This field and microscope study explores the micromorphological changes occurring in humus profiles of the Canadian boreal forest which have been variously affected by factors of disturbance: fire, dehydration and overland flow after storms. It compares the materials and profiles of undisturbed sites to those of recently disturbed and recolonized sites. A wide range of changes was observed: cracking and abrasion of bark and roots, distortion and compression of softer tissues, accompanied by biotic consumption. Hyphae are the initiators of decay at points of breakage. Litter input in recent burn sites is frequently from recolonizing vegetation rather than tree litter. Most burned areas are coincident with a lesser degree of podzolization than exists in unburned areas. Compaction and intense pelletization are frequent in burned profiles and the term 'pyroturbation' is proposed for the disruptive effects of fire on the humus profile and the subjacent mineral layers. Concepts of the balance of litter supply and biotic consumption are advanced, as well as regressive and progressive development of the boreal humus profile.