Site characteristics
For full site characteristics, see Table S1. The ground vegetation was dominated by typical Subarctic dry heath dwarf shrubs, particularlyEmpetrum hermaphroditum, Vaccinium myrtillus and Vaccinium vitis-idea , with higher coverage of graminoids and forbs at the relatively wetter and more fertile sites and mosses and (rarely) lichens at the low fertility end (Table S1). The mean annual soil temperature (averaged from summer 2016 to summer 2017) across all sites ranged between 1.7±0.06 and 3.5±0.1 °C, while the mean annual growing season (averaged from 15/6 to 15/9 2017) soil temperature ranged between 6.7±0.1 and 11.1±0.2 °C. These figures are slightly lower but comparable to the coarse scaled (4x4 km) modelled Luftweb record for the mean growing season air temperature per transect for the period 2000-2014 (mean: 10.7±0.2 °C, Table S1), so we believe the variance in mean growing season soil temperature reflects the site air temperature variance relatively well. Early growing season volumetric soil moisture content ranged from 11±1.1 to 38±2.5 % across sites (Table S1). All sites were characterised by relatively acid (mean pH: 4.5±0.06) and nutrient poor soils. Dissolved inorganic N-content was low, and at the same level as dissolved P, while dissolved base cations (calcium, magnesium, sodium and potassium) were an order of magnitude more abundant than N and P (Table S1), suggesting no scarcity of other macronutrients than N.