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1.
  • Agrell, Jep, et al. (författare)
  • CO2 and O-3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria)
  • 2005
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 11:4, s. 588-599
  • Tidskriftsartikel (refereegranskat)abstract
    • Elevated levels of CO2 and O-3 affect plant growth and phytochemistry, which in turn can alter physiological performance of associated herbivores. Little is known, however, about how generalist insect herbivores respond behaviorally to CO2- and O-3-mediated changes in their host plants. This research examined the effects of elevated CO2 and O-3 levels on host plant preferences and consumption of forest tent caterpillar (FTC, Malacosoma disstria Hbn.) larvae. Dual choice feeding assays were performed with foliage from birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259). Trees were grown at the Aspen Free Air CO2 Enrichment (FACE) facility near Rhinelander, WI, USA, and had been exposed to ambient or elevated concentrations of CO2 and/or O-3. Levels of nutritional and secondary compounds were quantified through phytochemical analyses. The results showed that elevated O-3 levels increased FTC larval preferences for birch compared with aspen, whereas elevated CO2 levels had the opposite effect. In assays with the two aspen genotypes, addition of both CO2 and O-3 caused a shift in feeding preferences from genotype 259 to genotype 216. Consumption was unaffected by experimental treatments in assays comparing aspen and birch, but were increased for larvae given high O-3 foliage in the aspen genotype assays. Elevated levels of CO2 and O-3 altered tree phytochemistry, but did not explain shifts in feeding preferences. The results demonstrate that increased levels of CO2 and O-3 can alter insect host plant preferences both between and within tree species. Also, consequences of altered host quality (e.g., compensatory consumption) may be buffered by partial host shifts in situations when alternative plant species are available. Environmentally induced changes in host plant preferences may have the potential to alter the distribution of herbivory across plant genotypes and species, as well as competitive interactions among them.
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2.
  • Bondeau, Alberte, et al. (författare)
  • Modelling the role of agriculture for the 20th century global terrestrial carbon balance
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:3, s. 679-706
  • Forskningsöversikt (refereegranskat)abstract
    • In order to better assess the role of agriculture within the global climate-vegetation system, we present a model of the managed planetary land surface, Lund-Potsdam-Jena managed Land (LPJmL), which simulates biophysical and biogeochemical processes as well as productivity and yield of the most important crops worldwide, using a concept of crop functional types (CFTs). Based on the LPJ-Dynamic Global Vegetation Model, LPJmL simulates the transient changes in carbon and water cycles due to land use, the specific phenology and seasonal CO2 fluxes of agricultural-dominated areas, and the production of crops and grazing land. It uses 13 CFTs (11 arable crops and two managed grass types), with specific parameterizations of phenology connected to leaf area development. Carbon is allocated daily towards four carbon pools, one being the yield-bearing storage organs. Management (irrigation, treatment of residues, intercropping) can be considered in order to capture their effect on productivity, on soil organic carbon and on carbon extracted from the ecosystem. For transient simulations for the 20th century, a global historical land use data set was developed, providing the annual cover fraction of the 13 CFTs, rain-fed and/or irrigated, within 0.5 degrees grid cells for the period 1901-2000, using published data on land use, crop distributions and irrigated areas. Several key results are compared with observations. The simulated spatial distribution of sowing dates for temperate cereals is comparable with the reported crop calendars. The simulated seasonal canopy development agrees better with satellite observations when actual cropland distribution is taken into account. Simulated yields for temperate cereals and maize compare well with FAO statistics. Monthly carbon fluxes measured at three agricultural sites also compare well with simulations. Global simulations indicate a similar to 24% (respectively similar to 10%) reduction in global vegetation (respectively soil) carbon due to agriculture, and 6-9 Pg C of yearly harvested biomass in the 1990s. In contrast to simulations of the potential natural vegetation showing the land biosphere to be an increasing carbon sink during the 20th century, LPJmL simulates a net carbon source until the 1970s (due to land use), and a small sink (mostly due to changing climate and CO2) after 1970. This is comparable with earlier LPJ simulations using a more simple land use scheme, and within the uncertainty range of estimates in the 1980s and 1990s. The fluxes attributed to land use change compare well with Houghton's estimates on the land use related fluxes until the 1970s, but then they begin to diverge, probably due to the different rates of deforestation considered. The simulated impacts of agriculture on the global water cycle for the 1990s are similar to 5% (respectively similar to 20%) reduction in transpiration (respectively interception), and similar to 44% increase in evaporation. Global runoff, which includes a simple irrigation scheme, is practically not affected.
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3.
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4.
  • Friend, Andrew D., et al. (författare)
  • FLUXNET and modelling the global carbon cycle
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:3, s. 610-633
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of the net CO2 flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO2 source-sink patterns, CO2 flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO2 (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model-data comparison. Flux measurements made over four vegetation types were used to calibrate the land-surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land-surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO2 uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite-measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982-1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO2 measurements. Combining the PEM, DGVM, and inversion results suggests that CO2 fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO2 uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of -1.2 Pg [C] yr(-1) for 1982-1995 is compatible with the PEM- and DGVM-predicted trends in NPP. There is, thus, a convergence in understanding derived from process-based models, remote-sensing-based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night-time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land-atmosphere CO2 fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO2 uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO2 variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO2 exchange.
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5.
  • Haapala, Jaana K., et al. (författare)
  • Carbon dioxide balance of a fen ecosystem in northern Finland under elevated UV-B radiation
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:4, s. 943-954
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of elevated UV-B radiation on CO2 exchange of a natural flark fen was studied in open-field conditions during 2003-2005. The experimental site was located in SodankylA in northern Finland (67 degrees 22'N, 26 degrees 38'E, 179 m a.s.l.). Altogether 30 study plots, each 120 cm x 120 cm in size, were randomly distributed between three treatments (n=10): ambient control, UV-A control and UV-B treatment. The UV-B-treated plots were exposed to elevated UV-B radiation level for three growing seasons. The instantaneous net ecosystem CO2 exchange (NEE) and dark respiration (R-TOT) were measured during the growing season using a closed chamber method. The wintertime CO2 emissions were estimated using a gradient technique by analyzing the CO2 concentration in the snow pack. In addition to the instantaneous CO2 exchange, the seasonal CO2 balances during the growing seasons were modeled using environmental data measured at the site. In general, the instantaneous NEE at light saturation was slightly higher in the UV-B treatment compared with the ambient control, but the gross photosynthesis was unaffected by the exposure. The R-TOT was significantly lower under elevated UV-B in the third study year. The modeled seasonal (June-September) CO2 balance varied between the years depending on the ground water level and temperature conditions. During the driest year, the seasonal CO2 balance was negative (net release of CO2) in the ambient control and the UV-B treatment was CO2 neutral. During the third year, the seasonal CO2 uptake was 43 +/- 36 g CO2-C m(-2) in the ambient control and 79 +/- 45 g CO2-C m(-2) in the UV-B treatment. The results suggest that the long-term exposure to high UV-B radiation levels may slightly increase the CO2 accumulation to fens resulting from a decrease in microbial activity in peat. However, it is unlikely that the predicted development of the level of UV-B radiation would significantly affect the CO2 balance of fen ecosystems in future.
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6.
  • Jardine, Kolby J., et al. (författare)
  • Within-plant isoprene oxidation confirmed by direct emissions of oxidation products methyl vinyl ketone and methacrolein
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:3, s. 973-984
  • Tidskriftsartikel (refereegranskat)abstract
    • Isoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress-induced accumulation of reactive oxygen species (ROS), but the products of isoprene-ROS reactions in plants have not been detected. Using pyruvate-2-13C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (iox) in leaves and that iox/i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of iox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biotachemistryclimate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change.
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7.
  • Johansson, T, et al. (författare)
  • Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net radiative forcing
  • 2006
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 12:12, s. 2352-2369
  • Tidskriftsartikel (refereegranskat)abstract
    • Thawing permafrost in the sub-Arctic has implications for the physical stability and biological dynamics of peatland ecosystems. This study provides an analysis of how permafrost thawing and subsequent vegetation changes in a sub-Arctic Swedish mire have changed the net exchange of greenhouse gases, carbon dioxide (CO2) and CH4 over the past three decades. Images of the mire (ca. 17 ha) and surroundings taken with film sensitive in the visible and the near infrared portion of the spectrum, [i.e. colour infrared (CIR) aerial photographs from 1970 and 2000] were used. The results show that during this period the area covered by hummock vegetation decreased by more than 11% and became replaced by wet-growing plant communities. The overall net uptake of C in the vegetation and the release of C by heterotrophic respiration might have increased resulting in increases in both the growing season atmospheric CO2 sink function with about 16% and the CH4 emissions with 22%. Calculating the flux as CO2 equivalents show that the mire in 2000 has a 47% greater radiative forcing on the atmosphere using a 100-year time horizon. Northern peatlands in areas with thawing sporadic or discontinuous permafrost are likely to act as larger greenhouse gas sources over the growing season today than a few decades ago because of increased CH4 emissions.
