The effects of different management interventions on degraded rangelands in Iceland

Loss of vegetation and soil erosion are symptoms of widespread rangeland degradation across most of the Icelandic highlands. Areas at different stages of degradation coexist as a mosaic that includes both vegetated heathlands, and exposed gravelly deserts. Revegetation efforts have included fertilizer applications and grazing exclusion to increase plant biomass and reduce bare ground, but their effectiveness is predicted to differ depending on the stage of degradation for a certain area. In this study, we used a 4-year field experiment to test the predictions of a state-and-transition model for the Icelandic highlands. We measured the combined effects of grazing exclusion and factorial applications of nitrogen (N), phosphorus (P), and potassium (K) fertilizers, on plant biomass, species richness, amount of exposed bare ground and plant community composition in a dwarf-shrub heathland and a gravelly desert habitat. After 4 years: (1) grazing exclusion alone had no effect in either habitat; (2) fertilizers increased biomass in both habitats, especially in plots treated with NP or NK; (3) the combination of fertilizers and grazing exclusion produced the greatest amount of aboveground biomass, predominantly of forb and graminoid species. In the dwarf-shrub heath, the increase in biomass in fertilized and fenced plots also corresponded to a loss in species richness, whereas in the gravelly desert, increased biomass reduced the amount of bare ground without reducing species richness. Our results reinforce the importance in understanding the effects of different management interventions across ecological conditions to determine the most effective revegetation approach.