Fine scale distribution of ectomycorrhizal fungi and roots across substrate layers including coarse woody debris in a mixed forest.

• Ectomycorrhizal (ECM) fungi are widespread plant root symbionts in boreal forests, but information is lacking on the fine scale distribution of roots and fungi in substrate patches of different quality, including coarse woody debris (CWD). • Wood and soil cores were taken systematically both horizontally and vertically through decayed logs and underlying soil layers in a mixed forest. Root tips were counted and ECM fungi identified by morphotyping and sequencing. • The abundance of root tips and ECM fungi was highly variable on a 5-cm scale. Most species were replaced on a 50-cm scale. Detrended correspondence analysis demonstrated strong preference of resupinate thelephoroid and athelioid fungi and Sebacinaceae for CWD, whereas ascomycetes and euagarics appeared more frequently in mineral soil. Clavulicium delectabile was determined to be an ECM fungus for the first time. • ECM fungi occupy different niches and show variable distribution patterns. CWD plays an important role as a habitat both for roots and ECM fungi. We suggest sampling larger soil cores and selecting random root tips in future studies. Sequencing is a powerful tool in ECM community studies.