher competitive strength than species naturally occurring closer to the water table. Surprisingly, this effect was irrespective of the water table, indicating a minor role for capillary water transport. Cover change seemed to be related to differences in length growth, but not to water table or WCcap.The WCcap of species within a mixture did not differ, but was lower than the WCcap of the individual species growing in monoculture, indicating differences in ability to supply water to the capitula between mono- and mixed cultures. Subcapitulum bulk densities between mono- and mixed cultures did not differ, or were even lower in monocultures, but did differ between species within mixed cultures.Our results indicate that structural heterogeneity of the peat in mixed cultures has a negative effect on WCcap of both species. Furthermore, we show that sustained periods of drought cause species that naturally occur further above the water table to oust species that naturally occur closer to the water table, even if the water table remains high. Ultimately, the Sphagnum vegetation in raised bogs may shift from hollow to hummock species, evening out the natural microtopography of raised bogs.
Peat bogs play a large role in the global sequestration of C, and are often dominated by differen... more Peat bogs play a large role in the global sequestration of C, and are often dominated by different Sphagnum species. Therefore, it is crucial to understand how Sphagnum vegetation in peat bogs will respond to global warming. We performed a greenhouse experiment to study the effect of four temperature treatments (11.2, 14.7, 18.0 and 21.4°C) on the growth of four Sphagnum species: S. fuscum and S. balticum from a site in northern Sweden and S. magellanicum and S. cuspidatum from a site in southern Sweden. In addition, three combinations of these species were made to study the effect of temperature on competition. We found that all species increased their height increment and biomass production with an increase in temperature, while bulk densities were lower at higher temperatures. The hollow species S. cuspidatum was the least responsive species, whereas the hummock species S. fuscum increased biomass production 13-fold from the lowest to the highest temperature treatment in monocultures. Nutrient concentrations were higher at higher temperatures, especially N concentrations of S. fuscum and S. balticum increased compared to field values. Competition between S. cuspidatum and S. magellanicum was not influenced by temperature. The mixtures of S. balticum with S. fuscum and S. balticum with S. magellanicum showed that S. balticum was the stronger competitor, but it lost competitive advantage in the highest temperature treatment. These findings suggest that species abundances will shift in response to global warming, particularly at northern sites where hollow species will lose competitive strength relative to hummock species and southern species.
To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understa... more 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.
On raised bogs, the distribution of Sphagnum species is determined by their distance to the water... more On raised bogs, the distribution of Sphagnum species is determined by their distance to the water table, but occasionally species are able to survive outside their niche. Hollow species that persist in hummock vegetation are assumed to profit from the higher water content of the surrounding hummock species, although the mechanism responsible is unclear. In this study, we elucidated the role of lateral hummock water transport (LHWT) and precipitation on the water content of hollow species occurring in hummocks. This was tested using a full factorial field transplantation experiment with cores of Sphagnum cuspidatum in a high and a low hummock. Treatments included direct precipitation (present or absent) and LHWT (present or absent). Fresh weights of the cores were measured at regular time intervals. Our results show a relatively large effect of precipitation on the water content in both the high and low hummock, whereas LHWT only seemed to be an important source of water in the high hummock, which was relatively dry. Furthermore, LHWT played an important role only after large precipitation events, suggesting that lateral water transport is indirectly affected by rain. This study shows that precipitation alone can explain the persistence of hollow species in high hummocks, whereas it was less important for hollow species in low hummocks. Our data suggest that the survival and potential expansion of hollow species in higher hummocks strongly depends on the intensity and frequency of rain events. Changes in precipitation patterns may result in a loss of Sphagnum diversity in hummocks.
Despite their low primary production, ombrotrophic peatlands have a considerable potential to sto... more 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.
Page 1. The behaviour of the kleptoparasite, Pseudohypocera kerteszi (Diptera, Phoridae), in hive... more Page 1. The behaviour of the kleptoparasite, Pseudohypocera kerteszi (Diptera, Phoridae), in hives of stingless bees (Hymenoptera, Apidae) in Central America Bjorn JM Robroek1, Harriet de Jong2 & Marinus J. Sommeijer1 ...
Page 1. The development of Pseudohypocera kerteszi (Diptera, Phoridae), a kleptoparasite in nests... more Page 1. The development of Pseudohypocera kerteszi (Diptera, Phoridae), a kleptoparasite in nests of stingless bees (Hymenoptera, Apidae) in Central America Bjorn JM Robroek1, Harriet de Jong2, Henry Arce3 & Marinus J. Sommeijer1 ...
