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  • 1.
    Aunapuu, Maano
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dahlgren, Jonas
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Tarja
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Grellmann, Doris
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Rammul, Ullar
    Schneider, Michael
    Johansen, Bernt
    Hygen, Hans Olav
    Spatial patterns and dynamic responses of arctic food webs corroborate the exploitation ecosystems hypothesis (EEH)2008In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 171, no 2, p. 249-262Article in journal (Refereed)
    Abstract [en]

    According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community‐level trophic cascades, whereas unproductive ecosystems harbor food‐limited grazers, which regulate community‐level plant biomass. We tested this hypothesis along arctic‐alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation‐free islands. Corresponding transfer to herbivore‐free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co‐occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.

  • 2. Barrio, I. C.
    et al.
    Bueno, C. G.
    Gartzia, M.
    Soininen, E. M.
    Christie, K. S.
    Speed, J. D. M.
    Ravolainen, V. T.
    Forbes, B. C.
    Gauthier, G.
    Horstkotte, Tim
    Hoset, K. S.
    Høye, T. T.
    Jónsdóttir, I. S.
    Lévesque, E.
    Mörsdorf, M. A.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wookey, P. A.
    Hik, D. S.
    Biotic interactions mediate patterns of herbivore diversity in the Arctic2016In: Global Ecology and Biogeography, ISSN 1466-822X, E-ISSN 1466-8238, Vol. 25, no 9, p. 1108-1118Article in journal (Refereed)
    Abstract [en]

    Aim: Understanding the forces shaping biodiversity patterns, particularly for groups of organisms with key functional roles, will help predict the responses of ecosystems to environmental changes. Our aim was to evaluate the relative role of different drivers in shaping the diversity patterns of vertebrate herbivores, a group of organisms exerting a strong trophic influence in terrestrial Arctic ecosystems. This biome, traditionally perceived as homogeneous and low in biodiversity, includes wide variation in biotic and physical conditions and is currently undergoing major environmental change. Location: The Arctic (including the High Arctic, Low Arctic and Subarctic) MethodsWe compiled available data on vertebrate (birds and mammals) herbivore distribution at a pan-Arctic scale, and used eight variables that represent the most relevant hypotheses for explaining patterns of species richness. We used range maps rasterized on a 100kmx100km equal-area grid to analyse richness patterns of all vertebrate herbivore species combined, and birds and mammalian herbivores separately. Results: Overall, patterns of herbivore species richness in the Arctic were positively related to plant productivity (measured using the normalized difference vegetation index) and to the species richness of predators. Greater species richness of herbivores was also linked to areas with a higher mean annual temperature. Species richness of avian and mammalian herbivores were related to the distance from the coast, with the highest avian richness in coastal areas and mammalian richness peaking further inland. Main conclusions: Herbivore richness in the Arctic is most strongly linked to primary productivity and the species richness of predators. Our results suggest that biotic interactions, with either higher or lower trophic levels or both, can drive patterns of species richness at a biome-wide scale. Rapid ongoing environmental changes in the Arctic are likely to affect herbivore diversity through impacts on both primary productivity and changes in predator communities via range expansion of predators from lower latitudes.

  • 3. Barrio, Isabel C.
    et al.
    Lindén, Elin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    te Beest, Mariska
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Rocha, Adrian
    Soininen, Eeva M.
    Alatalo, Juha M.
    Andersson, Tommi
    Asmus, Ashley
    Boike, Julia
    Brathen, Kari Anne
    Bryant, John P.
    Buchwal, Agata
    Bueno, C. Guillermo
    Christie, Katherine S.
    Denisova, Yulia V.
    Egelkraut, Dagmar
    Ehrich, Dorothee
    Fishback, LeeAnn
    Forbes, Bruce C.
    Gartzia, Maite
    Grogan, Paul
    Hallinger, Martin
    Heijmans, Monique M. P. D.
    Hik, David S.
    Hofgaard, Annika
    Holmgren, Milena
    Høye, Toke T.
    Huebner, Diane C.
    Jonsdottir, Ingibjorg Svala
    Kaarlejärvi, Elina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium.
    Kumpula, Timo
    Lange, Cynthia Y. M. J. G.
    Lange, Jelena
    Levesque, Esther
    Limpens, Juul
    Macias-Fauria, Marc
    Myers-Smith, Isla
    van Nieukerken, Erik J.
    Normand, Signe
    Post, Eric S.
    Schmidt, Niels Martin
    Sitters, Judith
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium.
    Skoracka, Anna
    Sokolov, Alexander
    Sokolova, Natalya
    Speed, James D. M.
    Street, Lorna E.
    Sundqvist, Maja K.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. The Center for Macroecology, Evolution and Climate, The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark.
    Suominen, Otso
    Tananaev, Nikita
    Tremblay, Jean-Pierre
    Urbanowicz, Christine
    Uvarov, Sergey A.
    Watts, David
    Wilmking, Martin
    Wookey, Philip A.
    Zimmermann, Heike H.
    Zverev, Vitali
    Kozlov, Mikhail V.
    Publisher Correction to: Background invertebrate herbivory on dwarf birch (Betula glandulosa-nana complex) increases with temperature and precipitation across the tundra biome (vol 40, pg 2265, 2017)2018In: Polar Biology, ISSN 0722-4060, E-ISSN 1432-2056, Vol. 41, no 8, p. 1653-1654Article in journal (Refereed)
    Abstract [en]

    The above mentioned article was originally scheduled for publication in the special issue on Ecology of Tundra Arthropods with guest editors Toke T. Hoye . Lauren E. Culler. Erroneously, the article was published in Polar Biology, Volume 40, Issue 11, November, 2017. The publisher sincerely apologizes to the guest editors and the authors for the inconvenience caused.

  • 4. Barrio, Isabel C.
    et al.
    Lindén, Elin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Te Beest, Mariska
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Rocha, Adrian
    Soininen, Eeva M.
    Alatalo, Juha M.
    Andersson, Tommi
    Asmus, Ashley
    Boike, Julia
    Bråthen, Kari Anne
    Bryant, John P.
    Buchwal, Agata
    Bueno, C. Guillermo
    Christie, Katherine S.
    Denisova, Yulia V.
    Egelkraut, Dagmar
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ehrich, Dorothee
    Fishback, LeeAnn
    Forbes, Bruce C.
    Gartzia, Maite
    Grogan, Paul
    Hallinger, Martin
    Heijmans, Monique M. P. D.
    Hik, David S.
    Hofgaard, Annika
    Holmgren, Milena
    Høye, Toke T.
    Huebner, Diane C.
    Jónsdóttir, Ingibjorg Svala
    Kaarlejärvi, Elina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology, Vrije Universiteit Brussel (VUB), Brussels, Belgium.
    Kumpula, Timo
    Lange, Cynthia Y. M. J. G.
    Lange, Jelena
    Lévesque, Esther
    Limpens, Juul
    Macias-Fauria, Marc
    Myers-Smith, Isla
    van Nieukerken, Erik J.
    Normand, Signe
    Post, Eric S.
    Schmidt, Niels Martin
    Sitters, Judith
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology, Vrije Universiteit Brussel (VUB), Brussels, Belgium.
    Skoracka, Anna
    Sokolov, Alexander
    Sokolova, Natalya
    Speed, James D. M.
    Street, Lorna E.
    Sundqvist, Maja K.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. The Center for Macroecology, Evolution and Climate, The Natural History Museum of Denmark, University of Copenhagen, Copenhagen Ø, Denmark.
    Suominen, Otso
    Tananaev, Nikita
    Tremblay, Jean-Pierre
    Urbanowicz, Christine
    Uvarov, Sergey A.
    Watts, David
    Wilmking, Martin
    Wookey, Philip A.
    Zimmermann, Heike H.
    Zverev, Vitali
    Kozlov, Mikhail V.
    Background invertebrate herbivory on dwarf birch (Betula glandulosa-nana complex) increases with temperature and precipitation across the tundra biome2017In: Polar Biology, ISSN 0722-4060, E-ISSN 1432-2056, Vol. 40, no 11, p. 2265-2278Article in journal (Refereed)
    Abstract [en]

    Chronic, low intensity herbivory by invertebrates, termed background herbivory, has been understudied in tundra, yet its impacts are likely to increase in a warmer Arctic. The magnitude of these changes is however hard to predict as we know little about the drivers of current levels of invertebrate herbivory in tundra. We assessed the intensity of invertebrate herbivory on a common tundra plant, the dwarf birch (Betula glandulosa-nana complex), and investigated its relationship to latitude and climate across the tundra biome. Leaf damage by defoliating, mining and gall-forming invertebrates was measured in samples collected from 192 sites at 56 locations. Our results indicate that invertebrate herbivory is nearly ubiquitous across the tundra biome but occurs at low intensity. On average, invertebrates damaged 11.2% of the leaves and removed 1.4% of total leaf area. The damage was mainly caused by external leaf feeders, and most damaged leaves were only slightly affected (12% leaf area lost). Foliar damage was consistently positively correlated with mid-summer (July) temperature and, to a lesser extent, precipitation in the year of data collection, irrespective of latitude. Our models predict that, on average, foliar losses to invertebrates on dwarf birch are likely to increase by 6-7% over the current levels with a 1 degrees C increase in summer temperatures. Our results show that invertebrate herbivory on dwarf birch is small in magnitude but given its prevalence and dependence on climatic variables, background invertebrate herbivory should be included in predictions of climate change impacts on tundra ecosystems.

