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1.
  • Akiyama, Kazunori, et al. (författare)
  • First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
  • 2019
  • Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of similar to 40 mu as, consistent with the size and shape of the lensed photon orbit encircling the "shadow" of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others' work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.
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2.
  • Akiyama, Kazunori, et al. (författare)
  • First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
  • 2019
  • Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.
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3.
  • Akiyama, Kazunori, et al. (författare)
  • First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
  • 2019
  • Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 +/- 3 mu as and constrain its fractional width to be
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4.
  • Porth, Oliver, et al. (författare)
  • The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
  • 2019
  • Ingår i: Astrophysical Journal, Supplement Series. - : American Astronomical Society. - 1538-4365 .- 0067-0049. ; 243:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high- precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.
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5.
  • Jelenkovic, Aline, et al. (författare)
  • Zygosity Differences in Height and Body Mass Index of Twins From Infancy to Old Age : A Study of the CODATwins Project
  • 2015
  • Ingår i: Twin Research and Human Genetics. - : Cambridge University Press. - 1832-4274 .- 1839-2628. ; 18:5, s. 557-570
  • Tidskriftsartikel (refereegranskat)abstract
    • A trend toward greater body size in dizygotic (DZ) than in monozygotic (MZ) twins has been suggested by some but not all studies, and this difference may also vary by age. We analyzed zygosity differences in mean values and variances of height and body mass index (BMI) among male and female twins from infancy to old age. Data were derived from an international database of 54 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), and included 842,951 height and BMI measurements from twins aged 1 to 102 years. The results showed that DZ twins were consistently taller than MZ twins, with differences of up to 2.0 cm in childhood and adolescence and up to 0.9 cm in adulthood. Similarly, a greater mean BMI of up to 0.3 kg/m(2) in childhood and adolescence and up to 0.2 kg/m(2) in adulthood was observed in DZ twins, although the pattern was less consistent. DZ twins presented up to 1.7% greater height and 1.9% greater BMI than MZ twins; these percentage differences were largest in middle and late childhood and decreased with age in both sexes. The variance of height was similar in MZ and DZ twins at most ages. In contrast, the variance of BMI was significantly higher in DZ than in MZ twins, particularly in childhood. In conclusion, DZ twins were generally taller and had greater BMI than MZ twins, but the differences decreased with age in both sexes.
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6.
  • Silventoinen, Karri, et al. (författare)
  • The CODATwins Project : The Cohort Description of Collaborative Project of Development of Anthropometrical Measures in Twins to Study Macro-Environmental Variation in Genetic and Environmental Effects on Anthropometric Traits
  • 2015
  • Ingår i: Twin Research and Human Genetics. - : Cambridge University Press. - 1832-4274 .- 1839-2628. ; 18:4
  • Tidskriftsartikel (refereegranskat)abstract
    • For over 100 years, the genetics of human anthropometric traits has attracted scientific interest. In particular, height and body mass index (BMI, calculated as kg/m2) have been under intensive genetic research. However, it is still largely unknown whether and how heritability estimates vary between human populations. Opportunities to address this question have increased recently because of the establishment of many new twin cohorts and the increasing accumulation of data in established twin cohorts. We started a new research project to analyze systematically (1) the variation of heritability estimates of height, BMI and their trajectories over the life course between birth cohorts, ethnicities and countries, and (2) to study the effects of birth-related factors, education and smoking on these anthropometric traits and whether these effects vary between twin cohorts. We identified 67 twin projects, including both monozygotic (MZ) and dizygotic (DZ) twins, using various sources. We asked for individual level data on height and weight including repeated measurements, birth related traits, background variables, education and smoking. By the end of 2014, 48 projects participated. Together, we have 893,458 height and weight measures (52% females) from 434,723 twin individuals, including 201,192 complete twin pairs (40% monozygotic, 40% same-sex dizygotic and 20% opposite-sex dizygotic) representing 22 countries. This project demonstrates that large-scale international twin studies are feasible and can promote the use of existing data for novel research purposes.
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8.
  • Phillips, Helen R. P., et al. (författare)
  • Global distribution of earthworm diversity
  • 2019
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 366:6464, s. 480-
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil organisms, including earthworms, are a key component of terrestrial ecosystems. However, little is known about their diversity, their distribution, and the threats affecting them. We compiled a global dataset of sampled earthworm communities from 6928 sites in 57 countries as a basis for predicting patterns in earthworm diversity, abundance, and biomass. We found that local species richness and abundance typically peaked at higher latitudes, displaying patterns opposite to those observed in aboveground organisms. However, high species dissimilarity across tropical locations may cause diversity across the entirety of the tropics to be higher than elsewhere. Climate variables were found to be more important in shaping earthworm communities than soil properties or habitat cover. These findings suggest that climate change may have serious implications for earthworm communities and for the functions they provide.
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9.
  • Aad, G, et al. (författare)
  • 2015
  • swepub:Mat__t
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10.
  • Asplund, Johan, et al. (författare)
  • Changes in functional traits of the terricolous lichen Peltigera aphthosa across a retrogressive boreal forest chronosequence
  • 2015
  • Ingår i: Lichenologist. - 0024-2829 .- 1096-1135. ; 47, s. 187-195
  • Tidskriftsartikel (refereegranskat)abstract
    • Changes in the functional traits of the terricolous lichen Peltigera aphthosa with declining soil fertility during ecosystem retrogression were investigated. A well-documented retrogressive chronosequence of 28 forested islands in northern Sweden that differ greatly in fire history and which spans 5000 years was used. The abundance of cephalodia increased, indicative of higher N-2-fixation rates resulting from lower N availability. Thallus C-13 values increased with ageing soils, in line with declining C-13 values of the humus substratum along this gradient. However, C-13 values were also driven by variation in factors that were at least partly independent of soil ageing. As such, C-13 values were mostly related to specific thallus mass (STM), possibly because a higher STM gives a thicker cortical layer and thus greater resistance to CO2 diffusion, leading to higher C-13 values. STM and other measured traits (i.e. thallus N, P, secondary compounds and water-holding capacity) were unresponsive to the gradient, despite these traits being very responsive to the same gradient in epiphytic lichen species.
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11.
  • Asplund, Johan, et al. (författare)
  • How lichens impact on terrestrial community and ecosystem properties
  • 2017
  • Ingår i: Biological Reviews. - : Wiley. - 1464-7931 .- 1469-185X. ; 92, s. 1720-1738
  • Forskningsöversikt (refereegranskat)abstract
    • Lichens occur in most terrestrial ecosystems; they are often present as minor contributors, but in some forests, drylands and tundras they can make up most of the ground layer biomass. As such, lichens dominate approximately 8% of the Earth's land surface. Despite their potential importance in driving ecosystem biogeochemistry, the influence of lichens on community processes and ecosystem functioning have attracted relatively little attention. Here, we review the role of lichens in terrestrial ecosystems and draw attention to the important, but often overlooked role of lichens as determinants of ecological processes. We start by assessing characteristics that vary among lichens and that may be important in determining their ecological role; these include their growth form, the types of photobionts that they contain, their key functional traits, their water-holding capacity, their colour, and the levels of secondary compounds in their thalli. We then assess how these differences among lichens influence their impacts on ecosystem and community processes. As such, we consider the consequences of these differences for determining the impacts of lichens on ecosystem nutrient inputs and fluxes, on the loss of mass and nutrients during lichen thallus decomposition, and on the role of lichenivorous invertebrates in moderating decomposition. We then consider how differences among lichens impact on their interactions with consumer organisms that utilize lichen thalli, and that range in size from microfauna (for which the primary role of lichens is habitat provision) to large mammals (for which lichens are primarily a food source). We then address how differences among lichens impact on plants, through for example increasing nutrient inputs and availability during primary succession, and serving as a filter for plant seedling establishment. Finally we identify areas in need of further work for better understanding the role of lichens in terrestrial ecosystems. These include understanding how the high intraspecific trait variation that characterizes many lichens impacts on community assembly processes and ecosystem functioning, how multiple species mixtures of lichens affect the key community- and ecosystem-level processes that they drive, the extent to which lichens in early succession influence vascular plant succession and ecosystem development in the longer term, and how global change drivers may impact on ecosystem functioning through altering the functional composition of lichen communities.