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8.
  • Jönsson, Anna Maria, et al. (författare)
  • Spatio-temporal impact of climate change on the activity and voltinism of the spruce bark beetle, Ips typographus
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:2, s. 486-499
  • Tidskriftsartikel (refereegranskat)abstract
    • The spruce bark beetle Ips typographus is one of the major insect pests of mature Norway spruce forests. In this study, a model describing the temperature-dependent thresholds for swarming activity and temperature requirement for development from egg to adult was driven by transient regional climate scenario data for Sweden, covering the period of 1961-2100 for three future climate change scenarios (SRES A2, A1B and B2). During the 20th century, the weather supported the production of one bark beetle generation per year, except in the north-western mountainous parts of Sweden where the climate conditions were too harsh. A warmer climate may sustain a viable population also in the mountainous part; however, the distributional range of I. typographus may be restricted by the migration speed of Norway spruce. Modelling suggests that an earlier timing of spring swarming and fulfilled development of the first generation will significantly increase the frequency of summer swarming. Model calculations suggest that the spruce bark beetle will be able to initiate a second generation in South Sweden during 50% of the years around the mid century. By the end of the century, when temperatures during the bark beetle activity period are projected to have increased by 2.4-3.8 degrees C, a second generation will be initiated in South Sweden in 63-81% of the years. The corresponding figures are 16-33% for Mid Sweden, and 1-6% for North Sweden. During the next decades, one to two generations per year are predicted in response to temperature, and the northern distribution limit for the second generation will vary. Our study addresses questions applicable to sustainable forest management, suggesting that adequate countermeasures require monitoring of regional differences in timing of swarming and development of I. typographus, and planning of control operations during summer periods with large populations of bark beetles.
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9.
  • Kasurinen, A, et al. (författare)
  • Below-ground responses of silver birch trees exposed to elevated CO2 and O-3 levels during three growing seasons
  • 2005
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 11:7, s. 1167-1179
  • Tidskriftsartikel (refereegranskat)abstract
    • Field-growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO2 and O-3 in open-top chambers for three consecutive growing seasons (1999-2001). At the beginning of the OTC experiment, all trees were 7 years old. We studied the single and interaction effects of CO2 and O-3 on silver birch below-ground carbon pools (i.e. effects on fine roots and mycorrhizas, soil microbial communities and sporocarp production) and also assessed whether there are any clonal differences in these below-ground CO2 and O-3 responses. The total mycorrhizal infection level of both clones was stimulated by elevated CO2 alone and elevated O-3 alone, but not when elevated CO2 was used in fumigation in combination with elevated O-3. In both clones, elevated CO2 affected negatively light brown/orange mycorrhizas, while its effect on other mycorrhizal morphotypes was negligible. Elevated O-3, instead, clearly decreased the proportions of black and liver-brown mycorrhizas and increased that of light brown/orange mycorrhizas. Elevated O-3 had a tendency to decrease standing fine root mass and sporocarp production as well, both of these O-3 effects mainly manifesting in clone 4 trees. CO2 and O-3 treatment effects on soil microbial community composition (PLFA, 2- and 3-OH-FA profiles) were negligible, but quantitative PLFA data showed that in 2001 the PLFA fungi : bacteria-ratio of clone 80 trees was marginally increased because of elevated CO2 treatments. This study shows that O-3 effects were most clearly visible at the mycorrhizal root level and that some clonal differences in CO2 and O-3 responses were observable in the below-ground carbon pools. In conclusion, the present data suggests that CO2 effects were minor, whereas increasing tropospheric O-3 levels can be an important stress factor in northern birch forests, as they might alter mycorrhizal morphotype assemblages, mycorrhizal infection rates and sporocarp production.
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10.
  • Kokfelt, Ulla, et al. (författare)
  • Ecosystem responses to increased precipitation and permafrost decay in subarctic Sweden inferred from peat and lake sediments
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:7, s. 1652-1663
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake-water, and changes in δ13C and C, N and δ15N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake-water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June–September (JJAS) precipitation and that the increase in lake-water TOC concentration reflects an increase in total annual precipitation. A significant depletion in 13C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake-sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.
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11.
  • Lasslop, Gitta, et al. (författare)
  • Separation of net ecosystem exchange into assimilation and respiration using a light response curve approach: critical issues and global evaluation
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:1, s. 187-208
  • Forskningsöversikt (refereegranskat)abstract
    • The measured net ecosystem exchange (NEE) of CO2 between the ecosystem and the atmosphere reflects the balance between gross CO2 assimilation [gross primary production (GPP)] and ecosystem respiration (R-eco). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light-response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of R-eco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher R-eco. Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 g C m(-2) yr(-1)), while errors arising from the random error (median absolute deviation: similar to 2 g C m(-2) yr(-1)) are negligible. Despite site-specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and R-eco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.
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12.
  • Lindroth, Anders, et al. (författare)
  • Storms can cause Europe-wide reduction in forest carbon sink
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:2, s. 346-355
  • Tidskriftsartikel (refereegranskat)abstract
    • Disturbance of ecosystems is a major factor in regional carbon budgets, and it is believed to be partly responsible for the large inter-annual variability of the terrestrial part of the carbon balance. Forest fires have so far been considered as the most important disturbance but also other forms of disturbance such as insect outbreaks or wind-throw might contribute significantly to the largely unexplained inter-annual variability, at least in specific regions. The effect of wind-throw has not yet been estimated because of lack of data on how carbon fluxes are affected. The Gudrun storm, which hit Sweden in January 2005, resulted in ca. 66 million m(3) of wind-thrown stem wood on an area of ca. 272 000 ha. Using a model (BIOME-BGC) calibrated to CO2 flux measurements at two sites, the annual net ecosystem productivity during the first year after the storm was estimated to be in the range -897 to -1259 g C m(-2) yr(-1). This is a much higher loss compared with harvested (clear-cut) forests in Europe, which ranged between ca. -420 and -100 g m(-2) yr(-1). The reduction in the carbon sink scaled to the whole wind-thrown area was estimated at ca. 3 million tons C during the first year. By historical data on wind-throw in Europe combined with modelling, we estimated that the large Lothar storm in 1999 reduced the European carbon balance by ca. 16 million tons C, this is ca. 30% of the net biome production in Europe. We conclude that the impact of increased forest damage by more frequent storms in future climate change scenarios must be considered and that intermittent large wind-throw events may explain a part of the large inter-annual variability in the terrestrial carbon sink.
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13.
  • Lund, Magnus, et al. (författare)
  • Variability in exchange of CO2 across 12 northern peatland and tundra sites
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:9, s. 2436-2448
  • Tidskriftsartikel (refereegranskat)abstract
    • Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO2) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1-7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R-eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R-eco. After this correction neither R-eco (P = 0.162) nor GPP (P = 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R-eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R-eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R-eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R-eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.
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14.
  • Luyssaert, S., et al. (författare)
  • CO2 balance of boreal, temperate, and tropical forests derived from a global database
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:12, s. 2509-2537
  • Forskningsöversikt (refereegranskat)abstract
    • Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 degrees C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO2 carbon fluxes are not presently being adequately accounted for.
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15.
  • Malmer, Nils, et al. (författare)
  • Vegetation, climatic changes and net carbon sequestration in a North-Scandinavian subarctic mire over 30 years
  • 2005
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 11:11, s. 1895-1909
  • Tidskriftsartikel (refereegranskat)abstract
    • This study deals with changes in the plant cover and its net carbon sequestration over 30 years on a subarctic Sphagnum-mire with permafrost near Abisko, northernmost Sweden, in relation to climatic variations during the same period. Aerial colour infrared images from 1970 and 2000 were compared to reveal changes in surface structure and vegetation over the whole mire, while the plant populations were studied within a smaller, mainly ombrotrophic part. The results demonstrated two processes, namely (1) that wet sites dominated by graminoids expanded while hummock sites dominated by dwarf shrubs receded, and (2) that on the hummocks lichens expanded while evergreen dwarf shrubs and mosses decreased, both processes creating an instability in the surface structure. A successive degradation of the permafrost is the likely reason for the increase in wet areas, while the changes in the hummock vegetation might have resulted from higher spring temperatures giving rise to an intensified snow melt, exposing the vegetation to frost drought. Because of the vegetation changes, the annual litter input of carbon to the mire has increased slightly, by 4 g m2 a1 (7.3%), over these years while an increased erosion has resulted in a loss of 40–80 Mg carbon or 7–17 g m2 a1 for the entire mire over the same period. As the recalcitrant proportion of the litter has decreased, the decay rate in the acrotelm might be expected to increase in the future.
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16.