  • 5.
    Barthelemy, Helene
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stark, Sari
    Rovaniemi, Finland.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Strong Responses of Subarctic Plant Communities to Long-Term Reindeer Feces Manipulation2015In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 18, no 5, p. 740-751Article in journal (Refereed)
    Abstract [en]

    Deposition of feces is a key mechanism by which herbivores influence soil nutrient cycling and plant production, but the knowledge about its importance for plant production and community structure is still rudimental since experimental evidence is scarce. We thus performed a 7-year long reindeer feces manipulation experiment in two tundra vegetation types with contrasting nutrient availability and analyzed effects on plant community composition and soil nutrient availability. Despite feces being fairly nutrient poor, feces manipulation had strong effect on both the nutrient-poor heath and the nutrient-rich meadow. The strongest effect was detected when feces were added at high density, with a substantial increase in total vascular plant productivity and graminoids in the two communities. Doubling natural deposition of reindeer feces enhanced primary production and the growth of deciduous shrubs in the heath. By contrast, removal of feces decreased only the production of graminoids and deciduous shrubs in the heath. Although the response to feces addition was faster in the nutrient-rich meadow, after 7 years it was more pronounced in the nutrient-poor heath. The effect of feces manipulation on soil nutrient availability was low and temporarily variable. Our study provides experimental evidence for a central role of herbivore feces in regulating primary production when herbivores are abundant enough. Deposition of feces alone does, however, not cause dramatic vegetation shifts; to drive unproductive heath to a productive grass dominated state, herbivore trampling, and grazing are probably also needed.

  • 6.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Defoliation of a grass is mediated by the positive effect of dung deposition, moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment2019In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 128, no 10, p. 1515-1524Article in journal (Refereed)
    Abstract [en]

    Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing-tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily-grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long-lasting effects on plant productivity and ecosystem functioning after grazing has ceased.

  • 7.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Defoliation, soil grazing legacy, dung and moss cover influence growth and nutrient uptake of the common grass species, Festuca ovinaManuscript (preprint) (Other academic)
    Abstract [en]

    Herbivores can strongly influence plant growth directly through defoliation and the return of nutrients in the form of dung and urine but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. The relative importance of these driving mechanisms of plant response to herbivory are still poorly understood. In a common garden experiment, we studied the aboveground and belowground responses of Festuca ovina, a grazing tolerant grass common in arctic secondary grassland, to defoliation, reindeer dung addition, changes in soil microclimate induced by the presence or the absence of a moss cover, and soil grazing legacy. Defoliation strongly reduced shoot and root growth and plant nutrient uptake. Plants did thus not compensate for the tissue lost due to defoliation, even at a higher nutrient availability. By contrast, defoliation enhanced plant N concentration and decreased plant C to N ratio. Soil from heavily grazed sites and dung addition increased plant production, plant N concentrations and nutrient uptake, although the effects of dung addition were only small. Mosses had a strong negative effect of root biomass and reduced plant compensatory growth after defoliation. Interestingly mosses also had facilitative effects on aboveground plant growth in absence of defoliation and on plant nutrient uptake and N concentrations. Although plants suffered severely from defoliation, they were also strongly favoured by the increased nutrient availability associated with herbivory. After two years, plants produced as much biomass when all positive and negative effects of herbivores were considered (defoliation, soil communities and nutrient availability under heavily grazing, dung addition and no moss cover) as in the ungrazed conditions (no defoliation, soil communities and nutrient availability under lightly grazing, no dung addition, a thick moss cover). This study indicates that graminoids can tolerate high densities of herbivores, although it suffer from defoliation directly, and suggests that changes in plant quality following defoliation and grazing-induced changes in soil processes are two key mechanisms through which herbivores can control plant productivity in arctic secondary grasslands. Plant tolerance to herbivory will depends on how herbivores utilise a pasture area and on the balance between the positive and the negative effects of grazing on plant growth.

  • 8.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stark, Sari
    Arctic Center, University of Lapland, Rovaniemi, Finland.
    Kytöviita, Minna-Maarit
    Department of Biological and Environmental Science, University of Jyväskylä, Finland.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Grazing decreases N partitioning among coexisting plant species2017In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 31, no 11, p. 2051-2060Article in journal (Refereed)
    Abstract [en]

    1. Herbivores play a key role in shaping ecosystem structure and functions by influencing plant and microbial community composition and nutrient cycling.

    2. This study investigated the long-term effects of herbivores on plant resource acquisition. We explored differences in the natural delta N-15 signatures in plant, microbial and soil N pools, and examined mycorrhizal colonization in two tundra sites that have been either lightly or heavily grazed by reindeer for more than 50 years. The study examined changes in nutrient acquisition in five common tundra plants with contrasting traits and mycorrhiza status; the mycorrhizal dwarf shrubs, Betula nana, Vaccinium myrtillus and Empetrum hermaphroditum; a mycorrhizal grass, Deschampsia flexuosa, and a non-mycorrhizal sedge, Carex bigelowii.

    3. There were large variations in delta N-15 among coexisting plant species in the lightly grazed sites. This variation was dramatically reduced in the heavily grazed sites. At an individual species level, delta N-15 was higher in E. hermaphroditum and lower in C. bigelowii in the heavily grazed sites. Mycorrhizal colonization in B. nana and E. hermaphroditum roots were also lower in the heavily grazed sites. The delta N-15 signatures of the total soil N pool and of the microbial N pools were higher in the heavily grazed sites.

    4. Since the strong delta N-15 differentiation among plant species has been interpreted as a result of plants with different mycorrhizal types using different sources of soil nitrogen, we suggest that the lower variation in delta N-15 in heavily grazed sites indicates a lower niche differentiation in nitrogen uptake among plants. Reduced mycorrhizamediated nitrogen uptake by some of the species, a shift towards a more mineral nutrition due to higher nutrient turnover, and uptake of labile nitrogen from dung and urine in the heavily grazed sites could all contribute to the changes in plant delta N-15.

    5. We conclude that herbivores have the potential to influence plant nutrient uptake and provide the first data suggesting that herbivores decrease nutrient partitioning on the basis of chemical N forms among plant species. Reduced niche complementarity among species is potentially important for estimates of the effects of -herbivory on plant nutrient availability and species coexistence.

  • 9.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stark, Sari
    Arctic Center, University of Lapland Rovaniemi, Finland.
    Michelsen, Anders
    Department of Biology, Terrestrial Ecology, University of Copenhagen 2. 4Center for Permafrost (CENPERM), University of Copenhagen.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effect of herbivory on the fate of added 15N-urea in a grazed Arctic tundraManuscript (preprint) (Other academic)
    Abstract [en]

    Mammalian herbivores can strongly influence nitrogen cycling and herbivore urine could be an important component of the nutrient cycle in grazed ecosystems. Despite its potential role for ecosystem productivity and soil processes, the distribution of N from urine in the different ecosystem compartments is poorly understood. This study investigates the fate of 15N enriched urea applied above the plant canopy in two tundra sites either heavily or lightly grazed by reindeer for the last 50 years. We explored the fate of the 15N in the different ecosystem N pools at 2 weeks and 1 years following tracer addition. We hypothesized that cryptogams would take up most N under light grazing, but graminoids most N under heavy grazing. The 15N-urea was rapidly incorporated in cryptogams and aboveground parts of vascular plants, while the soil microbial pool and plant roots sequestered only a marginal proportion of the labelled N applied. Hence, urine addition supports a higher primary production in tundra since most of the nutrients released from urine could be assimilated by the aboveground components with little N reaching the belowground compartments. Mosses and lichens still constituted the largest sink of the 15N-urea 1 year after tracer addition at both levels of grazing intensity demonstrating their large ability to capture and retain N  from urine. Deciduous and evergreen shrubs were just as efficient as graminoids in taking up the 15N-urea. The total recovery of the labelled urea was lower in the heavily grazed sites, suggesting that reindeer reduce the N retention in the system. Rapid incorporation of the applied 15N-urea indicates that arctic plants can take advantage of a pulse of incoming N in the form of urea, which supports a higher primary production. However, whether urine also maintains a high production of forage plants depend on plant community composition, since most urea was recovered in non-forage plants for reindeer.

