Nicolas Bernier

Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest

Abstract 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.

The Forest Regeneration Puzzle Biological mechanisms in humus layer and forest vegetation dynamics

Forest ecosystem dynamics have long been explained by changes in floristic composition (Foster 1988) and nutrient availability (Vitousek 1984). The passage from pioneer to late-successional plant communities has frequently been modeled (McCook 1994) and, in a few instances, has been studied experimentally (Facelli and Facelli 1993). Interactions between plants (including their immediate environment as a passive partner) have generally been considered to be responsible for forest succession. However, the belowground components of any given ecosystem must also be taken into account before successional and steady-state patterns in plant communities can be fully understood (Cromack 1981, Miles 1985, Nilsson 1994). For example, managed coniferous forests

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

Abstract 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.) Karst.] in association with bilberry ( Vaccinium myrtillus L.), located at 1630 and 1880 m altitude (Mâcot-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 A1 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 A1 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.

La régénération naturelle : connaissances actuelles. le cas de l'épicéa en forêt de macot (Savoie)

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. La régénération naturelle, connaissances actuelles: le cas de l’épicéa en forêt de Macot (Savoie) Jean-François Ponge, Jean André, Nicolas Bernier, Christiane Gallet

Lumbricus terrestris L. distribution within an experimental humus mosaic in a mountain spruce forest

Abstract An experiment was designed at a mountain site to study the distribution of adult Lumbricus terrestris in relation to a small-scale mosaic of humus forms representative of different stages of a spruce forest ecosystem. Good agreement was found between distribution in the mosaic and that in the field. ANOVA tests demonstrated the strong influence of humus form on earthworm abundance when comparing a vermimull (high earthworm burrowing activity) taken from a spruce regeneration site (61.8 individuals m–2) with a leptomoder (no earthworm burrowing activity) taken from a 60-year-old spruce stand (6.2 individuals m–2). Other humus forms were intermediate (mean density 34.6 individuals m–2). The same pattern was found with individual biomass, but with lower significance. Main differences observed in the experimental design were attributed to the immediate carrying density of the humus forms. A distinction was made between humus profiles built up with or without spruce cover. In the latter case (regeneration site and bilberry heath), the immediate carrying capacity indicated by the experimental approach overestimated the field density by a factor of 4. Under spruce this overestimate was even higher (approximately 10 times too high in an adult spruce stand (160 years old) and 30 times too high under moss cover). The increase in density due to experimental conditions was not determined for leptomoder humus accumulated under the actively growing spruce stand (60 years old) since the earthworm density was near zero in both cases. Relationships between humus form and earthworm populations are discussed.