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12.
  • Asplund, Johan, et al. (författare)
  • Removal of secondary compounds increases invertebrate abundance in lichens
  • 2015
  • Ingår i: Fungal Ecology. - : Elsevier BV. - 1754-5048 .- 1878-0083. ; 18, s. 18-25
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigated how lichen carbon-based secondary compounds (CBSCs) affect abundance of invertebrates in five lichen species growing on the forest floor (Cladonia rangiferina, Cladonia stellaris) or on tree trunks (Evernia prunastri, Hypogymnia physodes, Pseudevemia furfuracea). To do this, CBSCs were removed by rinsing lichen thalli in acetone (which has no adverse effects on the lichens) and the lichens were re-transplanted in their natural habitat. After 4 months there was higher abundance of mites, springtails and spiders in the three epiphytic lichens that had their CBSC concentrations reduced. The increase in predatory spiders following CBSC reduction suggests that the compounds have multitrophic consequences. The acetone treatment reduced the number of nematodes in four of the lichen species. Given that lichens serve as important habitats for a diverse range of invertebrates, increased knowledge of how lichen CBSCs may regulate their abundance helps us to better understand the role that lichens and their defence compounds play in structuring forest food webs. (C) 2015 Elsevier Ltd and The British Mycological Society. All rights reserved.
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13.
  • Bokhorst, Stef Frederik, et al. (författare)
  • Contrasting responses of springtails and mites to elevation and vegetation type in the sub-Arctic
  • 2018
  • Ingår i: Pedobiologia. - : Elsevier. - 0031-4056 .- 1873-1511. ; 67, s. 57-64
  • Tidskriftsartikel (refereegranskat)abstract
    • Climate change is affecting the species composition and functioning of Arctic and sub-Arctic plant and soil communities. Here we studied patterns in soil microarthropod (springtails and mites) communities across a gradient of increasing elevation that spanned 450 m, across which mean temperature declined by approximately 2.5 degrees C, in sub-Arctic Sweden. Across this gradient we characterized microarthropod communities in each of two types of vegetation, i.e., heath and meadow, to determine whether their responses to declining temperature differed with vegetation type. Mite abundance declined with increasing elevation, while springtail abundance showed the opposite response. Springtail communities were dominated by larger species at higher elevation. Mite abundance was unaffected by vegetation type, while springtail abundance was 53% higher in the heath than meadow vegetation across the gradient. Springtails but not mites responded differently to elevation in heath and meadow vegetation; hemi-edaphic species dominated in the heath at higher elevation while epiedaphic species dominated in the meadow. Our results suggest that sub-Arctic mite and springtail communities will likely respond in contrasting ways to changes in vegetation and soil properties resulting from climate warming.
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14.
  • Bokhorst, Stef Frederik, et al. (författare)
  • Lichen physiological traits and growth forms affect communities of associated invertebrates
  • 2015
  • Ingår i: Ecology. - : Wiley. - 0012-9658 .- 1939-9170. ; 96, s. 2394-2407
  • Tidskriftsartikel (refereegranskat)abstract
    • While there has been much interest in the relationships between traits of primary producers and composition of associated invertebrate consumer communities, our knowledge is largely based on studies from vascular plants, while other types of functionally important producers, such as lichens, have rarely been considered. To address how physiological traits of lichens drive community composition of invertebrates, we collected thalli from 27 lichen species from southern Norway and quantified the communities of associated springtails, mites, and nematodes. For each lichen species, we measured key physiological thallus traits and determined whether invertebrate communities were correlated with these traits. We also explored whether invertebrate communities differed among lichen groups, categorized according to nitrogen-fixing ability, growth form, and substratum. Lichen traits explained up to 39% of the variation in abundances of major invertebrate groups. For many invertebrate groups, abundance was positively correlated with lichen N and P concentrations, N: P ratio, and the percentage of water content on saturation (WC), but had few relationships with concentrations of carbon-based secondary compounds. Diversity and taxonomic richness of invertebrate groups were sometimes also correlated with lichen N and N: P ratios. Nitrogen-fixing lichens showed higher abundance and diversity of some invertebrate groups than did non-N-fixing lichens. However, this emerged in part because most N-fixing lichens have a foliose growth form that benefits invertebrates, through improving the microclimate, independently of N concentration. Furthermore, invertebrate communities associated with terricolous lichens were determined more by their close proximity to the soil invertebrate pool than by lichen traits. Overall, our results reveal that differences between lichen species have a large impact on the invertebrate communities that live among the thalli. Different invertebrate groups show contrasting responses to traits that are indicative of thallus quality (nutrient concentrations), and thallus growth form is often an important determinant of the invertebrate community. Given the large diversity of lichen traits and growth forms that occur in many ecosystems, lichen-invertebrate communities may be an important contributor to overall community diversity in boreal forests.
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15.
  • Bokhorst, Stef Frederik, et al. (författare)
  • Micro-arthropod community responses to ecosystem retrogression in boreal forest
  • 2017
  • Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 110, s. 79-86
  • Tidskriftsartikel (refereegranskat)abstract
    • Explaining the variation in communities of soil organisms across plant communities or ecosystems remains a major challenge for ecologists. Several studies have explored how soil communities are affected along ecosystem successional gradients but most of these are based on relatively short term chronosequences. To address the impact of ecosystem age on micro-arthropod communities, we utilized a 5000 year old post-fire chronosequence, which consists of thirty lake islands differing greatly in time since fire in the boreal forested zone of northern Sweden. The Acari community did not change along this chronosequence, indicating that Acari rapidly(<60 yr) reach equilibrium after forest fire and that they are relatively unresponsive to subsequent long term changes in plant community composition and soil quality. The Collembola community composition, however, showed greater responsiveness to the chronosequence and this was best explained through their functional traits. Notably, the youngest (most recently burned) islands, which had the highest ecosystem productivity and fungal mass turnover, were dominated by soil-dwelling (eu-edaphic) Collembola species that are best positioned to take advantage of resource input to the soil. Although plant community characteristics did not emerge as powerful drivers of the Collembola community, we found that Collembola community composition was related to the quality (N and P) of the soil substrate, which reflects a long term legacy of the plant community. Collembola life history characteristics proved to be important for understanding how abundances of different taxa varied relative to one another across the gradients of plant diversity and substrate quality gradients that occur across long-term chronosequences. The causal connection between vertical stratification of Collembola and substrate quality is at present unclear but is likely to be related to their feeding preferences and microhabitat conditions. Because the soil-dwelling Collembola showed a strong decline in abundance with ecosystem retrogression while surface-dwelling Collembola did not these two life history groups may operate as functionally distinct groups within the soil food web across these long-term chronosequences. 2017 Elsevier Ltd. All rights reserved.
