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Increased temperatures and precipitation, reduction in sea ice and glacier ice, the increased levels of UV-radiation and the long-range tra- ported contaminants in arctic and alpine regions are stress factors that challenge terrestrial and aquatic ecosystems. The large natural variation in the physical parameters of these extreme environments is a key factor in structuring the biodiversity and biotic productivity, and the effect of the new stress factors can be critical for the population structures and the - teraction between species.

These changes may also have socio-economic effects if the changes affect the bio-production, which form the basis for the marine and terrestrial food chains. The book is uniquely multidisciplinary and provides examples of va- ous aspects of contemporary environmental change in arctic and alpine - gions. When the search was updated in , 6 new potentially relevant articles were found, 4 of which were included after screening on full text. Finally, quality assessment was made of the articles that had passed the screening process, and 60 of them were then excluded.

Common reasons for exclusion were inadequate methodological descriptions, and vegetation or reindeer data that were difficult to interpret. In 12 cases, articles were excluded since they reported data that could also be found elsewhere. Nearly all of the 40 articles included in this review were found in publication databases.

Of the 39 included publications that had been identified during the main searches in late , 37 were returned by at least one of the databases searched see Additional file 4. Since the full-text searches required about a week of extra work, mainly spent screening more than 6, titles and some abstracts, we conclude in retrospect that this effort did not pay off well enough to be justified.

The two included publications that had not been retrieved from any of the databases, Gaare et al. The report by Gaare et al. The searches that used non-English search strings returned very few potentially relevant publications that had not already been identified by other means. A total of about 30 articles in Swedish, Danish, Norwegian or Finnish were considered during the initial stages of this review, but a majority of them had been found in review bibliographies or through searches with English search terms, and none of them made it through both full-text screening and critical appraisal, with the one exception mentioned above.

All but two of the 40 articles finally included were published in or later. Year of publication of articles that passed abstract screening. Most of the studies 31 of them were conducted in treeless terrain such as tundra or alpine areas, but 2 studies were carried out in subalpine birch forests, and 8 studies reported data from both treeless areas and birch forests.


Sites where studies included in this review were carried out. Fennoscandian sites, districts or ranges where studies included in this review were carried out. Numbers refer to Norwegian reindeer-herding districts see Table B in Additional file 5. As a consequence of the uneven geographic distribution of the studies, the majority 26 of them dealt with herbivory by native Eurasian reindeer Rangifer tarandus tarandus , either semi-domesticated or wild.

The North American studies were all concerned with wild caribou R. The reindeer on Svalbard R. About half of the studies 21 of them were made in areas where reindeer herbivory mainly took place during summer, whereas winter grazing was the subject of 5 studies. In 13 cases, reindeer were present during several seasons or throughout the whole year.

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Note that the winter grazing considered in this review was confined to treeless areas and birch forests, as studies in coniferous forests were outside our scope. Of the 41 included studies, 30 had a CI design, based on sampling inside and outside reindeer exclosures, in districts with different reindeer densities, or at various distances from a border fence. BA design had been applied in 6 studies, 2 of which were based on remote sensing.


The remaining 5 studies had a BACI design based on sampling inside and outside exclosures. Quantitative data have been extracted from 35 of the 41 included studies. Almost three quarters of these outcomes 1, of them originate from four of the studies [ 23 , 24 , 62 , 63 ] and have been supplied as raw data.

Most of the extracted outcomes are comparisons of the cover cases , abundance cases , biomass cases , Shannon or Simpson diversity cases , or species richness cases of a group or species of vegetation that had been exposed to different levels of reindeer herbivory. For that reason, we eventually decided not to analyse single-species data in this review. Instead, we have focused on the comparisons of vegetational groups that were available. We begin with a narrative synthesis in order to provide context and background for the quantitative meta-analyses that follow.

An overview of the included studies can be found in Table A in Additional file 5 , with Table B in the same file providing data on the sites or regions where the studies were carried out. The tables are subdivided based on the geographic distribution of the studies. One of the columns in Table A summarises the effects of reindeer herbivory as reported by the respective authors of the studies. The results often show divergent responses of the vegetation. For instance, Olofsson et al.

Similarly, Ravolainen et al. Responses also varied between studies. In Finnmark, northern Norway, an interesting remote-sensing study was done on the recovery of lichen-dominated vegetation after a decline in reindeer densities [ 46 ]. In the s, the reindeer populations had more than doubled in the area [ 58 ]. Cover increased 8. The increase rate was inversely related to changes in reindeer densities, and positively related to mean summer precipitation.

