Heterogeneity in soil animal communities and the development of humus forms.

Two examples are given of strong relationships between the activity of soil animals and the development of humus forms in a mountain spruce forest and a lowland Scots pine stand. Both were based on field distributions of fauna and humus components. The vertical heterogeneity of soil animal communities in a moder humus profile under Scots pine is related to successional processes occurring during litter decomposition. Accumulation of faecal pellets by litter-dwelling animals influences in turn the development of the root system of pine and associated ectomycorrhizal fungi. The heterogeneity of humus forms in a mountain spruce forest is related to the developmental phases of spruce stands. Typical earthworm mull humus forms are restricted to regeneration sites with a low herbaceous vegetation. Typical moder humus forms with a thick OH horizon (dysmoder) are restricted to the pole phase of spruce stands. Between these two stages of the forest successional cycle, intermediary humus forms are found. They correspond to changes in soil animal communities, mostly burrowing earthworms, occurring together with changes in vegetation. These observations are discussed in terms of feed-back loops involving humus forms, soil animal and plant communities.