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16.
  • Bokhorst, Stef Frederik, et al. (författare)
  • Responses of communities of soil organisms and plants to soil aging at two contrasting long-term chronosequences
  • 2017
  • Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 106, s. 69-79
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil fertility and vegetation are major drivers of soil communities. Soil community responses to vegetation development and associated changes in soil fertility have been mostly reported for chronosequences that span time scales from decades to centuries. Here we evaluated soil communities for two contrasting chronosequences, the Franz Josef chronosequence in southern New Zealand caused by glacial retreat and spanning 120,000 years, and the Cooloola chronosequence in eastern Australia caused by aeolian movement of sand that spans 700,000 years. Both chronosequences feature later-phase retrogressive stages characterized by reduced nutrient availability and plant stature. We hypothesized that soil communities would mirror the patterns of vegetation across these long-term chronosequences with organism biomasses, abundances and diversity increasing throughout early stages of the succession and declining at retrogression stages. The hypothesis was not consistently supported. Bacterial and fungal biomass increased across the youngest chronosequence stages but remained unchanged across the later stages, while fungal-to-bacterial ratios increased throughout. Microbial biomass was related to soil nitrogen concentrations across both chronosequences. Invertebrate abundance and richness increased during the early stages of ecosystem development in both chronosequences, but different groups peaked at different stages at each chronosequence, and not all invertebrate groups declined during the retrogressive stages. Invertebrate groups had no consistent correlations with biotic or abiotic ecosystem properties across either chronosequence. Our study demorBtrates that soil organisms track changes in plant biomass and richness and soil fertility during the initial stages of both chronosequences, but with increasing age of the chronosequences, these relationships weaken and other factors drive the soil community. Possible explanations for the different patterns in soil communities at the two chronosequences include that they differ strongly in soil organic matter, nutrient concentrations and abundances of soil organisms (all of which are much higher at Franz Josef than Cooloola), overlaid with different macroclimate and geology, so that different factors are likely to restrict the presence of particular organisms across both chronosequences. As such, while soil fertility and vegetation are widely recognized as important drivers of the soil community, the manner in which these factors directly and indirectly shape the soil community can vary greatly across organism groups, among chronosequences, and over the time scales that ecosystems develop. (C) 2016 Elsevier Ltd. All rights reserved.
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17.
  • Cavard, Xavier, et al. (författare)
  • Disentangling Effects of Time Since Fire, Overstory Composition and Organic Layer Thickness on Nutrient Availability in Canadian Boreal Forest
  • 2019
  • Ingår i: Ecosystems. - : Springer Science and Business Media LLC. - 1432-9840 .- 1435-0629. ; 22, s. 33-48
  • Tidskriftsartikel (refereegranskat)abstract
    • Wildfire is the primary abiotic disturbance in the boreal forest, and its long-term absence can lead to large changes in ecosystem properties, including the availability and cycling of nutrients. These effects are, however, often confounded with the effects of successional changes in vegetation toward nutrient-conservative species. We studied a system of boreal forested lake islands in eastern Canada, where time since last fire ranged from 50 to 1500years, and where the relative abundance of the most nutrient-conservative tree species, black spruce, was largely independent of time since last fire. This allowed us to disentangle the effects of time since fire and the dominant vegetation on ecosystem properties, including nutrient stocks and concentrations. Effects of time since fire independent of vegetation composition mostly involved an increase in the thickness of the organic layer and in nitrogen concentration in both soil and leaves. Domination by black spruce had strong negative effects on nutrient concentrations and was associated with a shift toward more fungi and Gram-positive bacteria in the soil microbial community. Path modeling showed that phosphorus concentration was inversely related to organic layer thickness, which was in turn related to both time since fire and black spruce abundance, while nitrogen was more directly related to time since fire and the composition of the overstory. We conclude that discriminating between the effects of vegetation and time since fire is necessary for better understanding and predicting the long-term changes that occur in forest nutrient availability and ecosystem properties.
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20.
  • De Long, Jonathan, et al. (författare)
  • Contrasting Responses of Soil Microbial and Nematode Communities to Warming and Plant Functional Group Removal Across a Post-fire Boreal Forest Successional Gradient
  • 2016
  • Ingår i: Ecosystems (New York. Print). - : Springer Science and Business Media LLC. - 1432-9840 .- 1435-0629. ; 19:2, s. 339-355
  • Tidskriftsartikel (refereegranskat)abstract
    • Global warming is causing increases in surface temperatures and has the potential to influence the structure of soil microbial and faunal communities. However, little is known about how warming interacts with other ecosystem drivers, such as plant functional groups or changes associated with succession, to affect the soil community and thereby alter ecosystem functioning. We investigated how experimental warming and the removal of plant functional groups along a post-fire boreal forest successional gradient impacted soil microbial and nematode communities. Our results showed that warming altered soil microbial communities and favored bacterial-based microbial communities, but these effects were mediated by mosses and shrubs, and often varied with successional stage. Meanwhile, the nematode community was generally unaffected by warming and was positively affected by the presence of mosses and shrubs, with these effects mostly independent of successional stage. These results highlight that different groups of soil organisms may respond dissimilarly to interactions between warming and changes to plant functional groups, with likely consequences for ecosystem functioning that may vary with successional stage. Due to the ubiquitous presence of shrubs and mosses in boreal forests, the effects observed in this study are likely to be significant over a large proportion of the terrestrial land surface. Our results demonstrate that it is crucial to consider interactive effects between warming, plant functional groups, and successional stage when predicting soil community responses to global climate change in forested ecosystems.
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21.
  • De Long, Jonathan, et al. (författare)
  • Effects of elevation and nitrogen and phosphorus fertilization on plant defence compounds in subarctic tundra heath vegetation
  • 2016
  • Ingår i: Functional Ecology. - : Wiley. - 0269-8463 .- 1365-2435. ; 30:2, s. 314-325
  • Tidskriftsartikel (refereegranskat)abstract
    • Plant chemical and structural defence compounds are well known to impact upon herbivory of fresh leaves and influence decomposition rates after leaf senescence. A number of theories predict that alleviating nutrient limitation and reducing other environmental stressors will result in decreased production of plant chemical defences. In this study, we measured plant defence properties [total polyphenols (TP), condensed tannins (CT) and lignin concentrations, and protein complexation capacity (PCC)] in both fresh and senesced plant leaves in a fully factorial N and P fertilization experiment set-up at each of three elevations along an elevational gradient in Swedish subarctic tundra heath vegetation. Further, we performed a decomposition of variance analysis on community-weighted averages (CWAs) of plant defence properties to determine the relative contributions of interspecific and intraspecific variation to the total variation observed in response to elevation and nutrient addition. We hypothesized that N fertilization would reduce plant defence properties and that this reduction would be greater at higher elevations, while the effects of P fertilization would have no effect at any elevation. At the community level, N addition reduced CT and PCC in both fresh and senesced leaves and TP in senesced leaves, while P addition had few effects, broadly in line with our hypothesis. The effects of N addition frequently varied with elevation, but in contrast to our hypothesis, the said effects were strongest at the lowest elevations. The effects of N addition and the interactive effect of N with elevation were primarily driven by intraspecific, rather than interspecific, variation. Our findings suggest that as temperatures warm and N availability increases due to global climate change, secondary metabolites in subarctic heath vegetation will decline particularly within species. Our results highlight the need to consider the effects of both nutrient availability and temperature, and their interaction, in driving subarctic plant defence.
