Bjørn Solheim

Universitetet i Tromsø, Tromsø, Troms, Norway

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Publications (17)77.36 Total impact

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    ABSTRACT: Over the past 100 years, Arctic temperatures have increased at almost twice the global average rate. One consequence is the acceleration of glacier retreat, exposing new habitats that are colonized by microorganisms whose diversity and function are unknown. Here, we characterized bacterial diversity along two approximately parallel chronosequences in an Arctic glacier forefield that span six time points following glacier retreat. We assessed changes in phylotype richness, evenness and turnover rate through the analysis of 16S rRNA gene sequences recovered from 52 samples taken from surface layers along the chronosequences. An average of 4500 sequences was obtained from each sample by 454 pyrosequencing. Using parametric methods, it was estimated that bacterial phylotype richness was high, and that it increased significantly from an average of 4000 (at a threshold of 97% sequence similarity) at locations exposed for 5 years to an average of 7050 phylotypes per 0.5 g of soil at sites that had been exposed for 150 years. Phylotype evenness also increased over time, with an evenness of 0.74 for 150 years since glacier retreat reflecting large proportions of rare phylotypes. The bacterial species turnover rate was especially high between sites exposed for 5 and 19 years. The level of bacterial diversity present in this High Arctic glacier foreland was comparable with that found in temperate and tropical soils, raising the question whether global patterns of bacterial species diversity parallel that of plants and animals, which have been found to form a latitudinal gradient and be lower in polar regions compared with the tropics.
    Molecular Ecology 03/2010; 19 Suppl 1:54-66. · 6.28 Impact Factor
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    ABSTRACT: During primary colonization of rock substrates by plants, mineral weathering is strongly accelerated under plant roots, but little is known on how it affects soil ecosystem development before plant establishment. Here we show that rock mineral weathering mediated by chemolithoautotrophic bacteria is associated to plant community formation in sites recently released by permanent glacier ice cover in the Midtre Lovénbreen glacier moraine (78 degrees 53'N), Svalbard. Increased soil fertility fosters growth of prokaryotes and plants at the boundary between sites of intense bacterial mediated chemolithotrophic iron-sulfur oxidation and pH decrease, and the common moraine substrate where carbon and nitrogen are fixed by cyanobacteria. Microbial iron oxidizing activity determines acidity and corresponding fertility gradients, where water retention, cation exchange capacity and nutrient availability are increased. This fertilization is enabled by abundant mineral nutrients and reduced forms of iron and sulfur in pyrite minerals within a conglomerate type of moraine rock. Such an interaction between microorganisms and moraine minerals determines a peculiar, not yet described model for soil genesis and plant ecosystem formation with potential past and present analogues in other harsh environments with similar geochemical settings.
    Environmental Microbiology 10/2009; 12(2):293-303. · 6.24 Impact Factor
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    ABSTRACT: Succession is defined as changes in biological communities over time. It has been extensively studied in plant communities, but little is known about bacterial succession, in particular in environments such as High Arctic glacier forelands. Bacteria carry out key processes in the development of soil, biogeochemical cycling and facilitating plant colonization. In this study we sampled two roughly parallel chronosequences in the foreland of Midre Lovén glacier on Svalbard, Norway and tested whether any of several factors were associated with changes in the structure of bacterial communities, including time after glacier retreat, horizontal variation caused by the distance between chronosequences and vertical variation at two soil depths. The structures of soil bacterial communities at different locations were compared using terminal restriction fragment length polymorphisms of 16S rRNA genes, and the data were analyzed by sequential analysis of log-linear statistical models. Although no significant differences in community structure were detected between the two chronosequences, statistically significant differences between sampling locations in the surface and mineral soils could be demonstrated even though glacier forelands are patchy and dynamic environments. These findings suggest that bacterial succession occurs in High Arctic glacier forelands but may differ in different soil depths.
