n. Lower duff is composed of highly decomposed organic material with little or no recognizable plant structure and is equivalent to the humus layer (H-layer) or Oa organic soil horizon.
Lower duff is also called the Oa soil horizon and the humus layer (or H-layer). The classification of the duff layer into upper duff and lower duff has not yet been standardized, and a number of criteria have been used to define the boundary between the two layers. Brown and others (1991) stratified the duff layer into shallow duff and lower duff, which include deep pockets of rotten wood.
Jandt and others (2005) stratified duff from boreal forest sites into upper and lower layers based on physical characteristics. They characterized lower duff as a dark, humic layer of mostly decomposed organic material and some soil particles. The Fuel Characteristic Classification System (FCCS) defines lower duff as the humus layer, equivalent to the Oi organic soil horizon (Fire and Environmental Applications Team 2007).