Long-term cessation of grazing reduces net carbon uptake in northern grassland and heathland

In northern grassland and heathland, livestock grazing has historically been the main land use. Today, traditional grazing is diminishing and ceased in many places with so far unknown long-term effects for ecosystem carbon cycling due to the scarcity of long-term studies. We compared CO 2 fluxes in exclosures where grazing was ceased multiple decades ago (20–83 years) with adjacent grazed land across a broad range of semi-natural grassland and heathland in Iceland. All 32 study sites, both grazed plots and the exclosures, were net carbon sinks during the growing season. However, long-term cessation of grazing led to considerable changes in ecosystem CO 2 fluxes, with overall 37% less net carbon uptake (NEE 600 ) in the exclosures (‘Exclosure effect’). Grassland was a substantially larger carbon sink and had a stronger ‘Exclosure effect’ than heathland. Across all sites, calculated Light Use Efficiency and Biomass Use Efficiency were 22% and 66% lower, respectively, in the exclosures compared to grazed plots. At several grassland sites, cessation of grazing led to a shift in vegetation to heathland. At these sites, NEE 600 was 72% lower in the exclosures. The greatest reductions in NEE 600 occurred in the first 20–30 years following cessation of grazing, and further reductions with advancing age of the exclosures (> 50 years) were found when grassland shifted into heathland. Our results showed that long-term cessation of grazing results in a reduced carbon sink strength, highlighting the key role of grazing animals for vegetation dynamics and carbon cycling in grassland and heathland. • 32 contrasts of grazed vs ungrazed (20–83 yrs) sub-arctic grassland and heathland. • Net carbon uptake was 37% lower in long-term ungrazed vs grazed land. • Long-term grazing cessation modified CO 2 fluxes, vegetation and soil microclimate. • Long-term grazing cessation can cause a shift from grassland into heathland. • Grassland is a larger carbon sink than heathland in sub-arctic regions.