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23.
  • De Long, Jonathan, et al. (författare)
  • Understory plant functional groups and litter species identity are stronger drivers of litter decomposition than warming along a boreal forest post-fire successional gradient
  • 2016
  • Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 98, s. 159-170
  • Tidskriftsartikel (refereegranskat)abstract
    • Increasing surface temperatures due to climate change have the potential to alter plant litter mass loss and nutrient release during decomposition. However, a great deal of uncertainty remains concerning how ecosystem functioning may be affected by interactions between warming and other drivers, such as plant functional group composition and environmental context. In this study, we investigated how vascular plant litter decomposition and nutrient release were affected by experimental warming, moss removal and shrub removal along a post-fire boreal forest successional gradient. Our results show that litter decomposition and nutrient loss were primarily driven by understory plant functional group removal. The removal of mosses generally reduced litter mass loss and increased litter phosphorus (P) loss, while shrub removal typically increased litter mass loss and in one litter species reduced immobilization of P. Litter nitrogen (N) loss was unaffected by plant functional group removal. Warming interacted with successional stage and species identity of the litter decomposed, but these effects were uncommon and generally weak. As climate change advances, moss cover is expected to decrease, while shrub cover is expected to increase. Taken together with our results, this suggests that lower moss cover will decrease leaf litter decomposition rates and increase P release from litter, while increasing shrub cover will decrease decomposition rates and may reduce P release from litter. Our results demonstrate that in the short term, the direct effects of warming and successional stage will play a relatively minor role in driving litter decomposition processes in the boreal forest. In the long term, as the climate warms, temperature and its indirect effects via changes in the understory vegetation will play an important role in driving litter decomposition, thereby potentially altering C storage and nutrient cycling. 
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24.
  • Engelbrecht Clemmensen, Karina, et al. (författare)
  • Carbon sequestration is related to mycorrhizal fungal community shifts during long-term succession in boreal forests
  • 2015
  • Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 205, s. 1525-1536
  • Tidskriftsartikel (refereegranskat)abstract
    • Boreal forest soils store a major proportion of the global terrestrial carbon (C) and below-ground inputs contribute as much as above-ground plant litter to the total C stored in the soil. A better understanding of the dynamics and drivers of root-associated fungal communities is essential to predict long-term soil C storage and climate feedbacks in northern ecosystems. We used 454-pyrosequencing to identify fungal communities across fine-scaled soil profiles in a 5000yr fire-driven boreal forest chronosequence, with the aim of pinpointing shifts in fungal community composition that may underlie variation in below-ground C sequestration. In early successional-stage forests, higher abundance of cord-forming ectomycorrhizal fungi (such as Cortinarius and Suillus species) was linked to rapid turnover of mycelial biomass and necromass, efficient nitrogen (N) mobilization and low C sequestration. In late successional-stage forests, cord formers declined, while ericoid mycorrhizal ascomycetes continued to dominate, potentially facilitating long-term humus build-up through production of melanized hyphae that resist decomposition. Our results suggest that cord-forming ectomycorrhizal fungi and ericoid mycorrhizal fungi play opposing roles in below-ground C storage. We postulate that, by affecting turnover and decomposition of fungal tissues, mycorrhizal fungal identity and growth form are critical determinants of C and N sequestration in boreal forests.See also the Commentary by Christopher W. Fernandez and Peter G. Kennedy
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25.
  • Fanin, Nicolas, et al. (författare)
  • Consistent effects of biodiversity loss on multifunctionality across contrasting ecosystems
  • 2018
  • Ingår i: Nature ecology & evolution. - : Springer Science and Business Media LLC. - 2397-334X. ; 2, s. 269-278
  • Tidskriftsartikel (refereegranskat)abstract
    • Understanding how loss of biodiversity affects ecosystem functioning, and thus the delivery of ecosystem goods and services, has become increasingly necessary in a changing world. Considerable recent attention has focused on predicting how biodiversity loss simultaneously impacts multiple ecosystem functions (that is, ecosystem multifunctionality), but the ways in which these effects vary across ecosystems remain unclear. Here, we report the results of two 19-year plant diversity manipulation experiments, each established across a strong environmental gradient. Although the effects of plant and associated fungal diversity loss on individual functions frequently differed among ecosystems, the consequences of biodiversity loss for multifunctionality were relatively invariant. However, the context-dependency of biodiversity effects also worked in opposing directions for different individual functions, meaning that similar multifunctionality values across contrasting ecosystems could potentially mask important differences in the effects of biodiversity on functioning among ecosystems. Our findings highlight that an understanding of the relative contribution of species or functional groups to individual ecosystem functions among contrasting ecosystems and their interactions (that is, complementarity versus competition) is critical for guiding management efforts aimed at maintaining ecosystem multifunctionality and the delivery of multiple ecosystem services.
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26.
  • Fanin, Nicolas, et al. (författare)
  • Effects of plant functional group removal on structure and function of soil communities across contrasting ecosystems
  • 2019
  • Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 22, s. 1095-1103
  • Tidskriftsartikel (refereegranskat)abstract
    • Loss of plant diversity has an impact on ecosystems worldwide, but we lack a mechanistic understanding of how this loss may influence below-ground biota and ecosystem functions across contrasting ecosystems in the long term. We used the longest running biodiversity manipulation experiment across contrasting ecosystems in existence to explore the below-ground consequences of 19 years of plant functional group removals for each of 30 contrasting forested lake islands in northern Sweden. We found that, against expectations, the effects of plant removals on the communities of key groups of soil organisms (bacteria, fungi and nematodes), and organic matter quality and soil ecosystem functioning (decomposition and microbial activity) were relatively similar among islands that varied greatly in productivity and soil fertility. This highlights that, in contrast to what has been shown for plant productivity, plant biodiversity loss effects on below-ground functions can be relatively insensitive to environmental context or variation among widely contrasting ecosystems.
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27.
  • Fanin, Nicolas, et al. (författare)
  • The ratio of Gram-positive to Gram-negative bacterial PLFA markers as an indicator of carbon availability in organic soils
  • 2019
  • Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 128, s. 111-114
  • Tidskriftsartikel (refereegranskat)abstract
    • Despite recent progress in understanding soil microbial responses to carbon (C) limitation, the functional shifts in microbial community structure associated with decreasing soil C availability and changes in organic matter chemistry remain poorly known. It has been proposed that Gram-negative (GN) bacteria use more plant-derived C sources that are relatively labile, while Gram-positive (GP) bacteria use C sources derived from soil organic matter that are more recalcitrant. Because these two groups may differ in how they influence the fate of different C forms in soils, it is important to understand how they vary across ecosystems that differ in their vegetation cover and ecosystem productivity or across environmental gradients. In this study, we used a 19-year plant functional group removal experiment across a long term post-fire chronosequence to assess how microbial community structure (assessed using phospholipids fatty acids; PLFAs) and the association of bacterial functional groups (specifically, the GP:GN ratio) responded to changes in organic matter chemistry (measured via nuclear magnetic resonance; NMR). We found that the GP:GN ratio increased upon removal of shrubs and tree roots and with decreasing ecosystem productivity along the chronosequence, thus showing the greater dependence of GN than GP bacteria on more labile plant-derived C. Overall, GN bacteria were associated with simple C compounds (alkyls) whereas GP bacteria were more strongly associated with more complex C forms (carbonyls). Therefore, we conclude that the GP:GN ratio has potential as a useful indicator of the relative C availability for soil bacterial communities in organic soils, and can be used as a coarse indicator of energy limitation in natural ecosystems.
