Étude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F1 d'un moder sous Pinus sylvestris
Résumé
This paper is the third in a series, the aim of which is to demonstrate the value of micromorphological investigations in soil ecological studies. After microstratification of a soil sample from a Scots pine stand (Forêt d'Orléans, Loiret, France), the F1 layer was studied using the following procedures: 1. Sorting the different components under adissecting microscope, 2. Counting and measuring soil animaIs, observing their digestive tract by clearing or after dissection, and by mounting their faeces, 3. Thin-sectioning different types of vegetation,observing these sections under a light microscope with an appropriate stain procedure. The principal results from this study concern the de composition of pine needles, twigs and bark, ecto-mycorrhizae and their associate mycelia (Cenococcum geophilum and several basidiomycetes), decomposition of mosses (Pseudoscleropodium pururn), bracken leaves and animal bodies in the F1 layer. All the observed needles are comminuted by members of the soil fauna, with different modalities according to their alimentary behaviour. Macrofaunal species (primarily the epigeic lumbricid worm Dendrobaena octaedra) break the needles and ingest entire pieces of vegetation. They make holes which allow both the microfauna (amoebae, nematoda and also some testacea) and mycorrhizal fungi to enter. The same phenomenon can be observed in moss stems and bracken petioles. Lumbricidae are able to crush the pine material and to mix it with other organic elements. Unlike them, woodlice and slugs do not crush the needles, which can be found without any structural change in their faeces,only protoplasm appearing to be digested. Macrofaunal faeces do not seem to promote any bacterial development except at their periphery, unless the excrements are squashed by animal movements, in which case they become more suitable for bacterial contamination. Among the mesofaunal groups, oribatid mites (mainly phthiracarids and euphthiracarids), enchytraeid worms and diptera larvae (sciarids) are very active in tunnelling the needles. Phthiracarid and euphthiracarid mites are specialized feeders on this type of food, while enchytraeids and also, but to a lesser extent, sciarid larvae are more omnivorous. Other oribatid species consume fungal mycelium almost exclusively. Among the Collembola (springtails), the Isotomidae, represented by the three ubiquitous species Parisotoma notabilis, Folsomia manolachei and Isomiella minor, seem to consume only excremental material, while two strongly acidophilic species, Pseudosinella terricola and Willemia anophthalma, consume only fungal hyphae. Cell wall material is probably hydrolyzed, at least partly, by some mesofaunal groups but a distrinction must be made between the three categories: hyaline fungal walls, melanized (dematiaceous) fungal walls, and higher plant cell walls. Hyaline fungal walls seem to be digested by aIl groups except sciarid larvae. Melanized walls are transformed (and perhaps digested to some extent?) only by oribatid mites (mostly camisiid mites), while higher plant cell walls seem to be (at least partly) digested only by sciarid larvae, phthiracarid and euphthiracarid mites, with a distinct browning during passage through the post-colon in the latter two groups. Early wood in the pine twigs is attacked by soft-rot fungi which develop their typical cavitieswithin the S2 layer of tracheid walls. Similar decay pathways were also recorded in pieces of pine bark, moss leaves and bracken petioles.The decomposition of fine roots was also studied and the role of mycorrhizal fungi as prime root invaders at, or prior to root death, is discussed. Generally speaking there seems to be a regression in the development of bacteria (in comparison with the L1 and L2 layers), while mycorrhizal associated mycelia grow not only at the periphery of decaying fragments (as in the L layers), but also through the vegetation tissues.
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