Plant species diversity and grazing in the Scandinavian mountains - patterns and processes at different spatial scales

There is a long tradition of grazing by semi-domestic reindeer and sheep in alpine and sub-alpine Scandinavian habitats, but present management regimes are questioned from a conservation point of view. In this review we discuss plant diversity patterns in the Scandinavian mountains in a global regional and local perspective. The main objective was to identify processes that influence diversity at different spatial scales with a particular focus on grazing. In a global perspective the species pool of the Scandinavian mountains is limited, partly reflecting the general latitudinal decline of species but also historical and ecological factors operating after the latest glaciation. At the local scale, both productivity and disturbance are primary factors structuring diversity, but abiotic factors such as soil pH, snow distribution and temperature are also important. Although evidence is scarce, grazing favours local species richness in productive habitats, whereas species richness decreases with grazing when productivity is low. Regional patterns of plant diversity is set by, 1) the species pool, 2) the heterogeneity and fragmentation of communities, and 3) local diversity of each plant community. We suggest that local shifts in community composition depend both on the local grazing frequency and the return-time of the plant community after a grazing session. In addition, an increasing number of grazing-modified local patches homogenises the vegetation and is likely to reduce the regional plant diversity. The time scale of local shifts in community composition depends on plant colonisation and persistence, From a mechanistic point of view, diversity patterns at a regional scale also depend on the regional dynamics of single species. Colonisation is usually a slow and irregular process in alpine environments, whereas the capacity for extended local persistence is generally high. Although the poor knowledge of plant regional dynamics restricts our understanding of how grazing influences plant diversity, we conclude that grazing is a key process for maintaining biodiversity in the Scandinavian mountains.