  • Mills, Gina, et al. (författare)
  • Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990 - 2006) in relation to AOT40 - and flux-based risk maps
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1365-2486 .- 1354-1013. ; 17:1, s. 592-613
  • Tidskriftsartikel (refereegranskat)abstract
    • Records of effects of ambient ozone pollution on vegetation have been compiled for Europe for the years 1990–2006. Sources include scientific papers, conference proceedings, reports to research funders, records of confirmed ozone injury symptoms and an international biomonitoring experiment coordinated by the ICP Vegetation. The latter involved ozone-sensitive (NC-S) and ozone-resistant (NC-R) biotypes of white clover (Trifolium repens L.) grown according to a common protocol and monitored for ozone injury and biomass differences in 17 European countries, from 1996 to 2006. Effects were separated into visible injury or growth/yield reduction. Of the 644 records of visible injury, 39% were for crops (27 species), 38.1% were for (semi-) natural vegetation (95 species) and 22.9% were for shrubs (49 species). Owing to inconsistencies in reporting effort from year to year it was not possible to determine geographical or temporal trends in the data. Nevertheless, this study has shown effects in ambient air in 18 European countries from Sweden in the north to Greece in the south. These effects data were superimposed on AOT40 (accumulated ozone concentrations over 40 ppb) and POD3gen (modelled accumulated stomatal flux over a threshold of 3 nmol m−2 s−1) maps generated by the EMEP Eulerian model (50 km × 50 km grid) that were parameterized for a generic crop based on wheat and NC-S/NC-R white clover. Many effects were found in areas where the AOT40 (crops) was below the critical level of 3 ppm h. In contrast, the majority of effects were detected in grid squares where POD3gen (crops) were in the mid-high range (>12 mmol m−2). Overall, maps based on POD3gen provided better fit to the effects data than those based on AOT40, with the POD3gen model for clover fitting the clover effects data better than that for a generic crop.
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17.
  • Morales, Pablo, et al. (författare)
  • Changes in European ecosystem productivity and carbon balance driven by regional climate model output
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:1, s. 108-122
  • Forskningsöversikt (refereegranskat)abstract
    • Climate change resulting from the enhanced greenhouse effect together with the direct effect of increased atmospheric CO2 concentrations on vegetation growth are expected to produce changes in the cycling of carbon in terrestrial ecosystems. Impacts will vary across Europe, and regional-scale studies are needed to resolve this variability. In this study, we used the LPJ-GUESS ecosystem model driven by a suite of regional climate model (RCM) scenarios from the European Union (EU) project PRUDENCE to estimate climate impacts on carbon cycling across Europe. We identified similarities and discrepancies in simulated climate impacts across scenarios, particularly analyzing the uncertainties arising from the range of climate models and emissions scenarios considered. Our results suggest that net primary production (NPP) and heterotrophic respiration (Rh) will generally increase throughout Europe, but with considerable variation between European subregions. The smallest NPP increases, and in some cases decreases, occurred in the Mediterranean, where many ecosystems switched from sinks to sources of carbon by 2100, mainly as a result of deteriorating water balance. Over the period 1991-2100, modeled climate change impacts on the European carbon balance ranged from a sink of 11.6 Gt C to a source of 3.3 Gt C, the average annual sink corresponding with 1.85% of the current EU anthropogenic emissions. Projected changes in carbon balance were more dependent on the choice of the general circulation model (GCM) providing boundary conditions to the RCM than the choice of RCM or the level of anthropogenic greenhouse gases emissions.
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18.
  • Morales, Pablo, et al. (författare)
  • Comparing and evaluating process-based ecosystem model predictions of carbon and water fluxes in major European forest biomes
  • 2005
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 11:12, s. 2211-2233
  • Tidskriftsartikel (refereegranskat)abstract
    • Process-based models can be classified into: (a) terrestrial biogeochemical models (TBMs), which simulate fluxes of carbon, water and nitrogen coupled within terrestrial ecosystems, and (b) dynamic global vegetation models (DGVMs), which further couple these processes interactively with changes in slow ecosystem processes depending on resource competition, establishment, growth and mortality of different vegetation types. In this study, four models - RHESSys, GOTILWA+, LPJ-GUESS and ORCHIDEE - representing both modelling approaches were compared and evaluated against benchmarks provided by eddy-covariance measurements of carbon and water fluxes at 15 forest sites within the EUROFLUX project. Overall, model-measurement agreement varied greatly among sites. Both modelling approaches have somewhat different strengths, but there was no model among those tested that universally performed well on the two variables evaluated. Small biases and errors suggest that ORCHIDEE and GOTILWA+ performed better in simulating carbon fluxes while LPJ-GUESS and RHESSys did a better job in simulating water fluxes. In general, the models can be considered as useful tools for studies of climate change impacts on carbon and water cycling in forests. However, the various sources of variation among models simulations and between models simulations and observed data described in this study place some constraints on the results and to some extent reduce their reliability. For example, at most sites in the Mediterranean region all models generally performed poorly most likely because of problems in the representation of water stress effects on both carbon uptake by photosynthesis and carbon release by heterotrophic respiration (R-h). The use of flux data as a means of assessing key processes in models of this type is an important approach to improving model performance. Our results show that the models have value but that further model development is necessary with regard to the representation of the some of the key ecosystem processes.
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19.
  • Nilsson, Mats, et al. (författare)
  • Contemporary carbon accumulation in a boreal oligotrophic minerogenic mire - a significant sink after accounting for all C-fluxes
  • 2008
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 14:10, s. 2317-2332
  • Tidskriftsartikel (refereegranskat)abstract
    • Based on theories of mire development and responses to a changing climate, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the current net C-exchange in mires requires measurements of all relevant fluxes. Estimates of annual total carbon budgets in mires are still very limited. Here, we present a full carbon budget over 2 years for a boreal minerogenic oligotrophic mire in northern Sweden (64 degrees 11'N, 19 degrees 33'E). Data on the following fluxes were collected: land-atmosphere CO2 exchange (continuous Eddy covariance measurements) and CH4 exchange (static chambers during the snow free period); TOC (total organic carbon) in precipitation; loss of TOC, dissolved inorganic carbon (DIC) and CH4 through stream water runoff (continuous discharge measurements and regular C-concentration measurements). The mire constituted a net sink of 27 +/- 3.4 (+/- SD) g C m(-2) yr(-1) during 2004 and 20 +/- 3.4 g C m(-2) yr(-1) during 2005. This could be partitioned into an annual surface-atmosphere CO2 net uptake of 55 +/- 1.9 g C m(-2) yr(-1) during 2004 and 48 +/- 1.6 g C m(-2) yr(-1) during 2005. The annual NEE was further separated into a net uptake season, with an uptake of 92 g C m(-2) yr(-1) during 2004 and 86 g C m(-2) yr(-1) during 2005, and a net loss season with a loss of 37 g C m(-2) yr(-1) during 2004 and 38 g C m(-2) yr(-1) during 2005. Of the annual net CO2-C uptake, 37% and 31% was lost through runoff (with runoff TOC > DIC >> CH4) and 16% and 29% through methane emission during 2004 and 2005, respectively. This mire is still a significant C-sink, with carbon accumulation rates comparable to the long-term Holocene C-accumulation, and higher than the C-accumulation during the late Holocene in the region.
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20.
  • Norkko, J., et al. (författare)
  • A welcome can of worms? Hypoxia mitigation by an invasive species
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:2, s. 422-434
  • Tidskriftsartikel (refereegranskat)abstract
    • Invasive species and bottom-water hypoxia both constitute major global threats to the diversity and integrity of marine ecosystems. These stressors may interact with unexpected consequences, as invasive species that require an initial environmental disturbance to become established can subsequently become important drivers of ecological change. There is recent evidence that improved bottom-water oxygen conditions in coastal areas of the northern Baltic Sea coincide with increased abundances of the invasive polychaetes Marenzelleria spp. Using a reactive-transport model, we demonstrate that the long-term bioirrigation activities of dense Marenzelleria populations have a major impact on sedimentary phosphorus dynamics. This may facilitate the switch from a seasonally hypoxic system back to a normoxic system by reducing the potential for sediment-induced eutrophication in the upper water column. In contrast to short-term laboratory experiments, our simulations, which cover a 10-year period, show that Marenzelleria has the potential to enhance long-term phosphorus retention in muddy sediments. Over time bioirrigation leads to a substantial increase in the iron-bound phosphorus content of sediments while reducing the concentration of labile organic carbon. As surface sediments are maintained oxic, iron oxyhydroxides are able to persist and age into more refractory forms. The model illustrates mechanisms through which Marenzelleria can act as a driver of ecological change, although hypoxic disturbance or natural population declines in native species may be needed for them to initially become established. Invasive species are generally considered to have a negative impact; however, we show here that one of the main recent invaders in the Baltic Sea may provide important ecosystem services. This may be of particular importance in low-diversity systems, where disturbances may dramatically alter ecosystem services due to low functional redundancy. Thus, an environmental problem in one region may be either exacerbated or alleviated by a single species from another region, with potentially ecosystem-wide consequences.
