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Small-scale vegetation patterns are frequently the results of plant-plant interactions such as facilitation and competition. Facilitation should be particularly pronounced when both abiotic and biotic stresses are high, but few studies were conducted in such habitats. In heavily-grazed pastures on the eastern Tibetan Plateau, an area with both high abiotic stress and strong biotic disturbance, we made relevés of herb species both beneath and outside canopies of three shrub species (Spiraea alpina, Sibiraea angustata and Potentilla fruticosa) differing in palatability and canopy structure. Herb species richness (S), pooled cover (PC) of all species, number of flowering species (FS) and number of inflorescences of all species (IN) were greater outside than beneath the shrub canopies. Evenness (J), in contrast, was smaller outside, while Shannon’s diversity index (H) was the same. Differences in S and J between plots beneath and outside the shrub canopies were greater in the case of P. fruticosa than in the cases of S. angustata and S. alpina, but differences in PC, FS or IN did not depend on the shrub species. Among the common species (frequency ≥6), 47–85% were equally frequent beneath and outside the shrubs, 13–39% were more frequent outside and 3–13% were more frequent beneath the shrubs. For the rarest species (frequency < 6), however, more species occurred beneath than outside the shrubs. The ordination diagram showed a clear separation between the relevés outside and beneath the shrubs and a gradient from P. fruticosa via S. alpina to S. angustata, accompanied by a distinct decrease in the extent of the difference between the vegetation beneath and outside the shrub canopies. In conclusion, the three shrub species facilitated some species in the herb layer and each shrub species had a specific impact, related to its canopy structure and palatability but also to the grazing pressure, which was greater around the P. fruticosa shrubs than around S. alpina and S. angustata.

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84 2258 2268 Beaulieu, J., J. Gauthier and L. Rochefort. 1996. The growth response of graminoid plants to goose grazing in high arctic environment

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10 457 462 Belski, A. J. 1992. Effects of grazing, competition, disturbance and fire on species composition and diversity in grassland communities

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bison athabascae) grazing on a sub-hygric shrub meadow plant community type, Mackenzie Bison Sanctuary, Northwest Territories. M.S. Thesis. Univ. Alberta. Edmonton. The impacts of Wood Bison (Bison bison

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grazing 1 st communication: Yield and digestibility of organic matter. Czech J. Anim. Sci. 51,5. 205–213 pp. Tasi J. Effect of different agronomical measures on yield and quality of

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. and Campbell, B.D. 2007. Plant trait responses to grazing — a global synthesis. Global Change Biology 13: 313–341. Campbell B.D. Plant trait responses to grazing — a global

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Báldi, A., P. Batáry and S. Erdős. 2005. Effects of grazing intensity on bird assemblages and populations of Hungarian grasslands. Agricult. Ecosyst. Environ. 108

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. Kadmon. 1999. Effects of grazing and topography on long-term vegetation changes in a Mediterranean ecosystem in Israel. Plant Ecology 145:243-254. Effects of grazing and topography on long-term vegetation changes in a

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Díaz, S., I. Noy-meir and M. Cabido. 2001. Can grazing response of herbaceous plants be predicted from simple vegetative traits? J. Appl. Ecol. 38: 497–508. Cabido M

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We studied biomass and species composition changes of open perennial sand grassland ( Festucetum vaginatae ) as response to different levels of simulated grazing pressures. We conducted a factorial micro-plot field experiment on previously grazed grassland that has been abandoned for a long time. In a two-way factorial design of 12 treatments × 8 repeats, we performed clipping (twice a year for three years) and litter treatments (removing and adding litter once at the beginning of the experiment) to simulate components of grazing, namely the biomass removal and the reduction of the litter accumulation. We used field spectroscopy and visual canopy cover estimation to measure the effects on the amount of the above-ground green biomass and on the vegetation composition.

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