Ecological responses to sheep grazing in Norwegian mountains: Insight from long-term experimental approaches

Land-use change is an understudied facet of global environmental change. In the mountains of Norway, sheep grazing is the dominant land-use. There is high spatial and temporal variation in sheep grazing throughout Norway, and this is expected to influence ecological processes across spatial scales. The impact of sheep grazing has the potential to both interact-with and feedback-to climatic change. In this presentation, we combine grazing manipulation experiments with chronosequences of forest development to examine the ecological effects of sheep grazing on vegetation, soils and ecosystems. We will first present results from a long-term enclosure experiment to show how sheep grazing limits treeline advance and shrub expansion in low alpine vegetation. We will next demonstrate how grazing also buffers an elevational shift in the plant community composition, and how the response of plant diversity to sheep grazing varies along an elevational gradient. Using a dendroecological approach we will propose that the influence of grazing on important alpine processes is far greater than the influence of climatic variation. Species distribution modelling will then be used to identify regions where sheep grazing is most influential to the conservation of rare plant species. In the second part of the presentation we will contrast plant and ecosystem carbon pools in the presence of livestock grazing as well as absence in the decadal time-scale (using experimental exclosures) and centurial time-scale (using a natural exclosure experiment). We will show how the limitation of forest establishment by herbivores can lead to substantial differences in ecosystem carbon storage. Finally, we will discuss the impacts of sheep grazing in alpine ecosystems in context of the ecosystem services framework. Using output from a wide range of studies across multiple ecosystem processes and taxonomic groups, we will ask whether the provisioning of ecosystem services in the low alpine zone is greater at low sheep densities, high sheep densities or in the absence of grazing.