  • 10.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stark, Sari
    Michelsen, Anders
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Urine is an important nitrogen source for plants irrespective of vegetation composition in an Arctic tundra: Insights from a N-15-enriched urea tracer experiment2018In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 106, no 1, p. 367-378Article in journal (Refereed)
    Abstract [en]

    1. Mammalian herbivores can strongly influence nitrogen (N) cycling and herbivore urine could be a central component of the N cycle in grazed ecosystems. Despite its potential role for ecosystem productivity and functioning, the fate of N derived from urine has rarely been investigated in grazed ecosystems. 2. This study explored the fate of N-15-enriched urea in tundra sites that have been either lightly or intensively grazed by reindeer for more than 50years. We followed the fate of the N-15 applied to the plant canopy, at 2weeks and 1year after tracer addition, in the different ecosystem N pools. 3. N-15-urea was rapidly incorporated in cryptogams and in above-ground parts of vascular plants, while the soil microbial pool and plant roots sequestered only a marginal proportion. Furthermore, the litter layer constituted a large sink for the N-15-urea, at least in the short term, indicating a high biological activity in the litter layer and high immobilization in the first phases of organic matter decomposition. 4. Mosses and lichens still constituted the largest sink for the N-15-urea 1year after tracer addition at both levels of grazing intensity demonstrating their large ability to capture and retain N from urine. Despite large fundamental differences in their traits, deciduous and evergreen shrubs were just as efficient as graminoids in taking up the N-15-urea. The total recovery of N-15-urea was lower in the intensively grazed sites, suggesting that reindeer reduce ecosystem N retention. 5. Synthesis. The rapid incorporation of the applied N-15-urea indicates that arctic plants can take advantage of a pulse of incoming N from urine. In addition, N-15 values of all taxa in the heavily grazed sites converged towards the N-15 values for urine, bringing further evidence that urine is an important N source for plants in grazed tundra ecosystems.

  • 11.
    Becher, Marina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Berglund, Louise
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Klaminder, Jonatan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Decreased cryogenic disturbance: one of the potential mechanisms behind the vegetation change in the Arctic2018In: Polar Biology, ISSN 0722-4060, E-ISSN 1432-2056, Vol. 41, no 1, p. 101-110Article in journal (Other academic)
    Abstract [en]

    During the last few decades, the Arctic has experienced large-scale vegetation changes. Understanding the mechanisms behind this vegetation change is crucial for our ability to predict future changes. This study tested the hypothesis that decreased cryogenic disturbances cause vegetation change in patterned ground study fields (non-sorted circles) in Abisko, Sweden during the last few decades. The hypothesis was tested by surveying the composition of plant communities across a gradient in cryogenic disturbance and by reinvestigating plant communities previously surveyed in the 1980s to scrutinise how these communities changed in response to reduced cryogenic disturbance. Whereas the historical changes in species occurrence associated with decreased cryogenic disturbances were relatively consistent with the changes along the contemporary gradient of cryogenic disturbances, the species abundance revealed important transient changes highly dependent on the initial plant community composition. Our results suggest that altered cryogenic disturbances cause temporal changes in vegetation dynamics, but the net effects on vegetation communities depend on the composition of initial plant species.

  • 12.
    Becher, Marina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Klaminder, Jonatan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Cryogenic disturbance and its impact on carbon fluxes in a subarctic heathland2015In: Environmental Research Letters, ISSN 1748-9326, E-ISSN 1748-9326, Vol. 10, no 11, article id 114006Article in journal (Refereed)
    Abstract [en]

    Differential frost heave, along with the associated cryogenic disturbance that accompanies it, is an almost universal feature of arctic landscapes that potentially influences the fate of the soil carbon (C) stored in arctic soils. In this study, we quantify how gross ecosystem photosynthesis (GEP), soil respiration (Re) and the resulting net ecosystem exchange (NEE) vary in a patterned ground system (non-sorted circles) at plot-scale and whole-patterned ground scales in response to cryogenic disturbances (differential heave and soil surface disruption). We found that: (i) all studied non-sorted circles (n=15) acted as net CO2 sources (positive NEE); (ii) GEP showed a weaker decrease than Re in response to increased cryogenic disturbance/decreased humus cover, indicating that undisturbed humus-covered sites are currently the main source of atmospheric CO2 in the studied system. Interestingly, Re fluxes normalized to C pools indicated that C is currently respired more rapidly at sites exposed to cryogenic disturbances; hence, higher NEE fluxes at less disturbed sites are likely an effect of a more slowly degrading but larger total pool that was built up in the past. Our results highlight the complex effects of cryogenic processes on the C cycle at various time scales. 

  • 13. Berg, A.
    et al.
    Östlund, L.
    Moen, Jon
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    A century of logging and forestry in a reindeer herding area in northern Sweden2008In: Forest Ecology and Management, Vol. 256, p. 1009-1020Article in journal (Refereed)
    Abstract [en]

    Boreal forest ecosystems are generally highly sensitive to logging and other forestry activities. Thus, commercial forestry has had major effects on the forests and landscape structure in northern Sweden since the middle of the 19th Century, when it rapidly extended across the region. Lichens (which constitute up to 80% of reindeer forage in winter and early spring) have often been amongst the most severely affected ecosystem components. The overall aim of the present study was to analyze how forestry has influenced the potential supply of ground-growing lichens as winter forage for the reindeer in this region over the past ca. 100 years. For this purpose, we analysed changes in forest and stand structure in Scots pine-dominated (Pinus sylvestris L.) reindeer wintering areas in the southern part of the county Norrbotten (covering ca. 58,000 ha) using detailed historical forest inventories and management plans. We found that the amount of the forest types considered potentially good pasture (mainly middleaged and old pine forests) decreased during the first part of the 20th Century. However, the quality of grazing grounds was improved by forestry during this time mainly because selective logging made the forests more open which benefits lichen growth. During the last part of the 20th century forestry impaired the quality of grazing grounds in several ways, e.g. by clear-cutting and intensified use of various silviculturalmeasures. We conclude that ca. 30–50% of the winter grazing grounds have been lost in the study area because of intensive forest management during the last century. The spatially precise historical information about the affects of forestry on lichen pasture provided in this study can be used to direct forest management which will facilitate and promote reindeer herding in the future.

  • 14. Bjorkman, Anne D.
    et al.
    Myers-Smith, Isla H.
    Elmendorf, Sarah C.
    Normand, Signe
    Rueger, Nadja
    Beck, Pieter S. A.
    Blach-Overgaard, Anne
    Blok, Daan
    Cornelissen, J. Hans C.
    Forbes, Bruce C.
    Georges, Damien
    Goetz, Scott J.
    Guay, Kevin C.
    Henry, Gregory H. R.
    HilleRisLambers, Janneke
    Hollister, Robert D.
    Karger, Dirk N.
    Kattge, Jens
    Manning, Peter
    Prevey, Janet S.
    Rixen, Christian
    Schaepman-Strub, Gabriela
    Thomas, Haydn J. D.
    Vellend, Mark
    Wilmking, Martin
    Wipf, Sonja
    Carbognani, Michele
    Hermanutz, Luise
    Levesque, Esther
    Molau, Ulf
    Petraglia, Alessandro
    Soudzilovskaia, Nadejda A.
    Spasojevic, Marko J.
    Tomaselli, Marcello
    Vowles, Tage
    Alatalo, Juha M.
    Alexander, Heather D.
    Anadon-Rosell, Alba
    Angers-Blondin, Sandra
    te Beest, Mariska
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Berner, Logan
    Bjork, Robert G.
    Buchwal, Agata
    Buras, Allan
    Christie, Katherine
    Cooper, Elisabeth J.
    Dullinger, Stefan
    Elberling, Bo
    Eskelinen, Anu
    Frei, Esther R.
    Grau, Oriol
    Grogan, Paul
    Hallinger, Martin
    Harper, Karen A.
    Heijmans, Monique M. P. D.
    Hudson, James
    Huelber, Karl
    Iturrate-Garcia, Maitane
    Iversen, Colleen M.
    Jaroszynska, Francesca
    Johnstone, Jill F.
    Jorgensen, Rasmus Halfdan
    Kaarlejärvi, Elina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Klady, Rebecca
    Kuleza, Sara
    Kulonen, Aino
    Lamarque, Laurent J.
    Lantz, Trevor
    Little, Chelsea J.
    Speed, James D. M.
    Michelsen, Anders
    Milbau, Ann
    Nabe-Nielsen, Jacob
    Nielsen, Sigrid Scholer
    Ninot, Josep M.
    Oberbauer, Steven F.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Onipchenko, Vladimir G.
    Rumpf, Sabine B.
    Semenchuk, Philipp
    Shetti, Rohan
    Collier, Laura Siegwart
    Street, Lorna E.
    Suding, Katharine N.
    Tape, Ken D.
    Trant, Andrew
    Treier, Urs A.
    Tremblay, Jean-Pierre
    Tremblay, Maxime
    Venn, Susanna
    Weijers, Stef
    Zamin, Tara
    Boulanger-Lapointe, Noemie
    Gould, William A.
    Hik, David S.
    Hofgaard, Annika
    Jonsdottir, Ingibjorg S.
    Jorgenson, Janet
    Klein, Julia
    Magnusson, Borgthor
    Tweedie, Craig
    Wookey, Philip A.
    Bahn, Michael
    Blonder, Benjamin
    van Bodegom, Peter M.
    Bond-Lamberty, Benjamin
    Campetella, Giandiego
    Cerabolini, Bruno E. L.
    Chapin, F. Stuart, III
    Cornwell, William K.
    Craine, Joseph
    Dainese, Matteo
    de Vries, Franciska T.
    Diaz, Sandra
    Enquist, Brian J.
    Green, Walton
    Milla, Ruben
    Niinemets, Ulo
    Onoda, Yusuke
    Ordonez, Jenny C.
    Ozinga, Wim A.
    Penuelas, Josep
    Poorter, Hendrik
    Poschlod, Peter
    Reich, Peter B.
    Sande, Brody
    Schamp, Brandon
    Sheremetev, Serge
    Weiher, Evan
    Plant functional trait change across a warming tundra biome2018In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 562, no 7725, p. 57-+Article in journal (Refereed)
    Abstract [en]