    The ISME Journal 08/2009; 3(11):1258-68. · 8.95 Impact Factor
  • Bjørn Solheim, Matthias Zielke
    08/2007: pages 137-152;
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    ABSTRACT: Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.
    Ecology Letters 08/2007; 10(7):619-27. · 17.95 Impact Factor
  • Ecology Letters 01/2007; 10(7):619-627. · 17.95 Impact Factor
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    Ecology 01/2006; 182:121-135. · 5.18 Impact Factor
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    ABSTRACT: Recent global climate models predict a further significant loss of ozone in the next decades, with up to 50% depletion of the ozone layer over large parts of the Arctic resulting in an increase in ultraviolet-B radiation (UV-B) (280–315 nm) reaching the surface of the Earth. The percentage of total annual ecosystem N input due to biological nitrogen fixation by cyanobacteria might be as high as 80% and the contribution to total annual N uptake by plants up to 20%. A possible reduction of nitrogen fixation raises serious concerns about already nutrient impoverished plant communities. This review shows that nitrogen fixation by moss-associated cyanobacteria in arctic vegetation was dramatically reduced after six years of exposure to enhanced UV-B radiation. In subarctic vegetation, nitrogen fixation activity of moss-associated cyanobacteria was not affected by 6 years of enhanced UV-B radiation. However, a 50% increase of summer precipitation resulted in a 5- to 6-fold increase in activity. Long-term effects of UV-B radiation on nitrogen fixation activity have been examined only in two lichens, giving contrasting results. Peltigera aphthosa (L.) Willd., having external cephalodia, experienced a significant reduction, whereas Peltigera didactyla (With.) J.R. Laudon, having cyanobacteria in the photobiont layer below the upper cortex, did not experience any changes due to radiation regimes. The difference is probably related to the location of the cyanobacteria. While the Nostoc cells are protected by the fungal, melanized upper cortex in P. didactyla, they are exposed and unprotected in P. aphthosa, and their own synthesis of UV-B absorbing compounds appears to be low. Under certain environmental conditions, an increasing UV-B radiation will dramatically affect nitrogen fixation in arctic tundra vegetation, which in turn may have severe influence on the nitrogen budget in these environments. Further long-term studies are necessary to conclude if these effects are temporal and how concurrent climatic changes will influence the nitrogen balance of the ecosystem.
    Plant Ecology 01/2006; 182(1):109-118. · 1.53 Impact Factor
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    ABSTRACT: The response of tundra plants to enhanced UV-B radiation simulating 15 and 30% ozone depletion was studied at two high arctic sites (Isdammen and Adventdalen, 78° N, Svalbard). The set-up of the UV-B supplementation systems is described, consisting of large and small UV lamp arrays, installed in 1996 and 2002. After 7 years of exposure to enhanced UV-B radiation, plant cover, density, morphological (leaf fresh and dry weight, leaf thickness, leaf area, reproductive and ecophysiological parameters leaf UV-B absorbance, leaf phenolic content, leaf water content) were not affected by enhanced UV-B radiation. DNA damage in the leaves was not increased with enhanced UV-B in Salix polaris and Cassiope tetragona. DNA damage in Salix polaris leaves was higher than in leaves of C. tetragona. The length of male gametophyte moss plants of Polytrichum hyperboreum was reduced with elevated UV-B as well as the number of Pedicularis hirsuta plants per plot, but the inflorescence length of Bistorta vivipara was not significantly affected. We discuss the possible causes of tolerance of tundra plants to UV-B (absence of response to enhanced UV-B) in terms of methodology (supplementation versus exclusion), ecophysiological adaptations to UV-B and the biogeographical history of polar plants
    12/2005: pages 121-136;