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21.
  • Rinnan, Riikka, et al. (författare)
  • Fifteen years of climate change manipulations alter soil microbial communities in a subarctic heath ecosystem
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:1, s. 28-39
  • Forskningsöversikt (refereegranskat)abstract
    • Soil microbial biomass in arctic heaths has been shown to be largely unaffected by treatments simulating climate change with temperature, nutrient and light manipulations. Here, we demonstrate that more than 10 years is needed for development of significant responses, and that changes in microbial biomass are accompanied with strong alterations in microbial community composition. In contrast to slight or nonsignificant responses after 5, 6 and 10 treatment years, 15 years of inorganic NPK fertilizer addition to a subarctic heath had strong effects on the microbial community and, as observed for the first time, warming and shading also led to significant responses, often in opposite direction to the fertilization responses. The effects were clearer in the top 5 cm soil than at the 5-10 cm depth. Fertilization increased microbial biomass C and more than doubled microbial biomass P compared to the non-fertilized plots. However, it only increased microbial biomass N at the 5-10 cm depth. Fertilization increased fungal biomass and the relative abundance of phospholipid fatty acid (PLFA) markers of gram-positive bacteria. Warming and shading decreased the relative abundance of fungal PLFAs, and shading also altered the composition of the bacterial community. The long time lag in responses may be associated with indirect effects of the gradual changes in the plant biomass and community composition. The contrasting responses to warming and fertilization treatments show that results from fertilizer addition may not be similar to the effects of increased nutrient mineralization and availability following climatic warming.
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22.
  • Rinnan, Riikka, et al. (författare)
  • Temperature adaptation of soil bacterial communities along an Antarctic climate gradient: predicting responses to climate warming
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:11, s. 2615-2625
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil microorganisms, the central drivers of terrestrial Antarctic ecosystems, are being confronted with increasing temperatures as parts of the continent experience considerable warming. Here we determined short-term temperature dependencies of Antarctic soil bacterial community growth rates, using the leucine incorporation technique, in order to predict future changes in temperature sensitivity of resident soil bacterial communities. Soil samples were collected along a climate gradient consisting of locations on the Antarctic Peninsula ( Anchorage Island, 67 degrees 34'S, 68 degrees 08'W), Signy Island (60 degrees 43'S, 45 degrees 38'W) and the Falkland Islands (51 degrees 76'S 59 degrees 03'W). At each location, experimental plots were subjected to warming by open top chambers (OTCs) and paired with control plots on vegetated and fell-field habitats. The bacterial communities were adapted to the mean annual temperature of their environment, as shown by a significant correlation between the mean annual soil temperature and the minimum temperature for bacterial growth (T-min). Every 1 degrees C rise in soil temperature was estimated to increase T-min by 0.24-0.38 degrees C. The optimum temperature for bacterial growth varied less and did not have as clear a relationship with soil temperature. Temperature sensitivity, indicated by Q(10) values, increased with mean annual soil temperature, suggesting that bacterial communities from colder regions were less temperature sensitive than those from the warmer regions. The OTC warming (generally <1 degrees C temperature increases) over 3 years had no effects on temperature relationship of the soil bacterial community. We estimate that the predicted temperature increase of 2.6 degrees C for the Antarctic Peninsula would increase T-min by 0.6-1 degrees C and Q(10) (0-10 degrees C) by 0.5 units.
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23.
  • Rousk, Johannes, et al. (författare)
  • Temperature adaptation of bacterial communities in experimentally warmed forest soils
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:10, s. 3252-3258
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed understanding of the influence of temperature on soil microbial activity is critical to predict future atmospheric CO2 concentrations and feedbacks to anthropogenic warming. We investigated soils exposed to 3-4 years of continuous 5 degrees C-warming in a field experiment in a temperate forest. We found that an index for the temperature adaptation of the microbial community, T-min for bacterial growth, increased by 0.19 degrees C per 1 degrees C rise in temperature, showing a community shift towards one adapted to higher temperature with a higher temperature sensitivity (Q(10(5-15 degrees C)) increased by 0.08 units per 1 degrees C). Using continuously measured temperature data from the field experiment we modelled in situ bacterial growth. Assuming that warming did not affect resource availability, bacterial growth was modelled to become 60% higher in warmed compared to the control plots, with the effect of temperature adaptation of the community only having a small effect on overall bacterial growth (<5%). However, 3 years of warming decreased bacterial growth, most likely due to substrate depletion because of the initially higher growth in warmed plots. When this was factored in, the result was similar rates of modelled in situ bacterial growth in warmed and control plots after 3 years, despite the temperature difference. We conclude that although temperature adaptation for bacterial growth to higher temperatures was detectable, its influence on annual bacterial growth was minor, and overshadowed by the direct temperature effect on growth rates.
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24.
  • Schwalm, Christopher R., et al. (författare)
  • Assimilation exceeds respiration sensitivity to drought: A FLUXNET synthesis
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:2, s. 657-670
  • Tidskriftsartikel (refereegranskat)abstract
    • The intensification of the hydrological cycle, with an observed and modeled increase in drought incidence and severity, underscores the need to quantify drought effects on carbon cycling and the terrestrial sink. FLUXNET, a global network of eddy covariance towers, provides dense data streams of meteorological data, and through flux partitioning and gap filling algorithms, estimates of net ecosystem productivity (F-NEP), gross ecosystem productivity (P), and ecosystem respiration (R). We analyzed the functional relationship of these three carbon fluxes relative to evaporative fraction (EF), an index of drought and site water status, using monthly data records from 238 micrometeorological tower sites distributed globally across 11 biomes. The analysis was based on relative anomalies of both EF and carbon fluxes and focused on drought episodes by biome and climatic season. Globally P was approximate to 50% more sensitive to a drought event than R. Network-wide drought-induced decreases in carbon flux averaged -16.6 and -9.3 g C m-2 month-1 for P and R, i.e., drought events induced a net decline in the terrestrial sink. However, in evergreen forests and wetlands drought was coincident with an increase in P or R during parts of the growing season. The most robust relationships between carbon flux and EF occurred during climatic spring for F-NEP and in climatic summer for P and R. Upscaling flux sensitivities to a global map showed that spatial patterns for all three carbon fluxes were linked to the distribution of croplands. Agricultural areas exhibited the highest sensitivity whereas the tropical region had minimal sensitivity to drought. Combining gridded flux sensitivities with their uncertainties and the spatial grid of FLUXNET revealed that a more robust quantification of carbon flux response to drought requires additional towers in all biomes of Africa and Asia as well as in the cropland, shrubland, savannah, and wetland biomes globally.
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25.
  • Ellison, David, et al. (författare)
  • On the forest cover-water yield debate : from demand- to supply-side thinking
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:3, s. 806-820
  • Forskningsöversikt (refereegranskat)abstract
    • Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more difficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that forest cover is inextricably linked to precipitation. Forest-driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross-continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Seasonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydrologic cycle. While evidence supports both sides of the argument trees can reduce runoff at the small catchment scale at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforestation, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy-making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.
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26.
  • Weyhenmeyer, Gesa A., et al. (författare)
  • Nitrogen deposition induced changes in DOC:NO3-N ratios determine the efficiency of nitrate removal from freshwaters
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:8, s. 2358-2365
  • Tidskriftsartikel (refereegranskat)abstract
    • With a unique data set comprising 1041 boreal forested and low human impacted lakes included in three Swedish lake inventories for 1995, 2000 and 2005 and with time series for 12 of the lakes from 1988 to 2008 we show that nitrate-nitrogen (NO3-N) is accumulated in freshwaters along with increasing atmospheric nitrogen deposition (Ndep). At the same time we observe decreasing DOC : NO3-N ratios in the water column. We suggest that NO3-N is accumulated in freshwaters when denitrifying bacteria are limited by their energy source rather than the availability of NO3-N, i.e. at low DOC : NO3-N ratios. We obtained further support for a close relationship between Ndep driven DOC : NO3-N ratios and the efficiency of nitrate removal by using a published global data set on measured nitrate removal rates in unproductive reference streams. Owing to the currently decreasing Ndep in large regions of, for instance, Northern Europe, this process is now reversed, resulting in increasing DOC : NO3-N ratios and more efficient nitrate removal from freshwaters. Depending on NOx emissions, nitrogen limited regions may expand with an immediate effect on nitrate concentrations in freshwaters.
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27.