    The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

  • 15.
    Bognounou, Fidele
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hulme, Philip E.
    Oksanen, Lauri
    Suominen, Otso
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Role of climate and herbivory on native and alien conifer seedling recruitment at and above the Fennoscandian tree line2018In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 29, no 4, p. 573-584Article in journal (Refereed)
    Abstract [en]

    Questions: We investigated the importance of climate and herbivory on native and alien conifer colonization of the birch-dominated Fennoscandian tree line by addressing the following questions: (a) are tree line and tundra habitats similarly suitable for conifer seedling recruitment; (b) do ungulate and rodent herbivores differentially impact seedling recruitment; and (c) how does the role of habitat and herbivory on seedling recruitment vary across a marked climate gradient?

    Location: Northern Fennoscandia, Sweden (Vassijaure and Paddus), and Norway (Joatka and Seiland).

    Methods: We conducted an experiment to assess the emergence rate, survival probability and height development of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and Siberian larch (Larix sibirica) seedlings. Three experimental plots (i.e., open control, reindeer exclosure and complete vertebrate exclosure) were established in both tree line and tundra habitats at each of the four locations. Seeds of the three conifer species were sown in each plot in June 1999 during three consecutive years. The surviving seedlings were counted in August to September 1999, 2000, 2001, 2002 and 2007. The height of all seedlings was measured in 2007.

    Results: Our study reveals that Norway spruce, Scots pine and Siberian larch can regenerate from seed at and above the current tree line in northern Fennoscandia. Their performance was generally higher above tree line in tundra than at tree line, but depended on species identity, climate aridity and mammal herbivory, particularly by rodents. These results suggest that the species composition and latitudinal limit of the tree line in the future might depend not only on direct effects of the future climate on the current tree line species, but also on the intensity of alien and native conifer introductions, as well as changes in herbivore populations.

    Conclusion: If sufficient seeds of Norway spruce, Scots pine and Siberian larch should reach the current tree line, their performances will increase with a warmer and wetter climate, and this effect will be markedly modulated by herbivores (particularly rodents). Further work is required to extend these results to determine the ability of these conifers to become tree line-forming species in the future.

  • 16. Brooker, R.W.
    et al.
    Maestre, F.T.
    Callaway, R.M.
    Lortie, C.L.
    Cavieres, L.A.
    Kunstler, G.
    Liancourt, P.
    Tielborger, K.
    Travis, J.M.J.
    Anthelme, F.
    Armas, C.
    Coll, L.
    Corcket, E.
    Delzon, S.
    Forey, E.
    Kikvidze, Z.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Pugnaire, F.
    Quiroz, C.L.
    Saccone, P.
    Schiffers, K.
    Seifan, M.
    Touzard, B.
    Michalet, R.
    Facilitation in plant communities: the past, the present, and the future2008In: Journal of Ecology, Vol. 96, no 1, p. 18-34Article in journal (Refereed)
    Abstract [en]

    1. Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field.

    2. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant–plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research.

    3. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies.

    4. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity.

    5. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes.

    6. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms.

    7. Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers.

    8. Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plant–plant interactions in the design and interpretation of studies from the fields of restoration and global change ecology.

    9. Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists.

  • 17.
    Buckland, Philip I
    et al.
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    Johan, Olofsson
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    Engelmark, Roger
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    SEAD: Strategic Environmental Archaeology Database, planning report2006Report (Other academic)
    Abstract [en]

    This document lays out a strategy for the development of SEAD – A Strategic Environmental Archaeology Database, which will facilitate the digitisation and accessibility augmentation of MAL’s existing data from nearly thirty years of work in the fields of archaeology and environmental science. SEAD will also provide a framework for the entry of data from all future research and consultancy work at MAL, and allow guest researchers and external partners to contribute to, and work with the same data. The planned system will be implemented at both local and internet levels, and be designed with an aim towards broadening its scope with external partners in the future. SEAD will be made available online in order to increase the ease of access to environmental archaeology data and encourage an expansion of both the discipline and Sweden’s role in it. This is inline with current EU strategies on enhancing research infrastructure, and providing a greater insight into human-environment interactions for long term planning.

  • 18. Callaghan, Terry V
    et al.
    Jonasson, Christer
    Thierfelder, Tomas
    Yang, Zhenlin
    Hedenas, Henrik
    Johansson, Margareta
    Molau, Ulf
    Van Bogaert, Rik
    Michelsen, Anders
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gwynn-Jones, Dylan
    Bokhorst, Stef
    Phoenix, Gareth
    Bjerke, Jarle W
    Tommervik, Hans
    Christensen, Torben R
    Hanna, Edward
    Koller, Eva K
    Sloan, Victoria L
    Ecosystem change and stability over multiple decades in the Swedish subarctic: complex processes and multiple drivers2013In: Philosophical Transactions of the Royal Society of London. Biological Sciences, ISSN 0962-8436, E-ISSN 1471-2970, Vol. 368, no 1624Article in journal (Refereed)
    Abstract [en]

    The subarctic environment of northernmost Sweden has changed over the past century, particularly elements of climate and cryosphere. This paper presents a unique geo-referenced record of environmental and ecosystem observations from the area since 1913. Abiotic changes have been substantial. Vegetation changes include not only increases in growth and range extension but also counterintuitive decreases, and stability: all three possible responses. Changes in species composition within the major plant communities have ranged between almost no changes to almost a 50 per cent increase in the number of species. Changes in plant species abundance also vary with particularly large increases in trees and shrubs (up to 600%). There has been an increase in abundance of aspen and large changes in other plant communities responding to wetland area increases resulting from permafrost thaw. Populations of herbivores have responded to varying management practices and climate regimes, particularly changing snow conditions. While it is difficult to generalize and scale-up the site-specific changes in ecosystems, this very site-specificity, combined with projections of change, is of immediate relevance to local stakeholders who need to adapt to new opportunities and to respond to challenges. Furthermore, the relatively small area and its unique datasets are a microcosm of the complexity of Arctic landscapes in transition that remains to be documented.

  • 19.
    Dahlgren, Jonas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Oksanen, Tarja
    Olofsson, Johan
    Trophic cascades and direct herbivore impacts in a low arctic scrublandIn: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323Article in journal (Other academic)
  • 20.
    Dahlgren, Jonas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Department of Biology, Section of Ecology, University of Turku, Turku, Finland.
    Oksanen, Tarja
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hambäck, Peter A
    Department of Botany, Stockholm University, Stockholm, Sweden.
    Lindgren, Åsa
    Department of Botany, Stockholm University, Stockholm, Sweden.
    Plant defences to no avail?: Responses of plants of varying edibility to food web manipulations in a low arctic scrubland2009In: Evolutionary Ecology Research, ISSN 1522-0613, E-ISSN 1937-3791, Vol. 11, p. 1189-1203Article in journal (Refereed)
    Abstract [en]

    Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin,all plants are edible for some herbivores. Hence, the copious abundance of plant biomass,typical for terrestrial ecosystems, depends on the collective regulatory action of predators on the herbivore guild. According to the counterarguments of Polis and Strong, the defensive traits of terrestrial plants attenuate terrestrial trophic cascades to species-specific trickles,so elimination of predators might lead to increased abundance of inedible plants but will not influence community-level plant biomass.