To evaluate the effects of changes in water level and temperatures on performance of four Sphagnu... more To evaluate the effects of changes in water level and temperatures on performance of four Sphagnum mosses, S. magellanicum, S. rubellum, S. imbricatum and S. fuscum were grown at two water levels, −5 cm and −15 cm, and at two temperatures, 15°C and 20°C. These species differ in their position along the microtopographical gradient and in their geographical distribution. Height increment, subcapitulum bulk density, biomass production, capitulum water content and cumulative evaporation were measured. Height increment and biomass production of S. magellanicum was lower at low water table than at high water table, whereas height increment and biomass production of S. rubellum, S. imbricatum and S. fuscum were unaffected. Height increment of S. magellanicum, S. rubellum and S. imbricatum was higher at high temperature than at low temperature. Biomass production of only S. magellanicum and S. rubellum was higher at high temperature than at low temperature, corresponding with their more southern distribution. Cumulative evaporation of S. magellanicum and S. rubellum was lower at low water table and could be explained by hampered water transport towards the capitula. We conclude that changes in water table and temperature may alter the Sphagnum composition on raised bogs, which may result in changes to important ecosystem processes. Therefore, it is important that species composition and changes therein are taken into account when evaluating global change effects on raised bog ecosystems.
Soaks (areas of mesotrophic/minerotrophic vegetation within acid bog) add to the overall heteroge... more Soaks (areas of mesotrophic/minerotrophic vegetation within acid bog) add to the overall heterogeneity and biodiversity of raised bog landscapes due to the presence of flora and fauna communities not typically associated with acid bog systems. A field experiment was set up to investigate the potential to restore the minerotrophic and aquatic communities that previously occurred within a soak of an oceanic raised bog in Ireland, which has recently undergone acidification with the expansion of acid bog type vegetation. Three different treatments, control (intact sphagnaceous raft), permeable (sphagnaceous raft removed), and enclosed (sphagnaceous raft removed and plots isolated from surrounding surface water influence) were applied to a total of six plots (each measuring 4 × 4 m), each treatment consisting of two replicates. Within 3 years a sphagnaceous raft with similar vegetation to the surroundings had developed in both permeable plots, while aquatic communities similar to those that occurred at the site in the past had established within the enclosed pots. Our results show that with manipulation of local hydrology it is possible to recreate conditions suitable for aquatic plant communities that once characterized the site. The results also give an insight into the likely processes responsible for the initial terrestrialization of the entire soak over the past century. Application of the results in relation to the site and the widespread practice of restoring bog vegetation on degraded peatlands are discussed.
Sphagnum cuspidatum, S. magellanicum and S. rubellum are three co-occurring peat mosses, which na... more Sphagnum cuspidatum, S. magellanicum and S. rubellum are three co-occurring peat mosses, which naturally have a different distribution along the microtopographical gradient of the surface of peatlands. We set out an experiment to assess the interactive effects of water table (low: −10 cm and high: −1 cm) and precipitation (present or absent) on the CO2 assimilation and evaporation of these species over a 23-day period. Additionally, we measured which sections of the moss layer were responsible for light absorption and bulk carbon uptake. Thereafter, we investigated how water content affected carbon uptake by the mosses. Our results show that at high water table, CO2 assimilation of all species gradually increased over time, irrespective of the precipitation. At low water table, net CO2 assimilation of all species declined over time, with the earliest onset and highest rate of decline for S. cuspidatum. Precipitation compensated for reduced water tables and positively affected the carbon uptake of all species. Almost all light absorption occurred in the first centimeter of the Sphagnum vegetation and so did net CO2 assimilation. CO2 assimilation rate showed species-specific relationships with capitulum water content, with narrow but contrasting optima for S. cuspidatum and S. rubellum. Assimilation by S. magellanicum was constant at a relatively low rate over a broad range of capitulum water contents. Our study indicates that prolonged drought may alter the competitive balance between species, favoring hummock species over hollow species. Moreover, this study shows that precipitation is at least equally important as water table drawdown and should be taken into account in predictions about the fate of peatlands with respect to climate change.
McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies ... more McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies to explore the disappearance of Sphagnum imbricatum from a peat bog in northern England, Wales and Ireland, respectively. McClymont et al. used their results to argue that a combination of rapid water-table rise and increased aeolian nutrient input from surrounding (agricultural) areas may have caused the
McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies ... more McClymont et al. (The Holocene 18 (2008) 991—1002) present data on several environmental proxies to explore the disappearance of Sphagnum imbricatum from a peat bog in northern England, Wales and Ireland, respectively. McClymont et al. used their results to argue that a combination of rapid water-table rise and increased aeolian nutrient input from surrounding (agricultural) areas may have caused the
<div><i>Sphagnum</i> mosses are key-stone species in European raised bogs, and ... more <div><i>Sphagnum</i> mosses are key-stone species in European raised bogs, and as such play an important role in the functioning of the ecosystem and its response to environmental change. Within <i>Sphagnum</i>, species may vary widely in their characteristics, making the identity of the dominant <i>Sphagnum</i> species an important factor explaining ecosystem response. We conducted several glasshouse and field studies, aimed at investigating how the main <i>Sphagnum</i> species in European raised bog respond to changes in climatic variables such as temperature, water table and precipitation with special focus on their effect on interspecific competition. </div> <div>Our results showed that <i>Sphagnum</i> species differ in their response to water table, although the effect of water table was not as straightforward as hitherto expected. Despite differences in the potential to transport water from the water table towards their apical parts, co-occurring <i>Sphagnum</i> species possessed similar water contents. Nevertheless, we found species which naturally occur closest to the water table (hollow) to be ousted by species that naturally occur further from the water table (hummock), indicating that competition is related to interspecific (physiological) differences in the response to capitulum water content. We show that carbon assimilation of hollow species has a narrow optimum at relatively high capitulum water contents, whereas carbon assimilation of hummock species had its optimum under dryer conditions. </div> <div>Additionally, our results show that precipitation plays an important role in determining the water content of <i>Sphagnum</i> mosses. Especially at low water tables, precipitation was found to be an important driver for carbon assimilation. Particularly, the competitive ability of hollow species may largely be determined by precipitation. </div> <div>A considerable part of the competitive performance of <i>Sphagnum</i> mosses may be explained by the size of anisotropic peat moss vegetation. Our results show that the persistence of <i>Sphagnum</i> species which are transplanted generally increases with increasing patch size. Our results indicate that with increasing patch size the ability of <i>Sphagnum</i> mosses to create their own optimal micro-hydrology increases. Especially, under drier conditions, this ability may increase the competitive strength of species.</div> <div>Summarizing, competition between <i>Sphagnum</i> species is influenced by the photosynthetic capacity at distinct capitulum water content. In turn, the capitulum water content of <i>Sphagnum </i>species<i> </i>depends on its capillary system, its water holding capacity (i.e. size of hyaline cells) and on the surrounding vegetation. The results presented in this thesis show that if the availability of water decreases, due to decreasing water tables or prolonged dry periods, hummock species will benefit at the expense of hollow species. Additionally, vascular plants like ericoids will increase. Since <i>Sphagnum </i>species differ in their production rates, changes in the species composition in raised bogs may result in changes in the production of the vegetation, and ultimately change the functioning of these ecosystems. It is important that peatland scientist continue to monitor the effects of environmental changes on changes in the species composition of peatlands. Especially long term measurement may provide a lot of information, which will help us to understand the mechanisms of competition between <i>Sphagnum</i> species to a higher degree.</div>
This report contains opening remarks, kickoff address and keynote address, and 89 papers presente... more This report contains opening remarks, kickoff address and keynote address, and 89 papers presented at the First International Symposium on Explosive Detection Technology held November 13-15, 1991, in Atlantic City, New Jersey. The general papers deal with the outlook of civil ...
A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In the... more 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.
1Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately th... more 1Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately the functioning of the whole ecosystem. We investigated the effect of different water tables on the competition between six Sphagnum species in the glasshouse. The amount of precipitation (234 mm year−1) and precipitation frequency (every 2 weeks) were kept low to encourage water-table effects. Relevant species combinations and monocultures were grown at different water tables for a 16-month period. We studied changes in cover, height increment and capitulum water content (WCcap) in order to understand competitive responses.2Species naturally occurring further above the water table generally showed higher competitive strength than species naturally occurring closer to the water table. Surprisingly, this effect was irrespective of the water table, indicating a minor role for capillary water transport. Cover change seemed to be related to differences in length growth, but not to water table or WCcap.3The WCcap of species within a mixture did not differ, but was lower than the WCcap of the individual species growing in monoculture, indicating differences in ability to supply water to the capitula between mono- and mixed cultures. Subcapitulum bulk densities between mono- and mixed cultures did not differ, or were even lower in monocultures, but did differ between species within mixed cultures.4Our results indicate that structural heterogeneity of the peat in mixed cultures has a negative effect on WCcap of both species. Furthermore, we show that sustained periods of drought cause species that naturally occur further above the water table to oust species that naturally occur closer to the water table, even if the water table remains high. Ultimately, the Sphagnum vegetation in raised bogs may shift from hollow to hummock species, evening out the natural microtopography of raised bogs.Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately the functioning of the whole ecosystem. We investigated the effect of different water tables on the competition between six Sphagnum species in the glasshouse. The amount of precipitation (234 mm year−1) and precipitation frequency (every 2 weeks) were kept low to encourage water-table effects. Relevant species combinations and monocultures were grown at different water tables for a 16-month period. We studied changes in cover, height increment and capitulum water content (WCcap) in order to understand competitive responses.Species naturally occurring further above the water table generally showed higher competitive strength than species naturally occurring closer to the water table. Surprisingly, this effect was irrespective of the water table, indicating a minor role for capillary water transport. Cover change seemed to be related to differences in length growth, but not to water table or WCcap.The WCcap of species within a mixture did not differ, but was lower than the WCcap of the individual species growing in monoculture, indicating differences in ability to supply water to the capitula between mono- and mixed cultures. Subcapitulum bulk densities between mono- and mixed cultures did not differ, or were even lower in monocultures, but did differ between species within mixed cultures.Our results indicate that structural heterogeneity of the peat in mixed cultures has a negative effect on WCcap of both species. Furthermore, we show that sustained periods of drought cause species that naturally occur further above the water table to oust species that naturally occur closer to the water table, even if the water table remains high. Ultimately, the Sphagnum vegetation in raised bogs may shift from hollow to hummock species, evening out the natural microtopography of raised bogs.
Peat bogs play a large role in the global sequestration of C, and are often dominated by differen... more Peat bogs play a large role in the global sequestration of C, and are often dominated by different Sphagnum species. Therefore, it is crucial to understand how Sphagnum vegetation in peat bogs will respond to global warming. We performed a greenhouse experiment to study the effect of four temperature treatments (11.2, 14.7, 18.0 and 21.4°C) on the growth of four Sphagnum species: S. fuscum and S. balticum from a site in northern Sweden and S. magellanicum and S. cuspidatum from a site in southern Sweden. In addition, three combinations of these species were made to study the effect of temperature on competition. We found that all species increased their height increment and biomass production with an increase in temperature, while bulk densities were lower at higher temperatures. The hollow species S. cuspidatum was the least responsive species, whereas the hummock species S. fuscum increased biomass production 13-fold from the lowest to the highest temperature treatment in monocultures. Nutrient concentrations were higher at higher temperatures, especially N concentrations of S. fuscum and S. balticum increased compared to field values. Competition between S. cuspidatum and S. magellanicum was not influenced by temperature. The mixtures of S. balticum with S. fuscum and S. balticum with S. magellanicum showed that S. balticum was the stronger competitor, but it lost competitive advantage in the highest temperature treatment. These findings suggest that species abundances will shift in response to global warming, particularly at northern sites where hollow species will lose competitive strength relative to hummock species and southern species.
To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understa... more 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.
On raised bogs, the distribution of Sphagnum species is determined by their distance to the water... more On raised bogs, the distribution of Sphagnum species is determined by their distance to the water table, but occasionally species are able to survive outside their niche. Hollow species that persist in hummock vegetation are assumed to profit from the higher water content of the surrounding hummock species, although the mechanism responsible is unclear. In this study, we elucidated the role of lateral hummock water transport (LHWT) and precipitation on the water content of hollow species occurring in hummocks. This was tested using a full factorial field transplantation experiment with cores of Sphagnum cuspidatum in a high and a low hummock. Treatments included direct precipitation (present or absent) and LHWT (present or absent). Fresh weights of the cores were measured at regular time intervals. Our results show a relatively large effect of precipitation on the water content in both the high and low hummock, whereas LHWT only seemed to be an important source of water in the high hummock, which was relatively dry. Furthermore, LHWT played an important role only after large precipitation events, suggesting that lateral water transport is indirectly affected by rain. This study shows that precipitation alone can explain the persistence of hollow species in high hummocks, whereas it was less important for hollow species in low hummocks. Our data suggest that the survival and potential expansion of hollow species in higher hummocks strongly depends on the intensity and frequency of rain events. Changes in precipitation patterns may result in a loss of Sphagnum diversity in hummocks.
Despite their low primary production, ombrotrophic peatlands have a considerable potential to sto... more 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.
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