  • Wikner, Johan, et al. (författare)
  • Increased freshwater discharge shifts the trophic balance in the coastal zone of the northern Baltic Sea
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:8, s. 2509-2519
  • Tidskriftsartikel (refereegranskat)abstract
    • Increased precipitation is one projected outcome of climate change that may enhance the discharge of freshwater to the coastal zone. The resulting lower salinity, and associated discharge of both nutrients and dissolved organic carbon, may influence food web functioning. The scope of this study was to determine the net outcome of increased freshwater discharge on the balance between auto- and heterotrophic processes in the coastal zone. By using long-term ecological time series data covering 13 similar to years, we show that increased river discharge suppresses phytoplankton biomass production and shifts the carbon flow towards microbial heterotrophy. A 76% increase in freshwater discharge resulted in a 2.2 times higher ratio of bacterio- to phytoplankton production (Pb:Pp). The level of Pb:Pp is a function of riverine total organic carbon supply to the coastal zone. This is mainly due to the negative effect of freshwater and total organic carbon discharge on phytoplankton growth, despite a concomitant increase in discharge of nitrogen and phosphorus. With a time lag of 2 similar to years the bacterial production recovered after an initial decline, further synergistically elevating the microbial heterotrophy. Current climate change projections suggesting increased precipitation may therefore lead to increased microbial heterotrophy, thereby decreasing the transfer efficiency of biomass to higher trophic levels. This prognosis would suggest reduced fish production and lower sedimentation rates of phytoplankton, a factor of detriment to benthic fauna. Our findings show that discharge of freshwater and total organic carbon significantly contributes to the balance of coastal processes at large spatial and temporal scales, and that model's would be greatly augmented by the inclusion of these environmental drivers as regulators of coastal productivity.
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28.
  • Algesten, Grete, et al. (författare)
  • Role of lakes for organic carbon cycling in the boreal zone
  • 2004
  • Ingår i: Global Change Biology. - Oxford : Blackwell Scientific. - 1354-1013 .- 1365-2486. ; 10:1, s. 141-147
  • Tidskriftsartikel (refereegranskat)abstract
    • We calculated the carbon loss (mineralization plus sedimentation) and net CO2 escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437–48 263 km2). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO2 emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO2 emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO2 between the atmosphere and the boreal landscape.
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29.
  • Keuper, Frida, et al. (författare)
  • A race for space? : How Sphagnum fuscumstabilizes vegetation composition during long-termclimate manipulations
  • 2011
  • Ingår i: Global Change Biology. - : Blackwell. - 1354-1013 .- 1365-2486. ; 17:6, s. 2162-2171
  • Tidskriftsartikel (refereegranskat)abstract
    • Strong climate warming is predicted at higher latitudes this century, with potentially major consequences forproductivity and carbon sequestration. Although northern peatlands contain one-third of the world’s soil organiccarbon, little is known about the long-term responses to experimental climate change of vascular plant communities inthese Sphagnum-dominated ecosystems.We aimed to see how long-term experimental climate manipulations, relevantto different predicted future climate scenarios, affect total vascular plant abundance and species composition whenthe community is dominated by mosses. During 8 years, we investigated how the vascular plant community of aSphagnum fuscum-dominated subarctic peat bog responded to six experimental climate regimes, including factorialcombinations of summer as well as spring warming and a thicker snow cover. Vascular plant species composition inour peat bog was more stable than is typically observed in (sub)arctic experiments: neither changes in total vascularplant abundance, nor in individual species abundances, Shannon’s diversity or evenness were found in response tothe climate manipulations. For three key species (Empetrum hermaphroditum, Betula nana and S. fuscum) we alsomeasured whether the treatments had a sustained effect on plant length growth responses and how these responsesinteracted. Contrasting with the stability at the community level, both key shrubs and the peatmoss showed sustainedpositive growth responses at the plant level to the climate treatments. However, a higher percentage of mossencroachedE. hermaphroditum shoots and a lack of change in B. nana net shrub height indicated encroachment byS. fuscum, resulting in long-term stability of the vascular community composition: in a warmer world, vascular speciesof subarctic peat bogs appear to just keep pace with growing Sphagnum in their race for space. Our findings contributeto general ecological theory by demonstrating that community resistance to environmental changes does notnecessarily mean inertia in vegetation response.
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30.
  • Schulze, E D, et al. (författare)
  • The European carbon balance. Part 4: integration of carbon and other trace-gas fluxes
  • 2010
  • Ingår i: GLOBAL CHANGE BIOLOGY. - : Blackwell Publishing Ltd. - 1354-1013 .- 1365-2486. ; 16:5, s. 1451-1469
  • Forskningsöversikt (refereegranskat)abstract
    • Overviewing the European carbon (C), greenhouse gas (GHG), and non-GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr-1, and fossil fuel imports are 1.6 Pg yr-1. GPP is about 1.25% of solar radiation, containing about 360 x 1018 J energy - five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1 parts per thousand of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100-year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N2O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO-N, 70% of total N2O-N, and 95% of total NH3-N emissions of Europe. European soils are a net C sink (114 Tg yr-1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO2 C-equivalent yr-1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non-GHGs (NH3, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr-1. Nevertheless, for the European trace-gas balance, land-use intensity is more important than land-use change. This study shows that emissions of GHGs and non-GHGs significantly distort the C cycle and eliminate apparent C sinks.
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31.
  • Ström, Lotta, et al. (författare)
  • Hydrologic effects on riparian vegetation in a boreal river : an experiment testing climate change predictions
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17:1, s. 254-267
  • Tidskriftsartikel (refereegranskat)abstract
    • Climate change is expected to alter the magnitude and variation of flow in streams and rivers, hence providing new conditions for riverine communities. We evaluated plant ecological responses to climate change by transplanting turfs of riparian vegetation to new elevations in the riparian zone, thus simulating expected changes in water-level variation, and monitored the results over 6 years. Turfs moved to higher elevations decreased in biomass and increased in species richness, whereas turfs transplanted to lower elevations gained biomass but lost species. Transplanted plant communities responded slowly to the new hydrologic conditions. After 6 years, biomass of transplanted turfs was statistically indistinguishable from target level controls, but species richness and species composition of transplants were intermediate between original and target levels. By using projections of future stream flow according to IPCC climate change scenarios, we predict likely changes to riparian vegetation in boreal rivers. Climate-driven hydrologic changes are predicted to result in narrower riparian zones along the studied Vindel River in northern Sweden towards the end of the 21st century. Present riparian plant communities are projected to be replaced by terrestrial communities at high elevations as a result of lower-magnitude spring floods, and by amphibious or aquatic communities at low elevations as a result of higher autumn and winter flows. Changes to riparian vegetation may be larger in other boreal climate regions: snow melt fed spring floods are predicted to disappear in southern parts of the boreal zone, which would result in considerable loss of riparian habitat. Our study emphasizes the importance of long-term ecological field experiments given that plant communities often respond slowly and in a nonlinear fashion to external pressures.
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32.
  • Alatalo, Juha, 1966-, et al. (författare)
  • Response to simulated climatic change in an alpine and subarctic pollen-risk strategist, Silene acaulis
  • 1997
  • Ingår i: Global Change Biology. - : Wiley. - 1365-2486 .- 1354-1013. ; 3, s. 74-79
  • Tidskriftsartikel (refereegranskat)abstract
    • The aim of this study was to test if early no overing species respond with increased seed production to climate warming as is predicted for late-flowering seed-risk strategists. Experimental climate warming of about 3 degrees C was applied to two populations of the cushion-forming plant Silene acaulis (L.) Jacq. The experiment was run at one subarctic site and one alpine site for 2 years and 1 year, respectively, using open-top chambers (OTC). The 2-year temperature enhancement at the subarctic site had a marked effect on the flowering phenology. Cushions inside the OTC started flowering substantially earlier than control cushions. Both the male and female phases developed faster in the OTCs, and maturation of capsules occurred earlier. The cushions also responded positively in reproductive terms and produced more mature seeds and had a higher seed/ovule ratio. After 1 year temperature enhancement at the alpine site there was a weak trend for earlier flowering, but there was no significant difference in seed production or seed/ovule ratio.
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33.
  • Andert, Janet, et al. (författare)
  • Importance of denitrifiers lacking the genes encoding the nitrous oxide reductase for N2O emissions from soil
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17, s. 1497-1504
  • Tidskriftsartikel (refereegranskat)abstract
    • Analyses of the complete genomes of sequenced denitrifying bacteria revealed that approximately 1/3 have a truncated denitrification pathway, lacking the nosZ gene encoding the nitrous oxide reductase. We investigated whether the number of denitrifiers lacking the genetic ability to synthesize the nitrous oxide reductase in soils is important for the proportion of N2O emitted by denitrification. Serial dilutions of the denitrifying strain Agrobacterium tumefaciens C58 lacking the nosZ gene were inoculated into three different soils to modify the proportion of denitrifiers having the nitrous oxide reductase genes. The potential denitrification and N2O emissions increased when the size of inoculated C58 population in the soils was in the same range as the indigenous nosZ community. However, in two of the three soils, the increase in potential denitrification in inoculated microcosms compared with the noninoculated microcosms was higher than the increase in N2O emissions. This suggests that the indigenous denitrifier community was capable of acting as a sink for the N2O produced by A. tumefaciens. The relative amount of N2O emitted also increased in two soils with the number of inoculated C58 cells, establishing a direct causal link between the denitrifier community composition and potential N2O emissions by manipulating the proportion of denitrifiers having the nosZ gene. However, the number of denitrifiers which do not possess a nitrous oxide reductase might not be as important for N2O emissions in soils having a high N2O uptake capacity compared with those with lower. In conclusion, we provide a proof of principle that the inability of some denitrifiers to synthesize the nitrous oxide reductase can influence the nature of the denitrification end products, indicating that the extent of the reduction of N2O to N-2 by the denitrifying community can have a genetic basis.