    Question: Does the elimination of predators from a low arctic scrubland, with high-quality forage plants and poorly edible evergreen ericoids, lead to a reduction of community-level plant biomass or to an increased abundance of well-defended evergreen ericoids?

    Methods: In 1991, we introduced grey-sided voles (Myodes rufocanus) to islands, initially harbouring dense scrubland vegetation, and established permanent plots there. In 2000, we transplanted vegetation blocks from a large three-trophic-level island with voles and predators,to two-trophic-level islands with introduced voles but without resident predators, and also to vole-free one-trophic-level islands, and back to the three-trophic-level island. Vole densities were monitored by semi-annual live trapping. Vegetation was monitored by the point-frequency method.

    Results: In the absence of predators, vole densities increased 3.7-fold and the communitylevel plant biomass was decimated. The least palatable plant group, evergreen ericoids,suffered especially heavily, whereas palatable herbaceous plants increased in abundance. However, all three functional plant groups responded positively to the elimination of grey-sided voles.

    Conclusions: Our results corroborate the Green World Hypothesis, indicating that in the absence of predators, plant defences do not prevent runaway consumption of the vegetation.

  • 21.
    Dahlgren, Jonas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Department of Biology, Section of Ecology, University of Turku, Turku, Finland.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Tarja
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Plant defense at no cost?: The recovery of tundra scrubland following heavy grazing by grey-sided voles (Myodes rufocanus)2009In: Evolutionary Ecology Research, ISSN 1522-0613, E-ISSN 1937-3791, Vol. 11, p. 1205-1216Article in journal (Refereed)
    Abstract [en]

    Background: Evergreen ericaceous dwarf shrubs form a dominating component of low arctic and low alpine vegetation. They typically produce high contents of secondary chemicals such as phenolics. The primary function of these chemicals may be to defend the shrubs by making them less palatable to herbivores. Question: Does the production of secondary chemicals carry a fitness cost in terms of low growth rate and, therefore, low capacity to recover from past herbivory?

    Methods: In 2000, we constructed vole-proof exclosures on low arctic islands where vegetation had, since 1991, been heavily impacted by grey-sided voles. In 2000 and 2003,we surveyed the vegetation of the exclosures, of unfenced plots on the same islands, and of control plots on a vole-free island. We used the point-frequency method for vegetation surveys.

    Results: In the exclosures, the biomasses of most plant species increased, by and large, at the same pace. The two woody species, which increased most rapidly, were the maximally palatable bilberry (Vaccinium myrtillus) and the phenolics-laden, maximally unpalatable northern crowberry (Empetrum nigrum ssp. hermaprhoditum). The recovery rates of these species were similar.

    Conclusions: The high concentrations of phenolics typical for evergreen arctic dwarf shrubs do not carry any obvious cost in the form of reduced capacity for compensatory growth. The principle of trade-offs does not help to explain the variation in plant palatability.

  • 22.
    Dahlgren, Jonas
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Oksanen, Lauri
    Sjödin, Maria
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Interactions between gray-sided voles (Clethrionomys rufucanus) and bilberry (Vaccinium myrtillus), their main winter food plant.2007In: Oecologia, ISSN 0029-8549, Vol. 152, no 3, p. 525-32Article in journal (Refereed)
  • 23.
    Egelkraut, Dagmar
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Aronsson, Kjell-Åke
    Ájtte, Swedish Mountain and Sami Museum, Jokkmokk, Sweden.
    Allard, Anna
    Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Åkerholm, Marianne
    Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Stark, Sari
    Arctic Centre, University of Lapland, Rovaniemi, Finland.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Multiple feedbacks contribute to a centennial legacy of reindeer on tundra vegetation2018In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 21, no 8, p. 1545-1563Article in journal (Refereed)
    Abstract [en]

    Historical contingency is the impact of past events, like the timing and order of species arrival, on community assembly, and can sometimes result in alternative stable states of ecological communities. Large herbivores, wild and domestic, can cause profound changes in the structure and functioning of plant communities and therefore probably influence historical contingency; however, little empirical data on the stability of such shifts or subsequent drivers of stability are available. We studied the centennial legacy of reindeer (Rangifer tarandus) pressure on arctic tundra vegetation by considering historical milking grounds (HMGs): graminoid- and forb-dominated patches amid shrub-dominated tundra, formed by historical Sami reindeer herding practices that ended approximately 100 years ago. Our results show that the core areas of all studied HMGs remained strikingly stable, being hardly invaded by surrounding shrubs. Soil nitrogen concentrations were comparable to heavily grazed areas. However, the HMGs are slowly being reinvaded by vegetative growth of shrubs at the edges, and the rate of ingrowth increased with higher mineral N availability. Furthermore, our data indicate that several biotic feedbacks contribute to the stability of the HMGs: increased nutrient turnover supporting herbaceous vegetation, strong interspecific competition preventing invasion and herbivore damage to invading shrubs. In particular, voles and lemmings appear to be important, selectively damaging shrubs in the HMGs. We concluded that HMGs provide clear evidence for historical contingency of herbivore effects in arctic ecosystems. We showed that several biotic feedbacks can contribute to subsequent vegetation stability, but their relative importance will vary in time and space.

  • 24.
    Egelkraut, Dagmar
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University.
    Barthelemy, Hélène
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Reindeer trampling causes vegetation changes in tundra heathlands: results from a simulation experimentManuscript (preprint) (Other academic)
  • 25.
    Egelkraut, Dagmar
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kardol, Paul
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden.
    De Long, Jonathan R.
    Department of Terrestrial Ecology, Netherlands Institute of Ecology, Wageningen, The Netherlands.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    The role of plant-soil feedbacks in stabilizing a reindeer-induced vegetation shift in subarctic tundra2018In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 32, no 8, p. 1959-1971Article in journal (Refereed)
    Abstract [en]

    1. Herbivory can drive vegetation into different states of productivity and community composition, and these changes may be stable over time due to historical contingency effects. Interactions with abiotic and biotic soil components can contribute to such long-term legacies in plant communities through stabilizing positive feedbacks.

    2. We studied the role of plant-soil feedbacks in maintaining vegetation changes caused by historical (similar to 1350-1900 AD) reindeer herding in northern Sweden. These historical milking grounds (HMGs) consist of meadow plant communities formed in naturally nutrient-poor heath or naturally nutrient-rich shrub-dominated vegetation and are still clearly visible in the landscape, a century after active use ceased.

    3. We selected two phytometer species: the forb Potentilla crantzii as representative of HMG vegetation, and the dwarf shrub Betula nana, as representative of control vegetation. We grew both species under glasshouse conditions on soils derived from replicated HMG and paired control plots, using live soils and sterilized (-radiation)-inoculated soils, to separate between biotic and abiotic soil effects.

    4. A net negative plant-soil feedback for B.nana biomass in its home (i.e., control) soil and a net positive feedback for P.crantzii in its home (i.e., HMG) soil in heath habitat was partly driven by the soil biotic community. However, abiotic differences in mineral nitrogen (N) concentrations between control and HMG soils were a stronger driver of differences in plant growth. Positive feedbacks maintaining a high mineral nutrient availability are thus important, especially in nutrient-poor habitats.

    5. The positive plant responses to higher soil mineral N concentrations, combined with positive biotic plant-soil feedbacks, might shift the competitive balance in favour of typical HMG plant species, thereby contributing to stability of HMG plant communities. Our data indicate that herbivore-driven changes in the interactions between plants and both biotic and abiotic components of the soil persist over long temporal scales.

  • 26. Ehrich, Dorothée
    et al.
    Schmidt, Niels M.
    Gauthier, Gilles
    Alisauskas, Ray
    Angerbjörn, Anders
    Clark, Karin
    Ecke, Frauke
    Eide, Nina E.
    Framstad, Erik
    Frandsen, Jay
    Franke, Alastair
    Gilg, Olivier
    Giroux, Marie-Andrée
    Henttonen, Heikki
    Hörnfeldt, Birger
    Ims, Rolf A.
    Kataev, Gennadiy D.
    Kharitonov, Sergey P.
    Killengreen, Siw T.
    Krebs, Charles J.
    Lanctot, Richard B.
    Lecomte, Nicolas
    Menyushina, Irina E.
    Morris, Douglas W.
    Morrisson, Guy
    Oksanen, Lauri
    Oksanen, Tarja
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Pokrovsky, Ivan G.
    Popov, Igor Yu.
    Reid, Donald
    Roth, James D.
    Saalfeld, Sarah T.
    Samelius, Gustaf
    Sittler, Benoit
    Sleptsov, Sergey M.
    Smith, Paul A.
    Sokolov, Aleksandr A.
    Sokolova, Natalya A.
    Soloviev, Mikhail Y.
    Solovyeva, Diana V.
    Documenting lemming population change in the Arctic: Can we detect trends?2019In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209Article in journal (Refereed)
    Abstract [en]

    Lemmings are a key component of tundra food webs and changes in their dynamics can affect the whole ecosystem. We present a comprehensive overview of lemming monitoring and research activities, and assess recent trends in lemming abundance across the circumpolar Arctic. Since 2000, lemmings have been monitored at 49 sites of which 38 are still active. The sites were not evenly distributed with notably Russia and high Arctic Canada underrepresented. Abundance was monitored at all sites, but methods and levels of precision varied greatly. Other important attributes such as health, genetic diversity and potential drivers of population change, were often not monitored. There was no evidence that lemming populations were decreasing in general, although a negative trend was detected for low arctic populations sympatric with voles. To keep the pace of arctic change, we recommend maintaining long-term programmes while harmonizing methods, improving spatial coverage and integrating an ecosystem perspective.