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28.
  • Freschet, Gregoire, et al. (författare)
  • Climate, soil and plant functional types as drivers of global fine-root trait variation
  • 2017
  • Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 105, s. 1182-1196
  • Tidskriftsartikel (refereegranskat)abstract
    • 1. Ecosystem functioning relies heavily on below-ground processes, which are largely regulated by plant fine-roots and their functional traits. However, our knowledge of fine-root trait distribution relies to date on local-and regional-scale studies with limited numbers of species, growth forms and environmental variation.2. We compiled a world-wide fine-root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine-root trait variation. Most particularly, we tested the competing hypotheses that fine-root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness.3. We demonstrate that: (i) Climate conditions promoting soil fertility relate negatively to fine-root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine-root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (ii) Soil bulk density strongly influences species fine-root morphology, by favouring thicker, denser fine-roots; (iii) Fine-roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N-2-fixing capacity positively relates to root nitrogen; and (iv) Plants growing in pots have higher SRL than those grown in the field.4. Synthesis. This study reveals both the large variation in fine-root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine-root trait variation. High trait variation occurred at local scales, suggesting that wide-ranging below-ground resource economics strategies are viable within most climatic areas and soil conditions.
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29.
  • Freschet, Gregoire, et al. (författare)
  • Explaining within-community variation in plant biomass allocation: a balance between organ biomass and morphology above vs below ground?
  • 2015
  • Ingår i: Journal of Vegetation Science. - : Wiley. - 1100-9233 .- 1654-1103. ; 26, s. 431-440
  • Tidskriftsartikel (refereegranskat)abstract
    • QuestionsIt remains unresolved why, despite the obvious functional importance of leaves and roots, co-existing plant species can display highly contrasting biomass distributions of these organs. Building on the functional equilibrium' hypothesis, we hypothesize that co-existing species can each achieve balanced resource acquisition above vs below ground by trading off the biomass vs morphology of structures responsible for resource acquisition, i.e. leaves and fine roots.MethodsWe tested this hypothesis in a natural field setting by measuring plant above- and below-ground biomass and morphological traits associated with resource uptake - specific leaf area (SLA) and specific root length (SRL) - of 18 dominant angiosperm species from a sub-alpine plant community.LocationNew Zealand South Island.ResultsWe found a significant negative relationship between the species leaf mass to fine root mass ratio and the SLA to SRL ratio when we considered eudicot species only. The SLA to SRL ratio and plant size explained 31% and 34% of the total variation in the species leaf to fine root mass ratio respectively, and 45% when used in combination (P<0.05 in all cases). Within a given plant size, 90% of the variation among species in total leaf area was due to differences in SLA, whereas variation in the fine root mass fraction was responsible for 71% of the variation among species in fine root length.ConclusionsIn support of our hypothesis, part of the difference between co-occurring species in leaf and fine root biomass distribution could be explained by the variable morphologies of these organs as well as variation in plant size, independent of the plant economic strategy. We expect that this outcome may result from environmental and evolutionary constraints on plant species-average traits, as well as plastic responses to local environmental conditions. These findings help explain why a diversity of strategies for achieving balanced resource acquisition can co-exist within a single plant community.
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30.
  • Frost, Carol, et al. (författare)
  • Using Network Theory to Understand and Predict Biological Invasions
  • 2019
  • Ingår i: Trends in ecology & evolution. - : Elsevier BV. - 0169-5347 .- 1872-8383. ; 34, s. 831-843
  • Forskningsöversikt (refereegranskat)abstract
    • Understanding and predicting biological invasions is challenging because of the complexity of many interacting players. A holistic approach is needed with the potential to simultaneously consider all relevant effects and effectors. Using networks to describe the relevant anthropogenic and ecological factors, from community-level to global scales, promises advances in understanding aspects of invasion from propagule pressure, through establishment, spread, and ecological impact of invaders. These insights could lead to development of new tools for prevention and management of invasions that are based on species' network characteristics and use of networks to predict the ecological effects of invaders. Here, we review the findings from network ecology that show the most promise for invasion biology and identify pressing needs for future research.
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31.
  • Giesler, Reiner, et al. (författare)
  • Boreal Forests Sequester Large Amounts of Mercury over Millennial Time Scales in the Absence of Wildfire
  • 2017
  • Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 51:5, s. 2621-2627
  • Tidskriftsartikel (refereegranskat)abstract
    • Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the land air exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R-2 = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.
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32.
  • Gundale, Michael, et al. (författare)
  • Comparison of plant-soil feedback experimental approaches for testing soil biotic interactions among ecosystems
  • 2019
  • Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 221, s. 577-587
  • Tidskriftsartikel (refereegranskat)abstract
    • The study of interactions and feedbacks between plants and soils is a rapidly expanding research area, and a primary tool used in this field is to perform glasshouse experiments where soil biota are manipulated. Recently, there has been vigorous debate regarding the correctness of methods for carrying out these types of experiment, and specifically whether it is legitimate to mix soils from different sites or plots (mixed soil sampling, MSS) or not (independent soil sampling, ISS) to create either soil inoculum treatments or subjects. We performed the first empirical comparison of MSS vs ISS approaches by comparing growth of two boreal tree species (Picea abies and Pinus sylvestris) in soils originating from 10 sites near the boreal forest limit in northern Sweden, and 10 sites in the subarctic region where boreal forests may potentially expand as a result of climate change. We found no consistent differences in the conclusions that we reached whether we used MSS or ISS approaches. We propose that researchers should not choose a soil handling method based on arguments that one method is inherently more correct than the other, but rather that method choice should be based on correct alignment with specific research questions and goals.
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33.
  • Gundale, Michael, et al. (författare)
  • Differences in endophyte communities of introduced trees depend on the phylogenetic relatedness of the receiving forest
  • 2016
  • Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 104:5, s. 1219-1232
  • Tidskriftsartikel (refereegranskat)abstract
    • Plant species sometimes perform extraordinarily well when introduced to new environments, through achieving higher growth rates, individual biomasses or higher densities in their receiving communities compared to their native range communities. One hypothesis proposed to explain enhanced performance in species’ new environments is that their soil microbial communities may be different and provide greater benefit than microbial communities encountered in species’ native environments. However, detailed descriptions of soil biota associated with species in both their native and introduced environments remain scarce. We established a global network of sites in regions where the tree species Pinus contorta has been introduced (Chile, New Zealand, Finland, Scotland and Sweden), as well as native range sites where the introduced populations originated (Canada and USA). We conducted pyrosequencing analysis to compare the root fungal endophyte communities associated with P. contorta in its native environments and in introduced environments with phylogenetically similar and dissimilar tree species (i.e. P. sylvestris in Europe and Nothofagus spp. in the Southern Hemisphere). Fungal communities associated with P. contorta consistently differed between its introduced and native environments. In Europe, P. contorta associated with the same community as P. sylvestris, where one particular species (Piloderma sphaerosporum) was particularly abundant relative to Canadian sites. In the Southern Hemisphere, P. contorta fungal communities were composed primarily of North American taxa and exhibited very little overlap with fungal communities associated with native Nothofagus spp. Synthesis. Our work shows that plants exhibit considerable plasticity in their interaction with fungi, by associating with different fungal communities across native and introduced environments. Our work also indicates that fungal communities associated with introduced plants can assemble through different mechanisms, that is by associating with existing fungal communities of phylogenetically close species, or through reassembly of co-introduced and co-invading fungi. The identification of different fungal communities in a plant species new environment provides an important step forward in understanding how soil biota may impact growth and invasion when a species is introduced to new environments.
