| 1. |
- Olofsson, Johan, et al.
(författare)
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Effects of herbivory on competition intensity in two arctic-alpine tundra communities with different productivity
- 2002
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 96:2, s. 265-272
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Tidskriftsartikel (refereegranskat)abstract
- The effects of long-term (I I yr) exclusion of vertebrate herbivores on competition intensity and plant community structure ere studied using manipulative field experiment,; in two arctic-alpine plant communities with contrasting productivity : an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed. the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response Lis observed on the tall herb meadow. The intensity of competition. measured with a neighbour removal experiment, did not differ significantly between three of the four habitat x treatment combinations - snowbed exposures. meadow exclosures and open meadow plots but as significantly loader on open snowbed plots, Our results thud, suggest that the low competition intensity in the unproductive snow bed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats When herbivorous mammals have been excluded for a sufficiently long time to allots the build-up of plant biomass even in Unproductive habitat,. between-habitat differences in competition intensity disappear.
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| 2. |
- Olofsson, Johan
(författare)
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Influence of herbivory and abiotic factors on the distribution of tall forbs along a productivity gradient : a transplantation experiment
- 2001
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 94:2, s. 351-357
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Tidskriftsartikel (refereegranskat)abstract
- In arctic-alpine areas tall herb vegetation is restricted to sites with high productivity. At higher elevation, low prostate forbs and narrow-leaved graminoids dominate the vegetation in sites with a protecting snow cover during winter. In this study, I test whether herbivory or abiotic factors prevent tall forbs from growing at higher altitudes. Vegetation blocks from a tall herb meadow were transplanted to herbivore exclosures and open plots in a low-productive snowbed and a productive tall herb meadow. The tall forbs performed equally well in the exclosures on the low-productive snowbed as in the tall herb meadow, but decreased in the open plots on the low-productive snowbed. Thus, even if abiotic factors are ultimately causing many of the vegetation patterns observed in arctic-alpine plant communities, herbivory appears to be the main proximate factor responsible for the decreasing abundance of tall forbs along gradients of decreasing productivity in arctic-alpine areas.
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| 3. |
- Olofsson, Johan, et al.
(författare)
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Response of galling invertebrates on Salix lanata to reindeer herbivory
- 2000
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 91:3, s. 493-498
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Tidskriftsartikel (refereegranskat)abstract
- Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long-term grazing manipulation experiment. More then 30-yr-old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall-forming insects (Pontania glabrifons) and gall-forming mitts were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short-term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural delta N-15 value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Lear weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata. Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long-term grazing seems to increase the density of invertebrate herbivory in the same way as short-term grazing, even if the plant responses differ substantially.
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| 4. |
- Olofsson, Johan, et al.
(författare)
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Role of litter decomposition for the increased primary production in areas heavily grazed by reindeer : a litterbag experiment
- 2002
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 96:3, s. 507-515
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Tidskriftsartikel (refereegranskat)abstract
- Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands hake replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transplantation of litter between lightly grazed and heavily grazed areas. using the litterbag technique, We used litter from two of the most common species on the lightly grazed side, Betula nana and Empetrum nigrum, and two of the most common species on the heavily grazed side. Carex bigelowii and Deschampsia flexuosa, We found that herbivory improved litter quality by favouring species with easily decomposed litter. However. herbivore also improved litter quality by increasing the nitrogen content and lowering the C/N ratio of each species. Decomposition rates even correlated with the abundance of the plant category in question, Shrub litter decomposed faster in the lightly grazed area where shrubs were common, and graminoid litter decomposed faster in the heavily grazed area where graminoids were common. These results indicate that the decomposer micro-organisms are adapted to the most common litter types. This studs shoals that detailed information about the effect of herbivore on litter quality is important to understand differences between the short-term and long-term effects of herbivory on nutrient cycling and primary production.
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| 5. |
- Rammul, Üllar, et al.