  • 27.
    Engelmark, Roger
    et al.
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    Larsson, Thomas B.
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    Olofsson, Johan
    Umeå University, Faculty of Arts, Archaeology and Sami Studies.
    Rapport från Geo-Arkeologisk fältkurs i Rumänien 20042005Report (Other academic)
  • 28. Eskelinen, Anu
    et al.
    Kaarlejarvi, Elina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Plant Biology and Nature Management, Vrije Universiteit Brussel, Brussels, Belgium.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Herbivory and nutrient limitation protect warming tundra from lowland species' invasion and diversity loss2017In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, no 1, p. 245-255Article in journal (Refereed)
    Abstract [en]

    Herbivory and nutrient limitation can increase the resistance of temperature-limited systems to invasions under climate warming. We imported seeds of lowland species to tundra under factorial treatments of warming, fertilization, herbivore exclusion and biomass removal. We show that warming alone had little impact on lowland species, while exclusion of native herbivores and relaxation of nutrient limitation greatly benefitted them. In contrast, warming alone benefitted resident tundra species and increased species richness; however, these were canceled by negative effects of herbivore exclusion and fertilization. Dominance of lowland species was associated with low cover of tundra species and resulted in decreased species richness. Our results highlight the critical role of biotic and abiotic filters unrelated to temperature in protecting tundra under warmer climate. While scarcity of soil nutrients and native herbivores act as important agents of resistance to invasions by lowland species, they concurrently promote overall species coexistence. However, when these biotic and abiotic resistances are relaxed, invasion of lowland species can lead to decreased abundance of resident tundra species and diminished diversity.

  • 29. Hoset, Katrine S.
    et al.
    Kyro, Kukka
    Oksanen, Tarja
    Oksanen, Lauri
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Spatial variation in vegetation damage relative to primary productivity, small rodent abundance and predation2014In: Ecography, ISSN 0906-7590, E-ISSN 1600-0587, Vol. 37, no 9, p. 894-901Article in journal (Refereed)
    Abstract [en]

    The relative importance of top-down and bottom-up mechanisms in shaping community structure is still a highly controversial topic in ecology. Predatory top-down control of herbivores is thought to relax herbivore impact on the vegetation through trophic cascades. However, trophic cascades may be weak in terrestrial systems as the complexity of food webs makes responses harder to predict. Alternatively, top-down control prevails, but the top-level (predator or herbivore) changes according to productivity levels. Here we show how spatial variation in the occurrence of herbivores (lemmings and voles) and their predators (mustelids and foxes) relates with grazing damage in landscapes with different net primary productivity, generating two and three trophic level communities, during the 2007 rodent peak in northern Norway. Lemmings were most abundant on the unproductive high-altitude tundra, where few predators were present and the impact of herbivores on vegetation was strong. Voles were most common on a productive, south facing slope, where numerous predators were present, and the impacts of herbivores on vegetation were weak. The impact of herbivores on the vegetation was strong only when predators were not present, and this cannot be explained by between-habitat differences in the abundance of plant functional groups. We thus conclude that predators influence the plant community via a trophic cascade in a spatial pattern that support the exploitation ecosystems hypothesis. The responses to grazing also differed between plant functional groups, with implications for short and long-term consequences for plant communities.

  • 30.
    Johansson, Otilia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nordin, Annika
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Responses of epiphytic lichens to an experimental whole-tree nitrogen-deposition gradient2010In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 188, no 4, p. 1075-1084Article in journal (Refereed)
    Abstract [en]

    • Here, we examined the responses of the epiphytic lichens Alectoria sarmentosa and Platismatia glauca to increased atmospheric nitrogen (N) deposition in an old-growth boreal spruce forest, to assess the sensitivity of these species to N and define their critical N load. • Nitrogen deposition was simulated by irrigating 15 trees over a 3 yr period with water and isotopically labeled NH(4) NO(3) , providing N loads ranging from ambient to 50 kg N ha(-1)  yr(-1) . • Thallus N concentration increased in both species with increasing N load, and uptake rates of both NH(4) (+) and NO(3) (-) were similar. Photobiont concentration increased linearly with increased N in both species, saturating in A. sarmentosa in the third year at the highest N loads (25 and 50 kg ha(-1 ) yr(-1) ). The simulated N deposition decreased the phosphorus (P) concentration in A. sarmentosa, and increased the N : P ratio in both species. • Significant responses in lichen chemistry were detected to inputs of 12.5 kg N ha(-1)  yr(-1) or higher, suggesting that resources other than N limit lichens at higher N loads. However, the data also suggest that N saturation may be cumulative over time, even at low N.

  • 31.
    Johansson, Otilia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Lichen responses to nitrogen and phosphorus additions can be explained by the different symbiont responses2011In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 191, no 3, p. 795-805Article in journal (Refereed)
    Abstract [en]

    Responses to simulated nitrogen (N) deposition with or without added phosphorus (P) were investigated for three contrasting lichen species – the N-sensitive Alectoria sarmentosa, the more N-tolerant Platismatia glauca and the N2-fixing Lobaria pulmonaria– in a field experiment.

    To examine whether nutrient limitation differed between the photobiont and the mycobiont within the lichen, the biomass responses of the respective bionts were estimated.

    The lichenized algal cells were generally N-limited, because N-stimulated algal growth in all three species. The mycobiont was P-limited in one species (A. sarmentosa), but the growth response of the mycobionts was complex, as fungal growth is also dependent on a reliable carbon export from the photobiont, which may have been the reason for the decrease of the mycobiont with N addition in P. glauca.

    Our findings showed that P availability was an important factor when studying effects of N deposition, as P supply can both mitigate and intensify the negative effects of N on epiphytic lichens.

  • 32.
    Johansson, Otilia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nitrogen deposition drives lichen community changes through differential species responses2012In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 18, no 8, p. 2626-2635Article in journal (Refereed)
    Abstract [en]

    Nitrogen (N) deposition has increased globally over the last 150 years and further increases are predicted. Epiphytic lichens decline in abundance and diversity in areas with high N loads, and the abundance of lichens decreases along gradients of increased deposition. Thus, although N is an essential nutrient for lichens, excessive loads may be detrimental for them. However, these gradients include many correlated pollutants and the mechanisms behind the decline are thus poorly known. The aim of this study was to assess effects of N deposition, alone, on the epiphytic lichen community composition in a naturally N-poor boreal forest. For this purpose, whole spruce trees were fertilized daily with N at five levels, equivalent to 0.6, 6, 12.5, 25, and 50 kg N ha-1 yr-1, during four consecutive growing seasons (20062009), and changes in the abundance of lichens were monitored each autumn from the preceding year (2005). The studied lichen communities were highly dynamic and responded strongly to the environmental perturbation. N deposition detectably altered the direction of succession and reduced the species richness of the epiphytic lichen communities, even at the lowest fertilization application (6 kg N ha-1 yr-1). The simulated N deposition caused significant changes in the abundance of Alectoria sarmentosa, Bryoria spp., and Hypogymnia physodes, which all increased at low N loads and decreased at high loads, but with species-specific optima. The rapid decline of A. sarmentosa may have been caused by the added nitrogen reducing the stability of the lichen thalli, possibly due to increases in the photobiont: mycobiont ratio or parasitic fungal attacks. We conclude that increases in nitrogen availability, per se, could be responsible for the reductions in lichen abundance and diversity observed along deposition gradients, and those community responses may be due to physiological responses of the individual species rather than changes in competitive interactions.