  •  
34.
  • Berendse, F, et al. (författare)
  • Raised atmospheric CO2 levels and increased N deposition cause shifts in plant species composition and production in Sphagnum bogs
  • 2001
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 7:5, s. 591-598
  • Tidskriftsartikel (refereegranskat)abstract
    • Part of the missing sink in the global CO2 budget has been attributed to the positive effects of CO2 fertilization and N deposition on carbon sequestration in Northern Hemisphere terrestrial ecosystems. The genus Sphagnum is one of the most important groups of plant species sequestrating carbon in temperate and northern bog ecosystems, because of the low decomposability of the dead material it produces. The effects of raised CO2 and increased atmospheric N deposition on growth of Sphagnum and other plants were studied in bogs at four sites across Western Europe. Contrary to expectations, elevated CO2 did not significantly affect Sphagnum biomass growth. Increased N deposition reduced Sphagnum mass growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum. Such changes in plant species composition may decrease carbon sequestration in Sphagnum-dominated bog ecosystems.
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35.
  • Berger, Stella A, et al. (författare)
  • Water temperature and stratification depth independently shift cardinal events during plankton spring succession
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:7, s. 1954-1965
  • Tidskriftsartikel (refereegranskat)abstract
    • In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification-dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.
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36.
  • Blenckner, Thorsten, et al. (författare)
  • Large-scale climatic signatures in lakes across Europe : A meta-analysis
  • 2007
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 13:7, s. 1314-1326
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent studies have highlighted the impact of the winter North Atlantic Oscillation (NAO) on water temperature, ice conditions, and spring plankton phenology in specific lakes and regions in Europe. Here, we use meta-analysis techniques to test whether 18 lakes in northern, western, and central Europe respond coherently to winter climate forcing, and to assess the persistence of the winter climate signal in physical, chemical, and biological variables during the year. A meta-analysis approach was chosen because we wished to emphasize the overall coherence pattern rather than individual lake responses. A particular strength of our approach is that time-series from each of the 18 lakes were subjected to the same robust statistical analysis covering the same 23-year period. Although the strongest overall coherence in response to the winter NAO was exhibited by lake water temperatures, a strong, coherent response was also exhibited by concentrations of soluble reactive phosphorus and soluble reactive silicate, most likely as a result of the coherent response exhibited by the spring phytoplankton bloom. Lake nitrate concentrations showed significant coherence in winter. With the exception of the cyanobacterial biomass in summer, phytoplankton biomass in all seasons was unrelated to the winter NAO. A strong coherence in the abundance of daphnids during spring can most likely be attributed to coherence in daphnid phenology. A strong coherence in the summer abundance of the cyclopoid copepods may have been related to a coherent change in their emergence from resting stages. We discuss the complex nature of the potential mechanisms that drive the observed changes.
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37.
  • Bokhorst, Stef Frederik (författare)
  • Extreme winter warming events more negatively impact small rather than large soil fauna: shift in community composition explained by traits not taxa
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18, s. 1152-1162
  • Tidskriftsartikel (refereegranskat)abstract
    • Extreme weather events can have negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (210 similar to degrees C for 214 similar to days), but returning to cold winter climate exposes the ecosystem to lower temperatures by the loss of insulating snow. Soil animals, which play an integral part in soil processes, may be very susceptible to such events depending on the intensity of soil warming and low temperatures following these events. We simulated week-long extreme winter warming events using infrared heating lamps, alone or with soil warming cables for two consecutive years in a sub-Arctic dwarf shrub heathland. Minimum temperatures were lower and freeze-thaw cycles were 211 times more frequent in treatment plots compared with control plots. Following the second event, Acari populations decreased by 39%; primarily driven by declines of Prostigmata (69%) and the Mesostigmatic nymphs (74%). A community-weighted vertical stratification shift occurred from smaller soil dwelling (eu-edaphic) Collembola species dominance to larger litter dwelling (hemi-edaphic) species dominance in the canopy-with-soil warming plots compared with controls. The most susceptible groups to these winter warming events were the smallest individuals (Prostigmata and eu-edaphic Collembola). This was not apparent from abundance data at the Collembola taxon level, indicating that life forms and species traits play a major role in community assembly following extreme events. The observed shift in soil community can cascade down to the micro-flora affecting plant productivity and mineralization rates. Short-term extreme weather events have the potential to shift community composition through trait composition with potentially large consequences for ecosystem development.
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38.
  • De Frenne, Pieter, et al. (författare)
  • Temperature effects on forest herbs assessed by warming and transplant experiments along a latitudinal gradient
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17:10, s. 3240-3253
  • Tidskriftsartikel (refereegranskat)abstract
    • Slow-colonizing forest understorey plants are probably not able to rapidly adjust their distribution range following large-scale climate change. Therefore, the acclimation potential to climate change within their actual occupied habitats will likely be key for their short-and long-term persistence. We combined transplant experiments along a latitudinal gradient with open-top chambers to assess the effects of temperature on phenology, growth and reproductive performance of multiple populations of slow-colonizing understorey plants, using the spring flowering geophytic forb Anemone nemorosa and the early summer flowering grass Milium effusum as study species. In both species, emergence time and start of flowering clearly advanced with increasing temperatures. Vegetative growth (plant height, aboveground biomass) and reproductive success (seed mass, seed germination and germinable seed output) of A. nemorosa benefited from higher temperatures. Climate warming may thus increase future competitive ability and colonization rates of this species. Apart from the effects on phenology, growth and reproductive performance of M. effusum generally decreased when transplanted southwards (e. g., plant size and number of individuals decreased towards the south) and was probably more limited by light availability in the south. Specific leaf area of both species increased when transplanted southwards, but decreased with open-top chamber installation in A. nemorosa. In general, individuals of both species transplanted at the home site performed best, suggesting local adaptation. We conclude that contrasting understorey plants may display divergent plasticity in response to changing temperatures which may alter future understorey community dynamics.
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39.
  • Duarte, Helder, et al. (författare)
  • Can amphibians take the heat? : Vulnerability to climate warming in subtropical and temperate larval amphibian communities
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:2, s. 412-421
  • Tidskriftsartikel (refereegranskat)abstract
    • Predicting the biodiversity impacts of global warming implies that we know where and with what magnitude these impacts will be encountered. Amphibians are currently the most threatened vertebrates, mainly due to habitat loss and to emerging infectious diseases. Global warming may further exacerbate their decline in the near future, although the impact might vary geographically. We predicted that subtropical amphibians should be relatively susceptible to warming-induced extinctions because their upper critical thermal limits (CTmax) might be only slightly higher than maximum pond temperatures (Tmax). We tested this prediction by measuring CTmax and Tmax for 47 larval amphibian species from two thermally distinct subtropical communities (the warm community of the Gran Chaco and the cool community of Atlantic Forest, northern Argentina), as well as from one European temperate community. Upper thermal tolerances of tadpoles were positively correlated (controlling for phylogeny) with maximum pond temperatures, although the slope was steeper in subtropical than in temperate species. CTmax values were lowest in temperate species and highest in the subtropical warm community, which paradoxically, had very low warming tolerance (CTmaxTmax) and therefore may be prone to future local extinction from acute thermal stress if rising pond Tmax soon exceeds their CTmax. Canopy-protected subtropical cool species have larger warming tolerance and thus should be less impacted by peak temperatures. Temperate species are relatively secure to warming impacts, except for late breeders with low thermal tolerance, which may be exposed to physiological thermal stress in the coming years.
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40.