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34.
  •  
35.
  • Gundale, Michael, et al. (författare)
  • The effect of biochar management on soil and plant Community properties in a boreal forest
  • 2016
  • Ingår i: GCB Bioenergy. - : Wiley. - 1757-1693 .- 1757-1707. ; 8, s. 777-789
  • Tidskriftsartikel (refereegranskat)abstract
    • Biochar management has been proposed as a possible tool to mitigate anthropogenic CO2 emissions, and thus far its impacts in forested environments remain poorly understood. We conducted a large-scale, replicated field experiment using 0.05-ha plots in the boreal region in northern Sweden to evaluate how soil and vegetation properties and processes responded to biochar application and the disturbance associated with burying biochar in the soil. We employed a randomized block design, where biochar and soil mixing treatments were established in factorial combination (i.e., control, soil mixing only, biochar only, and biochar and soil mixing; n=6 plots of each). After two growing seasons, we found that biochar application enhanced net soil N mineralization rates and soil NH4+ concentrations regardless of the soil mixing treatment, but had no impact on the availability of NO3- , the majority of soil microbial community parameters, or soil resPiration. Meanwhile, soil mixing enhanced soil NO3- concentrations, but had negative impacts on net N mineralization rates and several soil microbial community variables. Many of the effects of soil mixing on soil nutrient and microbial community properties were less extreme when biochar was also added. Biochar addition had almost no effects on vegetation properties (except for a small reduction in species richness of the ground layer vegetation), while soil mixing caused significant reductions in graminoid and total ground layer vegetation cover, and enhanced seedling survival rates of P.sylvestris, and seed germination rates for four tree species. Our results suggest that biochar application can serve as an effective tool to store soil C in boreal forests while enhancing NH4+ availability. They also suggest that biochar may serve as a useful complement to site preparation techniques that are frequently used in the boreal region, by enhancing soil fertility and reducing nutrient losses when soils are scarified during site preparation.
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36.
  • Henriksson, Anna, et al. (författare)
  • Biotic resistance in freshwater fish communities : species richness, saturation or species identity?
  • 2015
  • Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 124:8, s. 1058-1064
  • Tidskriftsartikel (refereegranskat)abstract
    • Some communities are susceptible to invasions and some are not. Why? Elton suggested in 1958 that the ability of the community to withstand invading species - its biotic resistance - depends on the number of resident species. Later contributors have emphasized the habitat's ability to support species, as well as the contribution of individual species to the resistance. In this study we use information from 184 introductions of Arctic char into Swedish lakes to study both abiotic and biotic aspects of the resident community's ability to resist introductions. We find that the best model included the proportion of forest cover and the proportion of agricultural land cover in the watershed in combination with the presence versus absence of northern pike. Thus, the most important biotic factor to explain the outcome of introductions of Arctic char is the presence of northern pike, a large piscivore. This means that one single species explains the outcome of the introductions better than does the species richness or the saturation level of the community.
  •  
37.
  • Henriksson, Anna (författare)
  • Biotic resistance in freshwater fish communities
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Invasions of non-native species cause problems in ecosystems worldwide, and despite the extensive effort that has been put into research about invasions, we still lack a good understanding for why some, but not other, communities resist these invasions. In this doctoral thesis I test hypotheses on biotic resistance using a large dataset of more than 1000 both failed and successful introductions of freshwater fish into Swedish lakes. We have found that the classic species richness hypothesis is a poor descriptor of introduction success because it fails to acknowledge that resident species contribute to the resistance in different ways. We developed a new measure of biotic resistance, the weighted species richness, which takes into account that the resident species contributes to the resistance with different strength and sign. Further, we correlated performance traits of species in their role as an invader and as a resident species to predict how the biotic resistance of these communities would develop over time. We found a positive correlation between performance traits: Some species have high introduction success, they make a large contribution to the resistance, and they cause extinctions when introduced but do not go extinct themselves when other species establishes, whereas other species are weak performers in these respects. Thus, the biotic resistance of these communities should grow stronger as non-native species accumulates. These results give us clues about what type of communities that should be most sensitive to further invasions, i.e., communities harboring species weak performers. My results show that the biotic resistance of communities is an important factor in determining invasibility of a community. They also show that methods for quantifying resistance must take into account how interactions are structured in nature. What determine the biotic resistance of a community is the type of interactions that the resident species have with the invader and not the species richness of the community.
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38.
  • Henriksson, Anna, et al. (författare)
  • Strong invaders are strong defenders : implications for the resistance of invaded communities
  • 2016
  • Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 19:4, s. 487-494
  • Tidskriftsartikel (refereegranskat)abstract
    • Many ecosystems receive a steady stream of non-native species. How biotic resistance develops over time in these ecosystems will depend on how established invaders contribute to subsequent resistance. If invasion success and defence capacity (i.e. contribution to resistance) are correlated, then community resistance should increase as species accumulate. If successful invaders also cause most impact (through replacing native species with low defence capacity) then the effect will be even stronger. If successful invaders instead have weak defence capacity or even facilitative attributes, then resistance should decrease with time, as proposed by the invasional meltdown hypothesis. We analysed 1157 introductions of freshwater fish in Swedish lakes and found that species' invasion success was positively correlated with their defence capacity and impact, suggesting that these communities will develop stronger resistance over time. These insights can be used to identify scenarios where invading species are expected to cause large impact.
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39.
  • Henriksson, Anna, et al. (författare)
  • Weighted species richness outperforms species richness as predictor of biotic resistance
  • 2016
  • Ingår i: Ecology. - : Wiley. - 0012-9658 .- 1939-9170. ; 97:1, s. 262-271
  • Tidskriftsartikel (refereegranskat)abstract
    • The species richness hypothesis, which predicts that species-rich communities should be better at resisting invasions than species-poor communities, has been empirically tested many times and often poorly supported. In this paper we contrast the species richness hypothesis with four alternative hypotheses with the aim of finding better descriptors of invasion resistance. These alternative hypotheses state that resistance to invasions is determined by abiotic conditions, community saturation (i.e., the number of resident species relative to the maximum number of species that can be supported), presence/absence of key species, or weighted species richness. Weighted species richness is a weighted sum of the number of species, where each species' weight describes its contribution to resistance. We tested these hypotheses using data on the success of 571 introductions of four freshwater fish species into lakes throughout Sweden (i.e., Arctic char (Salvelinus alpinus), tench (Tinca tinca), zander (Sander lucioperca), and whitefish (Coregonus lavaretus)). We found that the weighted species richness best predicted invasion success. The weights describing the contribution of each resident species to community resistance varied considerably in both strength and sign. Positive resistance weights, which indicate that species repel invaders, were as common as negative resistance weights, which indicate facilitative interactions. This result can be contrasted with the implicit assumption of the original species richness hypothesis, that all resident species have negative effects on invader success. We argue that this assumption is unlikely to be true in natural communities, and thus that we expect that weighted species richness is a better predictor of invader success than the actual number of resident species.
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40.