  • 33.
    Johansson, Otilia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nitrogen drives lichen community changes through the different species responsesManuscript (preprint) (Other academic)
  • 34.
    Kaarlejarvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hoset, Katrine S.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Mammalian herbivores confer resilience of Arctic shrub-dominated ecosystems to changing climate2015In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, no 9, p. 3379-3388Article in journal (Refereed)
    Abstract [en]

    Climate change is resulting in a rapid expansion of shrubs in the Arctic. This expansion has been shown to be reinforced by positive feedbacks, and it could thus set the ecosystem on a trajectory toward an alternate, more productive regime. Herbivores, on the other hand, are known to counteract the effects of simultaneous climate warming on shrub biomass. However, little is known about the impact of herbivores on resilience of these ecosystems, that is, the capacity of a system to absorb disturbance and still remain in the same regime, retaining the same function, structure, and feedbacks. Here, we investigated how herbivores affect resilience of shrub-dominated systems to warming by studying the change of shrub biomass after a cessation of long-term experimental warming in a forest-tundra ecotone. As predicted, warming increased the biomass of shrubs, and in the absence of herbivores, shrub biomass in tundra continued to increase 4 years after cessation of the artificial warming, indicating that positive effects of warming on plant growth may persist even over a subsequent colder period. Herbivores contributed to the resilience of these systems by returning them back to the original low-biomass regime in both forest and tundra habitats. These results support the prediction that higher shrub biomass triggers positive feedbacks on soil processes and microclimate, which enable maintaining the rapid shrub growth even in colder climates. Furthermore, the results show that in our system, herbivores facilitate the resilience of shrub-dominated ecosystems to climate warming.

  • 35.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Baxter, Robert
    Hofgaard, Annika
    Hytteborn, Håkan
    Khitun, Olga
    Molau, Ulf
    Sjögersten, Sofie
    Wookey, Philip
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of warming on shrub abundance and chemistry drive ecosystem-level changes in a forest-tundra ecotone2012In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 15, no 8, p. 1219-1233Article in journal (Refereed)
    Abstract [en]

    Tundra vegetation is responding rapidly to on-going climate warming. The changes in plant abundance and chemistry might have cascading effects on tundra food webs, but an integrated understanding of how the responses vary between habitats and across environmental gradients is lacking. We assessed responses in plant abundance and plant chemistry to warmer climate, both at species and community levels, in two different habitats. We used a long-term and multisite warming (OTC) experiment in the Scandinavian forest-tundra ecotone to investigate (i) changes in plant community composition and (ii) responses in foliar nitrogen, phosphorus, and carbon-based secondary compound concentrations in two dominant evergreen dwarf-shrubs (Empetrum hermaphroditum and Vaccinium vitis-idaea) and two deciduous shrubs (Vaccinium myrtillus and Betula nana). We found that initial plant community composition, and the functional traits of these plants, will determine the responsiveness of the community composition, and thus community traits, to experimental warming. Although changes in plant chemistry within species were minor, alterations in plant community composition drive changes in community-level nutrient concentrations. In view of projected climate change, our results suggest that plant abundance will increase in the future, but nutrient concentrations in the tundra field layer vegetation will decrease. These effects are large enough to have knock-on consequences for major ecosystem processes like herbivory and nutrient cycling. The reduced food quality could lead to weaker trophic cascades and weaker top down control of plant community biomass and composition in the future. However, the opposite effects in forest indicate that these changes might be obscured by advancing treeline forests. © 2012 Springer Science+Business Media, LLC.

  • 36.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium.
    Eskelinen, Anu
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Herbivores rescue diversity in warming tundra by modulating trait-dependent species losses and gains2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 419Article in journal (Refereed)
    Abstract [en]

    Climate warming is altering the diversity of plant communities but it remains unknown which species will be lost or gained under warming, especially considering interactions with other factors such as herbivory and nutrient availability. Here, we experimentally test effects of warming, mammalian herbivory and fertilization on tundra species richness and investigate how plant functional traits affect losses and gains. We show that herbivory reverses the impact of warming on diversity: in the presence of herbivores warming increases species richness through higher species gains and lower losses, while in the absence of herbivores warming causes higher species losses and thus decreases species richness. Herbivores promote gains of short-statured species under warming, while herbivore removal and fertilization increase losses of short-statured and resource-conservative species through light limitation. Our results demonstrate that both rarity and traits forecast species losses and gains, and mammalian herbivores are essential for preventing trait-dependent extinctions and mitigate diversity loss under warming and eutrophication.

  • 37.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Eskelinen, Anu
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Herbivory prevents positive responses of lowland plants to warmer and more fertile conditions at high altitudes2013In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 27, no 5, p. 1244-1253Article in journal (Refereed)
    Abstract [en]

    Warm-adapted low elevation plants are expected to exhibit considerable range shifts to higher altitudes and latitudes as a result of climate warming and increased nutrient loads. However, empirical studies show that the magnitude and direction of plant responses are highly species- and site-specific, suggesting that several additional drivers interact with warmer climate.

    We experimentally tested the interactive effects of climate warming, mammalian herbivory and soil fertility on low elevation plants. Seedlings of three warm-adapted lowland forbs (Epilobium angustifolium, Silene dioica and Solidago virgaurea) were transplanted to an open tundra site with native mountain tundra vegetation, and the effects of full factorial combinations of herbivore exclosures, warming and fertilization on transplant survival, growth and flowering were studied for two growing seasons. We also investigated the response of native vegetation biomass to the same treatments and compared it with the responses of transplanted lowland forbs.

    Effects of both warming and fertilization on the transplanted lowland forbs strongly hinged on herbivore exclusion, resulting in 2–13-fold increase in biomass in warmed and fertilized plots without herbivores compared with warmed and fertilized plots with herbivores present, the magnitude depending on the species. While warm-adapted transplants benefited from warming, the native tundra plant community biomass did not respond to warming treatment.

    Our results show that grazing limits the growth of transplants under warmer and more productive conditions, indicating that the expansion of lowland plant species to higher altitudes with warming may be hampered by mammalian herbivory. Furthermore, our results also suggest that migration of warm-adapted species into lightly grazed high-altitude tundra ecosystems might transform these communities to be more responsive to warmer climate and nutrient loads. Studies that do not consider species' upward shifts from lower altitudes might thus have underestimated vegetation responses to global warming, as well as the potential of herbivory to influence these responses.

  • 38.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hoset, Katrine
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Short-term herbivory overweighs effects of long-term warming on plant communityManuscript (preprint) (Other academic)
  • 39.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Concurrent biotic interactions influence plant performance at their altitudinal distribution margins2014In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 123, no 8, p. 943-952Article in journal (Refereed)
    Abstract [en]

    Recent studies have shown that biotic interactions can shape species’ distributions, but empirical data on multiple biotic interactions are scarce. Therefore, we examined effects of plant-plant and plant-herbivore interactions on plant survival, growth and reproduction at different altitudes. For these purposes we conducted a factorial neighbor removal and large herbivore exclusion experiment with six transplant species (three tall forbs with their main distribution at low altitudes and three small forbs with their main distribution at high altitudes) on Låktačohkka Mountain, northern Sweden, replicated at two altitudes (ca. 600 and 900 m a.s.l.) and consequently a 2.1 °C difference in summer air temperatures. Overall transplant survival was 93%. Two out of three tall forbs grew better at low than at high altitudes, while no significant differences in growth between altitudes were found for any of the three small forbs. Since the main difference in abiotic conditions between the altitudes was most likely in temperature (as the sites were topographically and edaphically matched as closely as possible), this result indicates that climatic warming could induce upward migration of tall low-altitude forbs. Negative plant-plant interactions prevailed at both altitudes, and we found indications that competition may set the lower altitudinal limits of some small tundra forbs. Thus, increased competition in response to climate warming may potentially shift the lower margins of high-altitude forbs’ distributions upward. Large mammalian grazers reduced the growth of tall forbs and enhanced the flowering of small forbs, and grazers could thus at least partly counteract the anticipated warming-induced distribution shifts.

  • 40.
    Kivinen, Sonja
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Berg, Anna
    Moen, Jon
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Östlund, Lars
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Forest Fragmentation and Landscape Transformation in a Reindeer Husbandry Area in Sweden2012In: Environmental Management, ISSN 0364-152X, E-ISSN 1432-1009, Vol. 49, no 2, p. 295-304Article in journal (Refereed)
    Abstract [en]

    Reindeer husbandry and forestry are two main land users in boreal forests in northern Sweden. Modern forestry has numerous negative effects on the ground-growing and arboreal lichens that are crucial winter resources for reindeer husbandry. Using digitized historical maps, we examined changes in the forest landscape structure during the past 100 years, and estimated corresponding changes in suitability of forest landscape mosaics for the reindeer winter grazing. Cover of old coniferous forests, a key habitat type of reindeer herding system, showed a strong decrease during the study period, whereas clear-cutting and young forests increased rapidly in the latter half of the 20th century. The dominance of young forests and fragmentation of old-growth forests (decreased patch sizes and increased isolation) reflect decreased amount of arboreal lichens as well as a lowered ability of the landscape to sustain long-term persistence of lichens. The results further showed that variation in ground lichen cover among sites was mainly related to soil moisture conditions, recent disturbances, such as soil scarification and prescribed burning, and possibly also to forest history. In general, the results suggest that the composition and configuration of the forest landscape mosaic has become less suitable for sustainable reindeer husbandry.

