Reindeer grazing counterbalances the treeline expansion in the Scandinavian subarctic

Herbivory may offset climate change driven treeline expansion into the tundra. This study quantifies the effects of reindeer grazing on mountain birch recruitment and growth in the treeline ecotone in the Scandinavian sub-arctic in an area with contrasting grazing regimes for the past 20 years. We measured seedling density and the allometry of trees below, at, and above the treeline as well as vegetation composition along 20 transects crossing the treeline. Additionally, we investigated nutrient loading of soils and its effects on adult tree growth rate. Our results show that the treeline in the area grazed in winter may be responding to climate forcing by expanding diffusely into the tundra, while no treeline expansion was observed under the year-round grazing regime. High grazing pressure also reduced the numbers of tree basal shoots and the number of leaves below reindeer browsing height (<2 m). Additionally, we found a shift in ground layer vegetation composition in the area grazed year-round. Our results suggest that reindeer grazing at high density and when occurring during the growing season has the potential to stabilize the treeline locally, as well as significantly modify field layer vegetation composition in the treeline ecotone.