  • Jonsson, Micael, et al. (författare)
  • Direct and Indirect Drivers of Moss Community Structure, Function, and Associated Microfauna Across a Successional Gradient
  • 2015
  • Ingår i: Ecosystems. - : Springer Science and Business Media LLC. - 1432-9840 .- 1435-0629. ; 18:1, s. 154-169
  • Tidskriftsartikel (refereegranskat)abstract
    • Relative to vascular plants, little is known about what factors control bryophyte communities or how they respond to successional and environmental changes. Bryophytes are abundant in boreal forests, thus changes in moss community composition and functional traits (for example, moisture and nutrient content; rates of photosynthesis and respiration) may have important consequences for ecosystem processes and microfaunal communities. Through synthesis of previous work and new analyses integrating new and published data from a long-term successional gradient in the boreal forest of northern Sweden, we provide a comprehensive view of the biotic factors (for example, vascular plant productivity, species composition, and diversity) and abiotic factors (for example, soil fertility and light transmission) that impact the moss community. Our results show that different aspects of the moss community (that is, composition, functional traits, moss-driven processes, and associated invertebrate fauna) respond to different sets of environmental variables, and that these are not always the same variables as those that influence the vascular plant community. Measures of moss community composition and functional traits were primarily influenced by vascular plant community composition and productivity. This suggests that successional shifts in abiotic variables, such as soil nutrient levels, indirectly affect the moss community via their influence on vascular plant community characteristics, whereas direct abiotic effects are less important. Among the moss-driven processes, moss litter decomposition and moss productivity were mainly influenced by biotic variables (notably the community characteristics of both vascular plants and mosses), whereas moss functional traits (primarily specific leaf area and tissue nutrient concentrations) also were important in explaining moss di-nitrogen-fixation rates. In contrast, both abiotic and biotic variables were important drivers of moss microfaunal community structure. Taken together, our results show which abiotic and biotic factors impact mosses and their associated organisms, and thus highlight that multiple interacting factors need to be considered to understand how moss communities, associated food webs, and the ecosystem processes they influence will respond to environmental change.
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41.
  • Jonsson, Micael, et al. (författare)
  • Divergent responses of beta-diversity among organism groups to a strong environmental gradient
  • 2016
  • Ingår i: Ecosphere. - : Wiley. - 2150-8925 .- 2150-8925. ; 7:10
  • Tidskriftsartikel (refereegranskat)abstract
    • A limited understanding of how variation in the species composition among communities (i.e., beta-diversity) changes along natural environmental gradients, and the mechanisms responsible, inhibits our ability to understand large-scale biodiversity change resulting from either natural or anthropogenic drivers. Therefore, our aim was to test key drivers of beta-diversity patterns along a strong, natural environmental gradient for seven widely different organisms groups, that is, root-associated fungi, litter fungi, soil nematodes, vascular plants, epiphytic lichens, beetles, and spiders. Using previously published community-level data from boreal-forested islands, we calculated alpha-diversity and beta-diversity for each of the seven organism groups. Out of several available environmental variables, we identified four variables, that is, ecosystem age, total C storage, net primary productivity (NPP), and N-to-P ratio, as potential predictors of variation in beta-diversity. We found that ecosystem age was the variable with the highest overall importance. We then used two different methods to quantify the relative importance of stochastic and deterministic processes underlying patterns in beta-diversity along the ecosystem age gradient, and our detailed knowledge based on prior data collection in the study system to mechanistically explain among-group differences in these patterns. We found divergent responses in beta-diversity along the age gradient for the seven different organism groups, due to among-group differences in the relative importance of deterministic vs. stochastic community assembly, and attributed these results to reliance on resources from different energy channels that are not always related to NPP. Our results highlight the necessity to consider the importance of taxon-specific resources, and not only NPP, to obtain an understanding of beta-diversity patterns among organism groups and ecosystems, as well as large-scale patterns in biodiversity. They therefore also suggest that management and protection of beta-biodiversity in the landscape requires explicit consideration of a wide range of habitats.
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42.
  • Kardol, Paul, et al. (författare)
  • A test of the hierarchical model of litter decomposition
  • 2017
  • Ingår i: Nature ecology & evolution. - : Springer Science and Business Media LLC. - 2397-334X. ; 1, s. 1836-
  • Tidskriftsartikel (refereegranskat)abstract
    • Our basic understanding of plant litter decomposition informs the assumptions underlying widely applied soil biogeochemical models, including those embedded in Earth system models. Confidence in projected carbon cycle-climate feedbacks therefore depends on accurate knowledge about the controls regulating the rate at which plant biomass is decomposed into products such as CO2. Here we test underlying assumptions of the dominant conceptual model of litter decomposition. The model posits that a primary control on the rate of decomposition at regional to global scales is climate (temperature and moisture), with the controlling effects of decomposers negligible at such broad spatial scales. Using a regional-scale litter decomposition experiment at six sites spanning from northern Sweden to southern France-and capturing both within and among site variation in putative controls-we find that contrary to predictions from the hierarchical model, decomposer (microbial) biomass strongly regulates decomposition at regional scales. Furthermore, the size of the microbial biomass dictates the absolute change in decomposition rates with changing climate variables. Our findings suggest the need for revision of the hierarchical model, with decomposers acting as both local-and broad-scale controls on litter decomposition rates, necessitating their explicit consideration in global biogeochemical models.
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43.
  • Kardol, Paul, et al. (författare)
  • Biotic and abiotic plant-soil feedback depends on nitrogen-acquisition strategy and shifts during long-term ecosystem development
  • 2019
  • Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 107, s. 142-153
  • Tidskriftsartikel (refereegranskat)abstract
    • 1. Feedback between plants and soil is an important driver of plant community structure, but it remains unclear whether plant-soil feedback (PSF): (i) reflects changes in biotic or abiotic properties, (ii) depends on environmental context in terms of soil nutrient availability, and (iii) varies among plant functional groups. As soil nutrient availability strongly affects plant distribution and performance, soil chemical properties and plant nutrient-acquisition strategies might serve as important drivers of PSF.2. We used soils from young and old stages of a long-term soil chronosequence to represent sites where productivity is limited by nitrogen (N) and phosphorus (P) availability, respectively. We grew three N-fixing and three non-N-fixing plant species in soils conditioned by co-occurring conspecific or heterospecific species from each of these two stages. In addition, three soil treatments were used to distinguish biotic and abiotic effects on plant performance, allowing measurements of overall, biotic, and abiotic PSF.3. In young, N-poor soils, non-N-fixing plants grew better in soils from N-fixing plants than in their own soils (i.e., negative PSF). However, this difference was not only associated with improved abiotic conditions in soils from N-fixing plants but also with changes in soil biota.4. By contrast, no significant PSF was observed for N-fixing plants grown in young soils. Moreover, we did not observe any significant PSF for either N-fixing or non-N-fixing plants growing in old, P-impoverished soils.5. Synthesis. The direction and strength of plant-soil feedback (PSF) varied among N-acquisition strategies and soils differing in nutrient availability, with stronger plant-soil feedback in younger, N-poor soils compared to older, P-impoverished soils. Our results highlight the importance of considering soil nutrient availability, plant-mediated abiotic and biotic soil properties, and plant nutrient-acquisition strategies when studying plant-soil feedback, thereby advancing our mechanistic understanding of plant-soil feedback during long-term ecosystem development.
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44.
