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

Klopsch, C. and Thorhallsdottir, A.G. and van der Wal, R. and Bardgett, R.D. and Thorsteinsson, B. and Geirsdottir, A. (2026) Long-term cessation of grazing reduces net carbon uptake in northern grassland and heathland. Agriculture, Ecosystems and Environment, 407: 110441. ISSN 0167-8809

Abstract

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 CO2 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 CO2 fluxes, with overall 37% less net carbon uptake (NEE600) 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, NEE600 was 72% lower in the exclosures. The greatest reductions in NEE600 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.