Pulse grazing by reindeer ( Rangifer tarandus ) increases the phylogenetic diversity of vascular plant communities in the Fennoscandian tundra

Herbivore grazing is an important determinant of plant community assemblages. We used data from a multi-year experimental study in Northern Fennoscandia to analyze the effect of reindeer ( Rangifer tarandus ) grazing on plant community diversity including its phylogenetic structure. Similar to previous studies on low productivity ecosystems in this region, we found no effect of grazing on plant biodiversity and a transition from dwarf shrubs to graminoids with pulse (i.e. grazed every other year) and press (i.e. grazed every year) grazing. Interestingly, communities with pulse grazing were more phylogenetically dispersed than communities with press grazing and lightly grazed communities. We argue that the indirect effects of reindeer grazing (e.g. interspecific competition and soil nutrient availability) may work in opposition of the direct effects (i.e. physical removal of species) and that the phylogenetic structure of the resulting community will depend upon the relative strengths of the direct and indirect effects. This interpretation suggests that the indirect effects of reindeer grazing in our study region are more important than the direct effects in driving community assembly for plant communities that are exposed to pulse grazing. Our results have important implications for the management of reindeer in Fennoscandia if pulses of grazing can increase the phylogenetic diversity of plant communities.