  • Kardol, Paul, et al. (författare)
  • Browsing by an invasive herbivore promotes development of plant and soil communities during primary succession
  • 2016
  • Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 104, s. 1505-1517
  • Tidskriftsartikel (refereegranskat)abstract
    • Invasive herbivores can strongly affect ecosystems by reducing or removing native plant species, and early in primary successions they could have enduring consequences for plant community assembly and ecosystem functioning, although this has seldom been explored. Invasive brushtail possums (Trichosurus vulpecula) browse from ground levels to forest canopies in New Zealand, including on plant species common early in primary succession. We used exclosures (fenced on the top as well as the sides) and corresponding control plots to study effects of brushtail possums on primary successions on bare landslide surfaces in the South Island of New Zealand. We then conducted repeated measurements of the plant community over 11years and at the end also quantified soil abiotic and biotic properties. Above-ground, plant biomass increased from close to zero to >1000g dry mass m(-2) after 11years in both exclosures and control plots, with significantly more biomass of the N-2-fixing tree Carmichaelia odorata in the controls. Brushtail possums reduced the biomass in the control plots of small-stature grasses and sedges in the first 3years. Plant species richness was greater in control than exclosure plots throughout the experiment. Below-ground, soil carbon and nitrogen concentrations and the biomass of all major soil microbial groups were significantly greater in the control than exclosure plots, although soil nematode abundances were not. Below-ground effects could be attributed to the greater biomass of Carmichaelia in control plots.Synthesis. Our study shows that an invasive herbivore exerted strong effects on the rate and trajectory of primary succession by altering plant community assembly. Herbivore exclusion resulted in reduced biomass of a key N-2-fixing tree that drives forest development, with important below-ground consequences. These results support a view that restoration of ecosystems needs to consider the consequences of altering plant community structure before removing invasive herbivores.
  •  
45.
  • Kardol, Paul, et al. (författare)
  • Long-term effects of species loss on community properties across contrasting ecosystems
  • 2018
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 557, s. 710-713
  • Tidskriftsartikel (refereegranskat)abstract
    • Biodiversity loss can heavily affect the functioning of ecosystems, and improving our understanding of how ecosystems respond to biodiversity decline is one of the main challenges in ecology(1-4). Several important aspects of the longer-term effects of biodiversity loss on ecosystems remain unresolved, including how these effects depend on environmental context(5-7). Here we analyse data from an across-ecosystem biodiversity manipulation experiment that, to our knowledge, represents the world's longest-running experiment of this type. This experiment has been set up on 30 lake islands in Sweden that vary considerably in productivity and soil fertility owing to differences in fire history(8,9). We tested the effects of environmental context on how plant species loss affected two fundamental community attributes-plant community biomass and temporal variability-over 20 years. In contrast to findings from artificially assembled communities(10-12), we found that the effects of species loss on community biomass decreased over time; this decrease was strongest on the least productive and least fertile islands. Species loss generally also increased temporal variability, and these effects were greatest on the most productive and most fertile islands. Our findings highlight that the ecosystem-level consequences of biodiversity loss are not constant across ecosystems and that understanding and forecasting these consequences necessitates taking into account the overarching role of environmental context.
  •  
46.
  • Kardol, Paul, et al. (författare)
  • Plant-soil feedback and the maintenance of diversity in Mediterranean-climate shrublands
  • 2017
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 355, s. 173-176
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil biota influence plant performance through plant-soil feedback, but it is unclear whether the strength of such feedback depends on plant traits and whether plant-soil feedback drives local plant diversity. We grew 16 co-occurring plant species with contrasting nutrient-acquisition strategies from hyperdiverse Australian shrublands and exposed them to soil biota from under their own or other plant species. Plant responses to soil biota varied according to their nutrient-acquisition strategy, including positive feedback for ectomycorrhizal plants and negative feedback for nitrogen-fixing and nonmycorrhizal plants. Simulations revealed that such strategy-dependent feedback is sufficient to maintain the high taxonomic and functional diversity characterizing these Mediterranean-climate shrublands. Our study identifies nutrient-acquisition strategy as a key trait explaining how different plant responses to soil biota promote local plant diversity.
  •  
47.
  • Kardol, Paul, et al. (författare)
  • Soil fertility shapes belowground food webs across a regional climate gradient
  • 2017
  • Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 20, s. 1273-1284
  • Tidskriftsartikel (refereegranskat)abstract
    • Changes in soil fertility during pedogenesis affect the quantity and quality of resources entering the belowground subsystem. Climate governs pedogenesis, yet how climate modulates responses of soil food webs to soil ageing remains unexplored because of the paucity of appropriate model systems. We characterised soil food webs along each of four retrogressive soil chronosequences situated across a strong regional climate gradient to show that belowground communities are predominantly shaped by changes in fertility rather than climate. Basal consumers showed hump-shaped responses to soil ageing, which were propagated to higher-order consumers. There was a shift in dominance from bacterial to fungal energy channels with increasing soil age, while the root energy channel was most important in intermediate-aged soils. Our study highlights the over-arching importance of soil fertility in regulating soil food webs, and indicates that belowground food webs will respond more strongly to shifts in soil resources than climate change.
  •  
48.
  •  
49.
  • Kardol, Paul, et al. (författare)
  • Trophic cascades in the bryosphere: the impact of global change factors on top-down control of cyanobacterial N-2-fixation
  • 2016
  • Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 19, s. 967-976
  • Tidskriftsartikel (refereegranskat)abstract
    • Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem-level fluxes and processes, and the conditions under which top-down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss-inhabiting microfauna exerted top-down control of N-2-fixation by moss-associated cyanobacteria. However, the presence of higher trophic levels alleviated this top-down control, likely through feeding on bacterivorous microfauna. These effects of food-web structure on cyanobacterial N-2-fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N-limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning.
  •  
50.
  • Kumordzi, Bright Boye, et al. (författare)
  • Linkage of plant trait space to successional age and species richness in boreal forest understorey vegetation
  • 2015
  • Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 103, s. 1610-1620
  • Tidskriftsartikel (refereegranskat)abstract
    • Determining the changes in within- and between-species functional diversity in plant communities, and their contribution to overall species trait overlap, can enhance efforts at understanding mechanisms of species coexistence. However, little is known about how variation in species functional diversity influences variation in species trait overlap among contrasting environments. Here, we studied the understorey vegetation in a well-characterized 5000-year-old chronosequence involving 30 forested islands that differ greatly in size, soil fertility and species diversity. Across this chronosequence, we expected consistent changes in both within- and between-species functional diversity that would lead to decreasing overall species trait overlap with increasing successional age, species richness, understorey vegetation density and spatial heterogeneity of soil resources. For each island, we measured specific leaf area (SLA) of each of ten individuals of each plant species present. Using a variance decomposition method, we partitioned the total community functional diversity of SLA on each island into within- and between-species functional diversity. Further, we estimated overall species trait overlap as the ratio of within-species functional diversity to total functional diversity. Using regression analyses, we then explored relationships of within- and between-species functional diversity, and of overall species trait overlap, with several environmental variables across the 30 islands. Consistent with our hypotheses, overall species trait overlap decreased with successional age due to a statistically significant decrease in within-species functional diversity, and decreased with species richness due to a simultaneous decrease in within-species functional diversity and increase in between-species functional diversity. Against our predictions, overall species trait overlap increased in more competitive environments and did not change with increasing spatial heterogeneity of soil N or P.Synthesis. Our study suggests niche packing as a key mechanism for species coexistence in plant communities. Using SLA as an integrator of plant ecological strategy, we show that community successional age and species richness are significantly linked to trait space distribution of plant individuals of boreal forest understorey vegetation and therefore to local species coexistence. Our results also suggest that the trait space of dominant and subordinate species may respond differently to local environmental variables.
  •  
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