The humus forms during the life cycle of a spruce forest are described. A significant change in humus form may be attributed to plant and soil fauna changes. This phenomenon is considered to be fundamental for the renewal of the forest ecosystem. Forest dynamics is perceived as a biphasic cycle, (i) the tree growth phase with a shift from mull towards moder humus form, as a consequence of a decline in earthworm populations and (ii) a humus form improvement from moder towards earthworm mull humus, during the second half of the life of spruce trees. This results from a succession of earthworm species. The particular role of anecic species during the second phase has been highlighted, where they allow endogeic earthworm species and young spruce seedlings to install themselves in the regeneration site, the fall of parent trees not being considered as the chief factor governing humus changes. The life cycle of the spruce ecosystem can nevertheless be impaired by the development of a bilberry heath, with a mor humus form which is detrimental to the germination and growth of spruce seedlings. Earthworm populations of anecic and endogeic species are present in this case but without any burrowing activity.
