De l'influence du type d'humus sur le développement des plantules de sapins dans les Vosges

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anglais. Duchaufour has noted the relation between forest humus typesand natural regeneration of Fir in the Vosges : on the biologicallyactive humus of the Mull acide type, which maintain a high levelof fertility for the growth of adult trees, barely any seedlings areencountered, although they abund on the slowly decomposing humusof the Model,- or Mor types. Investigation of this phenomenonemphasized the decisive rôle played by the moss carpet which, byprotecting Moder and Mor against intense evaporation, maintainsthe humidity required by the growing seedlings ; on the contrary,Hull humus are either bare or covered with a herbaceous florawhich dries up the zone occupied by the roots of the young seedlings.Investigation pursued at the Soils Laboratory of the Ecole Nationaledes Eaux et Forêts, has shown that the Mull of the Vosgesforests already highly permeable to water because of its coarsestructure, may loose through drought and changes in the state ofits colloidal components, the capacity to absorb summer rainfallwhich then infiltrates at a rate four-times that prevailing in thespring. The soil colloids seem to recuperate slowly during autumnor winter their initial water-holding capacity.However, the few Fir seedlings that were found in the Hullhumus possessed peculiar characteristics of atrophy and necrosisin their root system. As these symptoms seemed evidence for toxicityof some nature, it appeared logical to investigate the activityof antiphytotic substances.Laboratory cultures of seedlings in the various humus types, wateredwith aqueous extracts of these same humus, did not revealany toxic activity of the Mull as long as the humidity of the culturemedia remained high ; but upon lowering the moisture contentof the humus, the necrosis of the roots already noted in the fieldstarted to appear and was followed by the death of the seedlings.The hypothesis of intoxication by water-soluble antiphytotic substanceshad therefore to be abandoned, since their activity should undoubtedlyhave increased as a consequence of the higher concentrationsarising from the extract-waterings.Chemical investigation of the soil revealed that a very largeamount of manganese, — particularly in its di-valent form availahle to plants, — was present in the toxic humus layer, even whenthis element was only found as traces in the sub-soil. The enrichmentis brought about by the adult trees which concentrate in theirfoliage the Manganese obtained by the deeper roots from the soil.The quantity of manganese in the various humus types, whenexpressed on a weight basis is already noticeably higher in Mullthan in Moder and Mor. This difference is in fact much more importantif it is considered that Moder and Mor have an apparentdensity 4 to 5 times lower than Therefore, and takinginto account the fact that seedling roots occupy a given volume ofsoil, exchangeable manganese :vas expressed as milliequivalents per100 cubic centimeters.EXCHANGEABLE MANGANESE (in me/100 cc)Mull ModerSurface horizon 0.14 - 1,60 Traces - 0.06Sub-soil 0.01 - 0.13 Traces - 0.01This accumulation of manganese in Hull is attributed to theintense biological activity cf this type of humus, characterized by arapid mineralization of the litter, and also to seasonal drying whichincreases the plant-available reduced manganese at the expense ofthe higher hydrated oxides.Sand cultures were set up to verify the action of increasingamounts of manganese, toxicity of the M++ icn appearing in Firat a concentration of 5.10 -6. Mortality is slow and progressive atlow concentrations, but very rapid above 50.10 -6: necrosis of theroots becomes evident and mortality is accelerated when the humidityof the culture media is lowered.The conclusion is therefore justified that disappearance of Firseedlings in the Mull humus types of the Vosges, is caused biologicallyby accumulation in the humus of exchangeable manganese,the toxic activity of which is considerably increased during periodsof drought.