(författare)
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Vole–vegetation interactions in an experimental, enemy free taiga floor system
- 2007
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 116:9, s. 1501-1513
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Tidskriftsartikel (refereegranskat)abstract
- Vole–vegetation interactions in a predation-free taiga environment of northern Fennoscandia were studied by transferring vegetation from natural Microtus habitats into a greenhouse, where three habitat islands of about 30 m2 were created. The ‘islands’ were subjected to simulated summer conditions and a paired female field vole, Microtusagrestis, was introduced to each ‘island’. The development of the female and her young was followed by recurrent live trapping. The development of the vegetation was followed by recurrent marking and censusing of plant shoots at intervals of five days. In the next growing season, two ‘islands’ were subjected to a new grazing treatment to study the impacts of repeated grazing on the vegetation and on the growth and reproduction of voles. Plant biomasses were harvested at the end of each trial. In all trials, the biomasses of graminoids and non-toxic herbs other than ferns, fireweeds and rosaceous plants were profoundly decimated. Even the biomass of a toxic herb Aconitum lycoctonum decreased largely at pace with the palatable herbs. The least preferred plant categories maintained their biomasses at control levels. Their neutral collective response was created by opposite species-level trends. Species typical for moist and nutrient-rich forests suffered from vole grazing, whereas the biomass of species adapted to disturbed habitats increased.In spite of the dramatic changes in the vegetation, the introduced female voles survived throughout the trials and reproduced normally. The young of their first litters survived well and reached the final weights typical for individuals starting to winter as immatures. We conclude that most of the plant biomass found on productive boreal forest floors is potential food for field voles and remains palatable for them even when subjected to recurrent, severe grazing. If nothing else than summer resources were limiting the growth of the field vole populations, the plants currently dominating moist and nutrient-rich taiga floors could not survive in this habitat.
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| 6. |
- Barthelemy, Hélène, 1986-, et al.
(författare)
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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 experiment
- 2019
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 128:10, s. 1515-1524
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Kaarlejärvi, Elina, 1980-, et al.
(författare)
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Concurrent biotic interactions influence plant performance at their altitudinal distribution margins
- 2014
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Ingår i: Oikos. - : John Wiley & Sons. - 0030-1299 .- 1600-0706. ; 123:8, s. 943-952
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Metcalfe, Dan, et al.
(författare)
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Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season
- 2015
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 124:12, s. 1632-1638
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Olofsson, Johan, et al.
(författare)
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On the balance between positive and negative plant interactions in harsh environments
- 1999
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Ingår i: Oikos. - 0030-1299 .- 1600-0706. ; 86:3, s. 539-543
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Tidskriftsartikel (refereegranskat)abstract
- Positive interactions between plants typically occur where the presence of a species ameliorates the abiotic environment for another. However, there is also a potential for resource competition to act at the same time, which creates a situation where the net outcome is a balance between positive and negative interactions. We present data from a nine-year study in two extreme high alpine habitats that was designed to test whether the effects of established Ranunculus glacialis individuals on germination and growth of Oxyria, digyna are primarily positive or negative at the altitudinal limit of vascular plants. We show net effects ranging from neutral to negative, but no positive effects were detected. We also argue that close associations between plants in these harsh environments may both ameliorate and deteriorate the abiotic environment, and that experimental manipulations are necessary to tell the difference.
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| 10. |
- Ramirez, Juan Ignacio, et al.
(författare)
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Reindeer grazing reduces climate-driven vegetation changes and shifts trophic interactions in the Fennoscandian tundra
- 2024
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Ingår i: Oikos. - : John Wiley & Sons. - 0030-1299 .- 1600-0706.
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Tidskriftsartikel (refereegranskat)abstract
- Herbivores drive shifts in plant species composition by interacting with vegetation through defoliation, trampling and nutrient addition: urine and faeces. As herbivore effects on vegetation accumulate over time, they might spillover to other trophic levels, but how and when this happens is poorly understood. Since it is methodologically demanding to measure biodiversity across spatial gradients, an alternative approach is to assess it through biodiversity indices of vascular plants. We employed the Index of biodiversity relevance developed for Swedish flora which provides an estimated number of organisms associated with a plant species, allowing the quantification of trophic community size. Values from this index were coupled with vegetation data from a network of 96 fenced and paired grazed plots across Fennoscandia. We analysed the role herbivory has on plant richness and diversity, and on the number of organisms that interact with the vegetation according to the index values. We also explored how herbivores influence the competitive effects of tall shrubs on other plants since the dominance of a vegetation type links directly to biodiversity. Plant diversity had no clear response to grazing. Overall vegetation and the vegetation subgroups herbs and non-fruit shrubs had higher biodiversity index values in fenced plots, indicating a higher number of plant–host interactions. Herb cover was negatively related to shrubs in both treatments but with a faster decline in the absence of herbivores. This study highlights the importance of maintaining herbivore populations in the Arctic to conserve the vegetation structure and biodiversity of the tundra. This method of coupling biodiversity indexes with vegetation data provides complementary information to the plant diversity, especially when methodological or time constraints prevent complete field inventories.
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