Site description
We selected 12 montane grassland sites across an 800 km north-south gradient of the United Kingdom (Figure 1). Sites were selected based on the following criteria: 1) grasslands had never received inorganic fertilisers or herbicides; 2) grazer exclusion plots had to be present for at least 10 years; 3) sites needed to be sufficiently far apart (>10km) to be independent from each other; 4) the main form of historical management at the site is extensive grazing by sheep. Sites were typically grazed by pure bred sheep at stocking densities of 1-2 ewes per hectare per year, although historical variation in grazing pressure across sites has resulted in mosaics of vegetation with patches of short and tall grass, interspersed with patches of dwarf-shrubs dominated by Calluna vulgaris , Vaccinium myrtillus ,Erica tetralix and Erica cinerea . All sites were visited and sampled once, between 28th of April and 7th June 2015. Fenced grazing exclosures varied in size from 25 m2 – 10.66 ha and in age since cessation of grazing from 10 to 65 years (Table 1, Table S1 and Figure 2). At the sites, we sampled 4 paired 5x5m plots where extensive grazing had been excluded by fencing and 4 adjacent grazed plots (Figure 2). At one location, Exmoor (site 11, Figure 2), we sampled three paired sites, and in the Peak District (site 9, Figure 2) we sampled six sites; therefore, the total number of plots was 98, half grazed and half ungrazed, from 12 distinct sites. The elevation from the sites varied between 300m and 700m asl (Table 1), and differed in underlying geology, climatic conditions, soil characteristics and dominant plant species (Table S1-4). In each plot we assessed the vegetation composition and biomass (see supplementary information for details). Soil samples were collected to determine soil abiotic properties (bulk density, water content, carbon (C), nitrogen (N), phosphorus (P) content, pH and the potential nitrogen-mineralization; for further details, see supplementary methods S6.