Data collection
In mid-June 2008, after two years of treatment, aboveground plant biomass was clipped at soil level in the inner 30 x 30 cm of each subplot and sorted to species. To estimate daily RGR of each species in monoculture, aboveground plant biomass was clipped at soil level within 10 x 10 cm quadrats in the outer 10 cm surrounding the inner 30 x 30 cm of each subplot during sequential harvests on days 53, 67, 88, 109, 116, 123, 130, 145, 152, 162, and 171 in the year 2008. Day 171 was the peak standing biomass. Each time different randomly chosen quadrats were clipped (Fig. S1). Harvested biomass samples were dried at 80°C and weighed. Soil cores were collected regularly during the growth season in 2008 and analysed for nitrate and ammonium concentrations (Labor für Boden- und Umweltanalytik, Thun, Switzerland). We measured plot level light interception ability in monoculture for each species and each nutrient treatment before the harvest in end-April 2008 as the percentage of transmitted photosynthetically active radiation (PAR) reaching the soil surface.