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    ABSTRACT: In the high Arctic, soils are commonly nutrient poor and nitrogen is often the limiting factor. There, cyanobacteria play an important role because they can overcome this limitation with their ability to fix atmospheric nitrogen and carbon dioxide. The present study is part of an investigation of the spatial and temporal succession of the cyanobacterial community during the process of colonisation of a deglaciated soil. In summer 2003, during three weeks of field work in Svalbard, seven sites along a chronosequence on the foreland of the retreating glacier Midtre Lovènbreen (Ny-Ålesund, 79°N/12°E), were sampled. The sites on the chronosequence had been previously characterised for the different time they came out of the ice. We analysed the biodiversity of the cyanobacterial community beginning from bare newly exposed soils near the glacier front down to mature vegetation that has developed for about 2000 years since ice retreated. A polyphasic approach, integrating morphological and molecular analyses of environmental samples and cultured strains was applied. Since now, 40 cyanobacterial strains were isolated and microphotographed; their morphological characterisation at genus level was carried out. The work in term of strain isolation, especially for site 7, is still in course. A phylogenetic analysis of selected cyanobacterial strains representing the morphological variability found in the environment was also done. Molecular fingerprints of the cyanobacterial community were compared with morphological observations and with the isolated strains obtained from the chronosequence. The obtained results aim to new insights on the diversity and role of cyanobacteria in the Arctic.
    Algological Studies 09/2005; 117(1):427-440.
  • Arctic Antarctic and Alpine Research - ARCT ANTARCT ALP RES. 01/2005; 37(3):372-378.
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    ABSTRACT: Stratospheric ozone depletion is most pronounced at high latitudes, and the concurring increased UV-B radiation might adversely affect plants from polar areas. However, vascular plants may protect themselves against UV-B radiation by UV-absorbing compounds located in the epidermis. In this 3-year study, epidermal UV-B (max 314nm) and UV-A (max 366nm) screening was assessed using a fluorescence method in 12 vascular species growing in their natural environment at Svalbard. The potential for acclimation to increased radiation was studied with artificially increased UV-B, simulating 11% ozone depletion. Open-top chambers simulated an increase in temperature of 2–3C in addition to the UV-B manipulation. Adaxial epidermal UV-B transmittance varied between 1.6 and 11.4%. Artificially increased UV-B radiation and temperature did not consistently influence the epidermal UV-B transmittance in any of the measured species, suggesting that they may not have the potential to increase their epidermal screening, or that the screening is already high enough at the applied UV-B level. We propose that environmental factors other than UV-B radiation may influence epidermal UV-B screening.
    Polar Biology 05/2004; 27(7):383-390. · 2.01 Impact Factor
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    Jarle W. Bjerke, Matthias Zielke, Bjørn Solheim
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    ABSTRACT: 3 fig. 1 tab. ["Samples of two arctic cyanolichens of the genus Peltigera, exposed in situ to ambient and enhanced UV-B radiation and ambient and increased temperatures, were collected in 2001, 5 yr after the establishment of the experimental set-up. Thallus dimensions and size, coverage of soralia, nitrogen fixation activity and levels of UV-C-absorbing substances were measured."]
    New Phytologist 06/2003; 159(2):361 - 367. · 6.74 Impact Factor
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    ABSTRACT: Effects of enhanced UV-B (representing a 15% ozone depletion) on cyanobacterial nitrogen fixation were measured at a high arctic site (Adventdalen, 79N, Svalbard) and a subarctic site (Abisko, 68N, Sweden). Nitrogen fixation potential (acetylene reduction) by cyanobacteria associated with the moss Sanionia uncinata in vegetation exposed to experimentally enhanced levels of UV-B for 3 and 4 years in the high arctic in Adventdalen was reduced by 50% compared to controls after 3 years. No reduction in nitrogen fixation potential was observed in cyanobacteria associated with the moss Hylocomium splendens when previously exposed to enhanced UV-B in Abisko for a 7-year period. However, in the same experiment a 50% increase in summer precipitation stimulated nitrogen fixation potential by up to 6-fold above the natural precipitation treatments both in cyanobacteria associated with vegetation exposed to natural and enhanced UV-B radiation. In contrast to the lack of UV effect on moss-associated nitrogen fixation at the subarctic site, nitrogen fixation potential by the dominant lichen species Peltigera aphthosa was reduced by 50% when measured after 8 years exposure to elevated UV-B treatment. Evidence from these studies highlights the importance of UV-B radiation for cyanobacterial nitrogen fixation in the Arctic and future impact on nitrogen availability in such plant communities.