  • 41.
    Linderholm, Johan
    et al.
    Umeå University, Faculty of Arts, Department of historical, philosophical and religious studies, Environmental Archaeology Lab.
    Olofsson, Johan
    Umeå University, Faculty of Arts, Department of historical, philosophical and religious studies, Environmental Archaeology Lab.
    Miljöarkeologisk markundersökning i Ängås, Västra Frölunda sn, Bohuslän2008Report (Other academic)
  • 42.
    Linderholm, Johan
    et al.
    Umeå University, Faculty of Arts, Department of historical, philosophical and religious studies, Environmental Archaeology Lab.
    Olofsson, Johan
    Umeå University, Faculty of Arts, Department of historical, philosophical and religious studies, Environmental Archaeology Lab.
    Miljöarkeologisk undersökning av markprofiler i fossila odlingslämningar, raä 217, Foss sn, Bohuslän.2008Report (Other academic)
  • 43. Metcalfe, Daniel B.
    et al.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season2015In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 124, no 12, p. 1632-1638Article in journal (Refereed)
    Abstract [en]

    Herbivores play a key role in the carbon (C) cycle of arctic ecosystems, but these effects are currently poorly represented within models predicting land-atmosphere interactions under future climate change. Although some studies have examined the influence of various individual species of herbivores on tundra C sequestration, few studies have directly compared the effects of different herbivore assemblages. We measured peak growing season instantaneous ecosystem carbon dioxide (CO2) exchange (photosynthesis, respiration and net ecosystem exchange) on replicated plots in arctic tundra which, for 14 years, have excluded different portions of the herbivore population (grazed controls, large mammals excluded, both small and large mammals excluded). Herbivory suppressed photosynthetic CO2 uptake, but caused little change in ecosystem respiration. Despite evidence that small mammals consume a greater portion of plant biomass in these ecosystems, the effect of excluding only large herbivores was indistinguishable from that of excluding both large and small mammals. The herbivory-induced decline in photosynthesis was not entirely attributable to a decline in leaf area but also likely reflects shifts in plant community composition and/or species physiology. One shrub species - Betula nana - accounted for only around 13% of total aboveground vascular plant biomass but played a central role in controlling ecosystem CO2 uptake and release, and was suppressed by herbivory. We conclude that herbivores can have large effects on ecosystem C cycling due to shifts in plant aboveground biomass and community composition. An improved understanding of the mechanisms underlying the distinct ecosystem impacts of different herbivore groups will help to more accurately predict the net impacts of diverse herbivore communities on arctic C fluxes.

  • 44. Oksanen, Lauri
    et al.
    Oksanen, Tarja
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dahlgren, Jonas
    Hambäck, Peter
    Ekerholm, Per
    Lindgren, Åsa
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Islands as test of the green world hypothesis2010In: Trophic Cascades: predators, prey, and the changing dynamics of nature / [ed] John Terborgh, James Estes, Washington DC: Island Press , 2010, p. 163-178Chapter in book (Other academic)
  • 45. Oksanen, Lauri
    et al.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Vertebrate Herbivory and Its Ecosystem Consequences2009In: Encyclopedia of Life Sciences, Chichester: John Wiley & Sons, Inc. , 2009Chapter in book (Other academic)
    Abstract [en]

    Herbivory is a process where animals obtain energy and nutrients from vegetative plant parts (leaves, stems, etc.). Herbivory is called grazing, browsing or folivory, depending on the size of the herbivore and the type of plant tissue consumed. The consumption is actually performed by microbes in the digestive system. The role of the animal is to prepare the material and to provide a good environment for the microbes. This requires specializations, making herbivores a distinct animal group. This article focuses on consequences of herbivory by vertebrates. Vertebrate herbivory can change forests to heathlands or grasslands and influence the composition and species richness of the herbaceous vegetation.

  • 46.
    Oksanen, Tarja
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Oksanen, L.
    Dahlgren, Jonas
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Arctic lemmings, Lemmus spp. and Dicrostonyx spp.:integrating ecological and evolutionary perspectives2008In: Evolutionary Ecology Research, Vol. 10, p. 415–434-Article in journal (Refereed)
    Abstract [en]

    Question: What do the evolution and the fluctuation patterns of arctic lemmings – Lemmus spp. and Dicrostonyx spp. – tell us about their population dynamics and the influence of lemmings on the ecology and evolution of arctic plants?

    Methods: We reviewed the literature concerning the evolution of arctic lemmings andanalysed their current fluctuation patterns with a focus on the following aspects: (1) changes in morphology related to feeding ecology; (2) per capita rate of population growth during the year preceding the peak (rp), predicted to be high for arvicoline rodents interacting with the winter forage plants and low for arvicoline rodents interacting with specialized predators; (3)the skew of logarithmically transformed density data (skd), predicted to be zero for arvicolines interacting with their winter forage plants, negative for arvicolines interacting with specialized predators, and positive for arvicolines interacting with both seasonally renewed and depletable winter forage plants (the ‘Barrow model’ of Turchin and Batzli, 2001). Moreover, we conducted spectral analysis of those density records, which were at least 15 years long. Here predator–arvicoline models predict that similar spectral density profiles, with statistically significant peaks, are obtained with untransformed and logarithmically transformed data, whereas arvicoline–plant models predict that such profiles are only obtained using logarithmically transformed data.

    Key insights: Arctic lemmings differ from other microtine rodents by having several features which increase their foraging efficiency under harsh conditions at the cost of reduced agility. These features were acquired rapidly at the dawn of the Pleistocene. Density fluctuations of all arctic lemming populations, for which sufficient data are available, correspond to the predictions of the ‘Barrow model’ and differ from the predictions of predator–arvicoline models. Our interpretation is as follows. When the Polar Sea froze, the primary productivity of northernmost Eurasia and North America was reduced, causing a shift from predation controlled to food-limited dynamics in microtine rodents. This change in population dynamics triggered an extraordinarily rapid change in the characteristics of lemmings and precipitated an intense, sustained lemming–vegetation interaction, as old as the tundra itself, which has probably played a major role in the evolution of arctic plants.

    Predictions: Increasing primary productivity along the southern (lower) boundary of the ranges of arctic lemmings should lead to their elimination by voles via apparent competition. Exclusion of lemmings should initiate dramatic changes in the vegetation of those tundra habitats, which have at least moderate snow cover and do not freeze in solid ice in winter. Exclusion of predators should have no impact on dynamics of inland populations of arctic lemmings.

  • 47.
    Oksanen, Tarja
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Department of Biology, Section of Ecology, University of Turku, Turku, Finland.
    Dahlgren, Jonas
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kyrö, Kukka
    Department of Biology, Section of Ecology, University of Turku, Turku, Finland.
    On the implications of currently available data concerning population fluctuations of arctic lemmings:  – reply to Gauthier et al. (2008)2009In: Evolutionary Ecology Research, ISSN 1522-0613, E-ISSN 1937-3791, Vol. 11, p. 485-487Article in journal (Refereed)
  • 48.
    Olofsson, J
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Oksanen, L
    Callaghan, T
    Hulme, PE
    Oksanen, T
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Suominen, O
    Herbivores inhibit climate-driven shrub expansion on the tundra2009In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 15, p. 2681-2693Article in journal (Refereed)
    Abstract [en]

    Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana, has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (40.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (o0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.

  • 49.
    Olofsson, J
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Stark, S
    Oksanen, L
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Reindeer influence ecosystem processes in the tundra2004In: Oikos, Vol. 105, p. 386-396Article in journal (Refereed)
  • 50.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of simulated reindeer grazing, trampling and waste products on nitrogen mineralization and production in a subarctic meadow2009In: Arctic, Antarctic and Alpine research, ISSN 1523-0430, E-ISSN 1938-4246, Vol. 41, no 3, p. 330-338Article in journal (Refereed)
    Abstract [en]

    An experiment was conducted in arctic tundra to evaluate the role of reindeer grazing, trampling, and feces and urine deposition in nutrient turnover and primary production. Grazing was simulated by mowing, trampling by the impact of a wooden pole, and waste product deposition by the application of fertilizer. In the first year, aboveground primary production increased with simulated grazing in the fertilized plots and decreased with simulated grazing in the unfertilized plots; this indicates a higher regrowth capacity at higher nutrient levels. However, nitrogen mineralization and primary production were mainly determined by the input or removal of nutrients and, therefore, decreased in plots that were grazed but not fertilized and increased in plots that were fertilized but not grazed. Simulated trampling decreased the depth of the moss layer and increased soil temperatures, but the higher temperatures increased N mineralization only in unmowed plots, and the increased nitrogen availability was not translated into increased primary production. Since aboveground and belowground net primary production in plots with simulated grazing was the same as in plots without simulated animal activity, this study indicates that an entire trophic level can be supported with no apparent effect on primary production.

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