| Summary: | Depending on grazing intensity and growing conditions, disturbances caused by large herbivores have strong effects on vascular plant communities. However, it is not well known how sheep grazing activity affects bryophyte communities in tundra ecosystems. The aim of this study was to assess the impacts of sheep grazing activity on the structure and composition of bryophyte communities in Iceland. Bryophyte communities were compared in three grazed and three ungrazed valleys, in two regions in Northwest and North Iceland. Sampling was stratified to allow for an investigation of different growing conditions with respect to exposure, elevation, and landform. Bryophyte layer depth, species diversity and abundance were measured, and species were grouped by growth form and life-history. This study found that growing conditions shape bryophyte communities in Iceland, and within certain conditions sheep grazing activity has a detectable impact. There were more pronounced effects on the bryophyte communities in the west than the east-facing slopes, and in low than high elevation. The bryophyte layer was significantly deeper in ungrazed than grazed valleys, which indicates that sheep grazing may have impacts on ecosystem function. Species diversity did not differ greatly but was somewhat higher in grazed valleys within slopes that had a west-facing exposure. Pleurocarpous mosses, the most abundant growth form, were more abundant in grazed than ungrazed valleys, suggesting this growth form tolerates herbivore disturbances relatively well. Competitive and stress tolerate species were more abundant in grazed valleys, suggesting they cope with disturbances caused by sheep grazing activity. While the main drivers in shaping bryophyte communities are the growing conditions, sheep grazing activity also has some impacts to these communities in Iceland. Þung beit stórra grasbíta raskar samfélögum háplantna, en þó mismikið eftir vaxtarskilyrðum. Það er hins vegar lítið vitað um hver beitaráhrifin eru á mosasamfélög í ...
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