    Oecologia 08/2002; 133(1):90-93. · 3.01 Impact Factor
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    ABSTRACT: Stratospheric ozone depletion is most pro- nounced at high latitudes, and the concurring increased UV-B radiation might adversely affect plants from polar areas. However, vascular plants may protect themselves against UV-B radiation by UV-absorbing compounds located in the epidermis. In this 3-year study, epidermal UV-B (kmax 314 nm) and UV-A (kmax 366 nm) screening was assessed using a fluorescence method in 12 vascular species growing in their natural environment at Svalbard. The potential for acclimation to increased radiation was studied with artificially increased UV-B, simulating 11% ozone depletion. Open-top chambers simulated an increase in temperature of 2-3� C in addition to the UV-B manipulation. Adaxial epidermal UV-B transmittance varied between 1.6 and 11.4%. Artificially increased UV-B radiation and temperature did not consistently influence the epidermal UV-B transmittance in any of the measured species, suggesting that they may not have the potential to increase their epidermal screening, or that the screening is already high enough at the applied UV-B level. We propose that environmental factors other than UV-B radiation may influence epidermal UV-B screening.
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    BJORN SOLHEIM
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    ABSTRACT: The effect of grazing and fertilization by barnacle geese (Branta leucopsis) on cyanobacterial nitrogen fixation and plant biomass productivity in a high arctic habitat was studied in a long- term experiment. In an area with high natural grazing pressure moss-dominated vegetation was exclosed from geese grazing over a period of six growing seasons and annually manipulated by the following treatments: (i) clipping of the vegetation, (ii) addition of goose droppings, (iii) a combination of (i) and (ii), and (iv) exclosing without additional treatment. In order to compare the treated vegetation with naturally grazed vegetation, samples from the nearby surrounding of the exclosures were included as an additional treatment in the analyses. Plant biomass and cyanobacterial nitrogen fixation activity of the vegetation were measured, and the community structure of moss-associated cyanobacteria was analyzed. In general, when droppings were added to the vegetation, plant biomass production was increased while nitrogen fixation activity decreased. In respect to the biomass this may be explained by an enhanced primary production caused by a higher availability of nutrients, while nitrogen compounds released from the droppings inhibit the nitrogen fixation. Furthermore, the clipping treatment caused an increase both in plant biomass and in nitrogen fixation activity and can, in case of the biomass production, be explained by an overcompensation effect that is known and previously described for grazed plants. In case of nitrogen fixation the removal of biomass reduced the net nitrogen content of the soil, which in turn has a stimulating effect on nitrogen fixation. Although the grazing by geese outside the exclosure was more intensive as than the clipping, it caused only a slight increase in nitrogen fixation. This may be explained by the inhibitory effect of nitrogen compounds both from droppings originating from the grazing geese and the trampling effect of the geese increasing mineralization of plant litter. Biomass production in this treatment was partly reduced, i.e. the biomass of mosses was unaffected while the grass biomass dropped to zero, and is explained by the geese's high preference for grass plants as forage. No treatment effect on the cyanobacterial community could be detected. In conclusion, grazing and fertilization by geese have a significant effects on plant biomass production and nitrogen fixation activity without significantly altering the cyanobacterial community structure. Whether these effects are positive or negative depends on the process affected and on the intensity of the grazing and fertilization. To predict the total impact of geese grazing and fertilization in terrestrial arctic habitats all factors has to be considered.

Publication Stats

284 Citations
77.36 Total Impact Points

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Institutions

  • 2002–2010
    • Universitetet i Tromsø
      Tromsø, Troms, Norway
  • 2006
    • Norwegian Institute for Nature Research
      Nidaros, Sør-Trøndelag, Norway
  • 2004
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany
  • 2003
    • University Centre in Svalbard
      Longyearbyen, Svalbard, Svalbard and Jan Mayen