We don't have synonyms for forest floor yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are first floor.
(N) :
The forest floor, also called detritus or duff, is the part of a forest ecosystem that mediates between the living, aboveground portion of the forest and the mineral soil, principally composed of dead and decaying plant matter such as rotting wood and shed leaves.
(N) :
The forest floor, also called detritus or duff, is the part of a forest ecosystem that mediates between the living, aboveground portion of the forest and the mineral soil, principally composed of dead and decaying plant matter such as rotting wood and shed leaves.