Modelling fine root biomass of boreal tree stands using site and stand variables

•Fine roots are different on drained peatlands compared to those on upland soils.•Nutrient poor soils in north have 5 times more fine roots per basal area than fertile soils in south.•C:N ratio of organic layer is a much better predictor for fine root mass on upland soils than drained peatlands. Quantification of fine root biomass is needed for estimating the role of roots as carbon (C) stores and sources of C input into the soil, as well as for modelling of forest nutrient cycling. Due to the laborious nature of root biomass determinations, there is a need to develop indirect methods that would allow fine root biomass to be estimated using data on easily measurable stand and site variables. We developed models for estimating tree fine root (diameter⩽2mm) biomass of boreal forests by compiling data from 95 Finnish forest stands (55 on upland soils and 40 on drained peatlands). Stand basal area predicted fine root biomass (R2-adj.=0.35, p