Comparative study of soil organic layers in two bilberry-spruce forest stands (Vaccinio-Piceetea): relation to forest dynamics

In: Geodema, 1993, 59 (1-4), pp.89-108. Morphological features of twelve humus profiles demonstrating the diversity of vegetation types present in subalpine forests were compared, together with soil fauna. Two forest stands of spruce [Picea abies (L,.) Karat.] in association with bilberry (Vaccinium myrtillus L,.), located at 1630 and 1880 m altitude (Macot-La Plagne, Tarentaise valley, Savoy, France), were studied. Morphological observations of small soil volumes were made on disturbed samples by the method of Ponge, but here transformed into quantitative data. Analysis of the data gave evidence of a large degree of heterogeneity of the form of humus in a given forest stand, but most of the observed variation might be explained by differences in vegetation, due to phases of the forest cycle and sylvicultural practices. Regeneration sites are characterized by the development of a herbaceous cover under which a mull humus is built through the activity of burrowing earthworm species. During the phase of intense growth of spruce organic matter accumulates in the top few centimeters. At this stage, the Al horizon previously formed under the action of endogeous earthworms becomes inactive but its crumby structure remains stable. Anecic worms (earthworms with a high amplitude of vertical movements) appear again under adult trees. As a result, changes in the form of humus are observed, with seemingly mull formation (burying of litter) but without true incorporation of organic matter to mineral matter. This humus was named dysmull. Thus soil conditions that prevail in the regeneration sites were developed to some extent under pre-existing adult trees. A parallel evolution of soil fauna, form. of humus and vegetation may thus be described. The above mentioned sequence is not the only one possible. When small gaps in the canopy are created by unsuitable sylvicultural practices such as selection thinning, the development of ericaceous species (bilberry at the higher montane and subalpine levels) may impede the natural forest cycle. Under bilberry, dramatic changes in the form of humus occur: disappearance of the Al horizon previously formed under spruce, accumulation of undecomposed organic matter and, at the subalpine level, podzolization. Accumulation of organic matter under bilberry is mainly due to mosses.