Soil cores taken from each subplot of each plot (i.e. 9 per plot), if possible down to 40 cm (Poland), but in other countries it was only possible to 20 cm (except Spain only 10 cm). Forest floor samples were taken separately from the soil cores. The cores were dividied into layers of 10 cm, and respective layers of each plot were pooled together, rendering one forest floor samples and 1 to 4 mineral samples per plot. Living fine roots (< 2 mm) were extrtacted from these samples, and classified as tree or understorey roots. After drying at 40 degrees the roots were weighed, and the biomass was calculated as g roots per m2, taken into account the stoniness of the mineral soil at each plot, as assessed by the metal rod penetration method (Tamminen & Starr 1994). Stoniness was calculated by determining the average penetration depth of a metal rod into the mineral soil (max 30 cm), estimating with the equation presented by Tamminen & Starr (1994): Buld density of forested mineral soils. Silva Fennica 28: 53-60.

Samples taken between May 2012 and October 2012