Effects of nutrient addition and sheep grazing on tundra rangelands in the Icelandic highlands

Overgrazing and soil erosion are widespread and chronic environmental problems in the Icelandic highlands. Restoration efforts have included the application of fertilisers and grazing exclusion to increase plant biomass and reduce bare ground, but the effects of fertilisers on plant community composition across ecological conditions remain unresolved. I measured the combined effects of grazing exclusion and fertilisation, including factorial applications of nitrogen (N), phosphorus (P) and potassium (K), in both vegetated and degraded areas at sites with contrasting soil conditions. After four years I found 1) above-ground biomass induced by fertiliser was counteracted by sheep grazing, and 2) joint application of fertiliser and grazing exclusion had contrasting effects on species diversity in vegetated habitats where NPK applications and grazing exclusion reduced species diversity, than in degraded habitats where NPK applications increased species diversity irrespective of grazing. These results contribute to improving restoration efforts in high latitude tundra rangelands.