The authors conclude that the rapid transition from barren ground to a flourishing lichen-dominated vegetation suggests that vegetation degradation by grazing and trampling is reversible [ 46 ]. We have performed quantitative syntheses meta-analyses of data extracted from 31 of the 41 included studies. Some of the 10 studies that appear in the narrative synthesis Additional file 5 but not in the quantitative one have only reported on responses of single species, or on species aggregations that were not, or were poorly, replicated in other studies, such as the data on leaf area index in Cahoon et al.

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There were also a few studies that could not be used in meta-analysis since lack of information on outcome deviances or sample sizes made it impossible to calculate effect sizes. Like the narrative synthesis, the meta-analyses unraveled a great divergence among responses to reindeer exposure data on a total of eight vegetation categories, such as lichens, graminoids, etc. Both significantly positive and significantly negative average responses could be found in all vegetation categories that were considered. However, most studies had large confidence intervals that included zero effect size.

Overall responses average standardised mean differences of all cover, abundance and biomass data combined were small. Vascular species richness also responded negatively to an increased grazing pressure. Summary of meta-analyses of vegetation responses to reindeer herbivory. Subdivisions are based on groups and aspects of vegetation. A position to the left of the green zero line indicates that the cover, biomass or abundance of vegetation is lower at high exposure to reindeer herbivory than at low exposure, and vice versa. Overall models combine data on cover, biomass and abundance.

Individual data are presented in forest plots in Additional file 6. Summary of meta-analyses of species richness responses to reindeer herbivory. Subdivisions are based on groups of vegetation. Individual data are presented in forest plots in Additional file 7. Funnel plots were created to visually check for systematic heterogeneity and publication bias in the data set. No publication bias was detected. However, responses of individual plant aspects were not significantly different from zero.

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Some studies show a high variation of effects. For this group too, responses varied within and between studies, vegetation categories and aspects. Woody plants showed a non-significant overall response to reindeer exposure. Again, however, responses varied within and between studies, with some individual comparisons showing significant negative responses and others significant positive responses. However, even for this group, which is well known to respond negatively to grazing and trampling, abundance showed a non-significant but positive response to reindeer exposure SMD: 0.

Bryophytes showed no overall response to reindeer herbivory, while the cover of bare ground showed an overall non-significant tendency to increase SMD: 0. We used weighted meta-regressions of effect sizes against four different ways of measuring intervention strength grazing pressure to see if they explained some of the divergence in the results. As measures of grazing pressure, we used the reindeer density in the high-exposure treatment, the absolute and relative differences between high and low densities, and the product of absolute density difference and duration of exposure difference i.

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None of the measures of grazing pressure showed any significant relationship with any of the effect sizes that we had calculated, suggesting either that the reindeer densities were too coarsely estimated, or that reindeer density is context-dependent. We illustrate the lack of pattern with meta-regressions for lichens all aspects combined and cover of bare ground see Additional file 8. Cover of lichens and bare ground at high and low exposures to grazing.

Each pair of symbols connected by a straight line represents data from an individual study or site. Sites with high reindeer densities already had very low lichen cover perhaps as a result of grazing or trampling before the studies began as well as a higher cover of bare ground, and did not respond to the exposure differences during the study period.

This shows that the composition of the vegetation is very important in determining the effects of herbivory. Meta-regressions of lichen and bare-ground cover against average cover of lichens and bare ground. Each symbol represents one study. Symbol sizes are proportional to the inverse variance of effect sizes. Meta-regression of vascular species richness against mean annual temperature.

We tested the effects of the most commonly reported effect modifiers on cover, since that was the aspect of vegetation most frequently represented in our data set. There was not enough data on other combinations of effect modifiers, aspects, and vegetation types to analyse them in a meaningful manner. The clearest differences were related to domestication — wild reindeer tended to have stronger impacts than semi-domesticated ones.

Wild reindeer had a positive effect on the cover of graminoids SMD: 0. We also tested effects of soil moisture dry, mesic, wet , habitat type tundra, forest-tundra ecotone, birch forest , seasonality of grazing summer-autumn-winter , and exposure type fencing, exclosures, area comparisons, etc. Overall we found large variations in the effects of reindeer on vegetation, and outcomes seem difficult to predict. However, we did identify a few patterns.

This indicates that forbs are both highly selected by reindeer and vulnerable. They may be an important indicator group of vascular plants. In conclusion, the effect of reindeer grazing on arctic-alpine vegetation appears to be context-dependent. What, then, could cause this context dependence, i. The distribution of reindeer encompasses large environmental gradients, ranging from low-productive to highly productive sites, from dry to wet environments and from forests to high alpine tundra, with an entire vegetation mosaic being present within these gradients.

All different vegetation types that occur here cannot be expected to respond in the same way to grazing and trampling. This is mirrored in our study, where vascular-plant species richness was found to be related to mean temperature. Further, the studies included in our analysis ranged from dry ridge vegetation to riparian herb meadows and mesic birch forests, with many different plant communities occurring within those vegetation types.