  • Eckersten, Henrik (författare)
  • Agroclimatic conditions in Europe under climate change
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17, s. 2298-2318
  • Forskningsöversikt (refereegranskat)abstract
    • To date, projections of European crop yields under climate change have been based almost entirely on the outputs of crop-growth models. While this strategy can provide good estimates of the effects of climatic factors, soil conditions and management on crop yield, these models usually do not capture all of the important aspects related to crop management, or the relevant environmental factors. Moreover, crop-simulation studies often have severe limitations with respect to the number of crops covered or the spatial extent. The present study, based on agroclimatic indices, provides a general picture of agroclimatic conditions in western and central Europe (study area lays between 8.5 degrees W-27 degrees E and 37-63.5 degrees N), which allows for a more general assessment of climate-change impacts. The results obtained from the analysis of data from 86 different sites were clustered according to an environmental stratification of Europe. The analysis was carried for the baseline (1971-2000) and future climate conditions (time horizons of 2030, 2050 and with a global temperature increase of 5 degrees C) based on outputs of three global circulation models. For many environmental zones, there were clear signs of deteriorating agroclimatic condition in terms of increased drought stress and shortening of the active growing season, which in some regions become increasingly squeezed between a cold winter and a hot summer. For most zones the projections show a marked need for adaptive measures to either increase soil water availability or drought resistance of crops. This study concludes that rainfed agriculture is likely to face more climate-related risks, although the analyzed agroclimatic indicators will probably remain at a level that should permit rainfed production. However, results suggests that there is a risk of increasing number of extremely unfavorable years in many climate zones, which might result in higher interannual yield variability and constitute a challenge for proper crop management.
  •  
41.
  • Eriksson, Tobias, et al. (författare)
  • Production and oxidation of methane in a boreal mire after a decade of increased temperature and nitrogen and sulfur deposition
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16, s. 2130-2144
  • Tidskriftsartikel (refereegranskat)abstract
    • Natural wetlands are the single largest source of atmospheric methane (CH(4)). Both a changed climate and deposition of anthropogenic nitrogen and sulfur can alter the production and oxidation of CH(4) respectively and thereby also CH(4) exchange. We used a long-term (12 years) factorial field experiment in a boreal oligotrophic mire to evaluate the effects of greenhouse cover and addition of ammonium nitrate and sodium sulfate on the production and oxidation of CH(4) by applying laboratory incubations of samples from five depths in the mire. The rates of CH(4) production were measured without amendments and after the addition of either glucose or sulfate. Twelve years of increased nitrogen deposition has changed the mire from a Sphagnum-dominated plant community to one dominated by sedges and dwarf shrubs. The deposition of nitrogen to the field plots caused increased production of CH(4) in incubations without amendments (34%), and also after amendments with glucose (40%) or sulfate (42%). This indicates increased substrate availability (without amendments) but also a greater abundance of methanogens (glucose amendment). The greenhouse cover caused a decrease in CH(4) production in incubations without amendments (34%), after glucose amendment (20%) and after sulfate amendment (31%). These responses indicate decreased substrate availability (without amendment) accompanied by the reduced abundance of methanogens (glucose amendment). The field application of sulfur had no effect on CH(4) production at the depth where maximal CH(4) production occurred. Closer to the mire surface, however, the rate of CH(4) production was significantly reduced by 32-45%. These results suggest that the deposition of sulfate has altered the vertical distribution of methanogens and sulfate-reducing bacteria. The oxidation of CH(4) was not significantly affected by any of the long-term field treatments.
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42.
  • Erlandsson, Martin, et al. (författare)
  • Thirty-five years of synchrony in the organic matter concentrations of Swedish rivers explained by variation in flow and sulphate
  • 2008
  • Ingår i: Global Change Biology. - : Blackwell Publishing. - 1354-1013 .- 1365-2486. ; 14:5, s. 1191-1198
  • Tidskriftsartikel (refereegranskat)abstract
    • Increasing concentrations of organic matter ( OM) in surface waters have been noted over large parts of the boreal/nemoral zone in Europe and North America. This has raised questions about the causes and the likelihood of further increases. A number of drivers have been proposed, including temperature, hydrology, as well as SO42 - and Cl (-) deposition. The data reported so far, however, have been insufficient to define the relative importance of different drivers in landscapes where they interact. Thirty-five years of monthly measurements of absorbance and chemical oxygen demand ( COD), two common proxies for OM, from 28 large Scandinavian catchments provide an unprecedented opportunity to resolve the importance of hypothesized drivers. For 21 of the catchments, there are 18 years of total organic carbon (TOC) measurements as well. Despite the heterogeneity of the catchments with regards to climate, size and land use, there is a high degree of synchronicity in OM across the entire region. Rivers go from widespread trends of decreasing OM to increasing trends and back again three times in the 35-year record. This synchronicity in decadal scale oscillations and long-term trends suggest a common set of dominant OM drivers in these landscapes. Here, we use regression models to test the importance of different potential drivers. We show that flow and SO42 - together can predict most of the interannual variability in OM proxies, up to 88% for absorbance, up to 78% for COD. Two other candidate drivers, air temperature and Cl (-) , add little explanatory value. Declines in anthropogenic SO42 - since the mid-1970s are thus related to the observed OM increases in Scandinavia, but, in contrast to many recent studies, flow emerges as an even more important driver of OM variability. Stabilizing SO42 - levels also mean that hydrology is likely to be the major driver of future variability and trends in OM.
  •  
43.
  • Finstad, Anders G., et al. (författare)
  • Competitive exclusion along climate gradients : energy efficiency influences the distribution of two salmonid fishes
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17:4, s. 1703-1711
  • Tidskriftsartikel (refereegranskat)abstract
    • We tested the importance of thermal adaptations and energy efficiency in relation to the geographical distribution of two competing freshwater salmonid fish species. Presence–absence data for Arctic char and brown trout were obtained from 1502 Norwegian lakes embracing both temperature and productivity gradients. The distributions were contrasted with laboratory-derived temperature scaling models for food consumption, growth and energy efficiency. Thermal performances of the two species were almost identical. However, Arctic char exhibited double the growth efficiency (per unit of food) and appear to have out-competed brown trout from cold, low-productivity lakes, perhaps by scramble competition. Brown trout, for which previous reports have shown to be aggressive and dominant, have likely excluded the more energy-efficient Arctic char from relatively warm, productive lakes, perhaps by contest competition. Competitive interaction changing in outcome with lake productivity, rather than thermal performance, is likely a major determinant of the range distribution of the two species. Our study highlights the need for more focus on choice of relevant ecophysiological traits in ecological climate impact studies and species distribution modelling.
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44.
  • Finstad, Anders G., et al. (författare)
  • Migrate or stay : terrestrial primary productivity and climate drive anadromy in Arctic char
  • 2012
  • Ingår i: Global Change Biology. - : John Wiley & Sons. - 1354-1013 .- 1365-2486. ; 18:8, s. 2487-2497
  • Tidskriftsartikel (refereegranskat)abstract
    • A shift in the magnitude and timing of animal migrations is one of the most documented ecological effects of climate change. Although migrations are largely driven by spatial variation in resource gradients, few studies connect expected changes in primary production with geographic patterns in migratory behavior. Here, we link lake primary production to the occurrence of sea migrations in the partially anadromous salmonid Arctic char (Salvelinus alpinus L.). We compiled presence/absence records of anadromous char populations spanning productivity and temperature gradients along the Norwegian coast. The probability of anadromy decreased with increasing migration distance, maximum slope of the migration route and lake productivity. There was a significant interaction between lake productivity and migration distance. The negative effect of longer migration distances was more severe in lakes with higher productivity, indicating reduced relative profitability of migration with increased feeding opportunities in freshwater. Lake productivity was mainly driven by terrestrial primary production in the catchment. We predicted future distributions of anadromous char given downscaled temperature and precipitation changes projected by two different emission scenarios and global climate models (GCMs). Projected increases in temperature and precipitation in 20712100 increased terrestrial primary production and, compared to the control scenario (19611990), decreased the range of anadromous populations. The prevalence of anadromy decreased by 53% in the HadAm3H GCM with the A2 emission scenario, 61% in HadAm3H with the B2 scenario and 22% in ECHAM4 with the B2 scenario. Cross-ecosystem studies (e.g., terrestrial to freshwater) are critical for understanding ecological impacts of climate change. In this case, climate-driven increases in terrestrial primary production are expected to increase primary production in lakes and ultimately reduce the prevalence of anadromy in Arctic char populations.
  •  
45.
  • Freeman, Michael (författare)
  • Reconciling the optimal and empirical approaches to modelling stomatal conductance
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17, s. 2134-2144
  • Tidskriftsartikel (refereegranskat)abstract
    • Models of vegetation function are widely used to predict the effects of climate change on carbon, water and nutrient cycles of terrestrial ecosystems, and their feedbacks to climate. Stomatal conductance, the process that governs plant water use and carbon uptake, is fundamental to such models. In this paper, we reconcile two long-standing theories of stomatal conductance. The empirical approach, which is most commonly used in vegetation models, is phenomenological, based on experimental observations of stomatal behaviour in response to environmental conditions. The optimal approach is based on the theoretical argument that stomata should act to minimize the amount of water used per unit carbon gained. We reconcile these two approaches by showing that the theory of optimal stomatal conductance can be used to derive a model of stomatal conductance that is closely analogous to the empirical models. Consequently, we obtain a unified stomatal model which has a similar form to existing empirical models, but which now provides a theoretical interpretation for model parameter values. The key model parameter, g(1), is predicted to increase with growth temperature and with the marginal water cost of carbon gain. The new model is fitted to a range of datasets ranging from tropical to boreal trees. The parameter g(1) is shown to vary with growth temperature, as predicted, and also with plant functional type. The model is shown to correctly capture responses of stomatal conductance to changing atmospheric CO2, and thus can be used to test for stomatal acclimation to elevated CO2. The reconciliation of the optimal and empirical approaches to modelling stomatal conductance is important for global change biology because it provides a simple theoretical framework for analyzing, and simulating, the coupling between carbon and water cycles under environmental change.
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46.
  • Gaudinski, JB, et al. (författare)
  • Use of stored carbon reserves in growth of temperate tree roots and leaf buds : analyses using radiocarbon measurements and modeling
  • 2009
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 15:4, s. 992-1014
  • Tidskriftsartikel (refereegranskat)abstract
    • Characterizing the use of carbon (C) reserves in trees is important for understanding regional and global C cycles, stress responses, asynchrony between photosynthetic activity and growth demand, and isotopic exchanges in studies of tree physiology and ecosystem C cycling. Using an inadvertent, whole-ecosystem radiocarbon ((14)C) release in a temperate deciduous oak forest and numerical modeling, we estimated that the mean age of stored C used to grow both leaf buds and new roots is 0.7 years and about 55% of new-root growth annually comes from stored C. Therefore, the calculated mean age of C used to grow new-root tissue is similar to 0.4 years. In short, new roots contain a lot of stored C but it is young in age. Additionally, the type of structure used to model stored C input is important. Model structures that did not include storage, or that assumed stored and new C mixed well (within root or shoot tissues) before being used for root growth, did not fit the data nearly as well as when a distinct storage pool was used. Consistent with these whole-ecosystem labeling results, the mean age of C in new-root tissues determined using 'bomb-(14)C' in three additional forest sites in North America and Europe (one deciduous, two coniferous) was less than 1-2 years. The effect of stored reserves on estimated ages of fine roots is unlikely to be large in most natural abundance isotope studies. However, models of root C dynamics should take stored reserves into account, particularly for pulse-labeling studies and fast-cycling roots (< 1 years).
  •  
47.
  • Gundale, Michael, et al. (författare)
  • Bryophytes attenuate anthropogenic nitrogen inputs in boreal forests
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17, s. 2743-2753
  • Tidskriftsartikel (refereegranskat)abstract
    • Productivity in boreal ecosystems is primarily limited by available soil nitrogen (N), and there is substantial interest in understanding whether deposition of anthropogenically derived reactive nitrogen (N-r) results in greater N availability to woody vegetation, which could result in greater carbon (C) sequestration. One factor that may limit the acquisition of N-r by woody plants is the presence of bryophytes, which are a significant C and N pool, and a location where associative cyanobacterial N-fixation occurs. Using a replicated stand-scale N-addition experiment (five levels: 0, 3, 6, 12, and 50 kg N ha(-1)yr(-1); n=6) in the boreal zone of northern Sweden, we tested the hypothesis that sequestration of Nr into bryophyte tissues, and downregulation of N-fixation would attenuate N-r inputs, and thereby limit anthropogenic N-r acquisition by woody plants. Our data showed that N-fixation per unit moss mass and per unit area sharply decreased with increasing N addition. Additionally, the tissue N concentrations of Pleuorzium schreberi increased and its biomass decreased with increasing N addition. This response to increasing N addition caused the P. schreberi N pool to be stable at all but the highest N addition rate, where it significantly decreased. The combined effects of changed N-fixation and P. schreberi biomass N accounted for 56.7% of cumulative N-r additions at the lowest N-r addition rate, but only a minor fraction for all other treatments. This 'bryophyte effect' can in part explain why soil inorganic N availability and acquisition by woody plants (indicated by their delta N-15 signatures) remained unchanged up to N addition rates of 12 kg ha(-1)yr(-1) or greater. Finally, we demonstrate that approximately 71.8% of the boreal forest experiences N-r deposition rates at or below 3 kg ha(-1)yr(-1), suggesting that bryophytes likely limit woody plant acquisition of ambient anthropogenic Nr inputs throughout a majority of the boreal forest.
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48.
  • Gunnarsson, Urban, et al. (författare)
  • Near-zero recent carbon accumulation in a bog with high nitrogen deposition in SW Sweden
  • 2008
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 14, s. 2152-2165
  • Tidskriftsartikel (refereegranskat)abstract
    • We present data on the accumulation of carbon and nitrogen into an open oceanic ombrotrophic bog, SW Sweden, with high levels of anthropogenic nitrogen deposition. The aim was to investigate if this peatland currently acts as a sink for atmospheric carbon. Peat cores were sampled from the top peat layer in five different vegetation types. Small pines were used to date the cores. The cores bulk density and carbon and nitrogen content were determined. A vegetation-classified satellite image was used to estimate the areal extent of the vegetation types and to scale up these results to bog level. The rate of current carbon input into the upper oxic acrotelm was 290 g m(-2) yr(-1), and there were no significant differences in accumulation rates among the vegetation types. This organic matter input to the acrotelm was almost completely decomposed before it was deposited for storage in the deeper peat layers (the catotelm) and only a small fraction (< , 1%) or 0.012 g m(-2) yr(-1) of the carbon would be left, assuming a residence time of 100 years in the acrotelm. Nitrogen accumulation rates differed between the vegetation classes, and the average input via primary production varied from 5.33 to 16.8 g m(-2) yr(-1). Current nitrogen input rates into the catotelm are much lower, 0-0.059 g m(-2) yr(-1), with the highest accumulation rates in lawn-dominated communities. We suggest that one of the main causes of the low carbon input rates is the high level of nitrogen deposition, which enhances decomposition and changes the vegetation from peat-forming Sphagnum-dominance to dominance by dwarf shrubs and graminoids.
  •  
49.
  • Hillebrand, Helmut, et al. (författare)
  • Warming leads to higher species turnover in a coastal ecosystem
  • 2010
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:4, s. 1181-1193
  • Tidskriftsartikel (refereegranskat)abstract
    • The responses of ecological communities and ecosystems to increased rates of environmental change will be strongly influenced by variation in the diversity of community composition. Yet, our understanding of how diversity is affected by rising temperatures is inconclusive and mainly based on indirect evidence or short-term experiments. In our study, we analyse the diversity and species turnover of benthic epilithic communities within the thermal flume of a nuclear power plant at the Swedish coast. This flume covers the range of predicted future temperature rises. Species composition was significantly different between control sites and sites with higher temperatures. We found that temperature had little effect on the number of species in three functional groups (macroinvertebrates, benthic diatoms, and macrophytes, which here comprise multicellular algae and macroscopic colonies of unicellular algae and cyanobacteria), neither at single sampling dates nor summed for the entire observation year. However, species turnover significantly increased with increasing temperature for diatoms, macrophytes and invertebrates. Different temperature regimes resulted in significantly different species composition and indicator species. Thus, increasing temperatures in the thermal flume increased temporal beta-diversity and decreased compositional stability of communities, although observed richness did not change at any point in time. We highlight the need to investigate the consequences of such declines in compositional stability for functional stability of ecosystem processes.
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50.
  • Hollesen, J., et al. (författare)
  • Future active layer dynamics and carbon dioxide production from thawing permafrost layers in Northeast Greenland
  • 2011
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 17:2, s. 911-926
  • Tidskriftsartikel (refereegranskat)abstract
    • Thawing permafrost and the resulting mineralization of previously frozen organic carbon (C) is considered an important future feedback from terrestrial ecosystems to the atmosphere. Here, we use a dynamic process oriented permafrost model, the CoupModel, to link surface and subsurface temperatures from a moist permafrost soil in High-Arctic Greenland with observed heat production and carbon dioxide (CO2) release rates from decomposition of previously frozen organic matter. Observations show that the maximum thickness of the active layer at the end of the summer has increased 1 cm yr-1 since 1996. The model is successfully adjusted and applied for the study area and shown to be able to simulate active layer dynamics. Subsequently, the model is used to predict the active layer thickness under future warming scenarios. The model predicts an increase of maximum active layer thickness from today 70 to 80-105 cm as a result of a 2-6 degrees C warming. An additional increase in the maximum active layer thickness of a few centimetres may be expected due to heat production from decomposition of organic matter. Simulated future soil temperatures and water contents are subsequently used with measured basal soil respiration rates in a respiration model to predict the corresponding depth-integrated CO2 production from permafrost layers between 0.7 and 2 m below the surface. Results show an increase from present values of < 40 g C m-2 yr-1 to between 120 and 213 g C m-2 yr-1 depending on the magnitude of predicted warming. These rates are more than 50% of the present soil CO2 efflux measured at the soil surface. Future modelling accounting for snow, vegetation and internal biological heat feedbacks are of interest in order to test the robustness of the above predictions and to describe the entire ecosystem response.
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