Comment distinguer dysmoder et mor ? L'exemple de la forêt domaniale de perche-trappe (orne)

Abstract

Difficulties encountered in the field in distinguishing mor from dysmoder humus are largely due to the inability to differentiate between the processes of organic accumulation arising from the excretory behaviour of small animals (deposition of faecal pellets on the OF horizons, that accumulate to form OH horizons) and those of gradual transformation of the litter without any significant contribution from the fauna (OM horizon). Observations of forms of humus present in the Perche-Trappe forest (Orne) have shed light on the distinction between these formations involving high levels of organic accumulation. Morphological criteria that can be observed in the field are correlated with the chemical and micro-morphological features of the surface horizons. Absence of fine organic material (micro-arthropod and enchytreid faecal pellets) in the fragmented litter and a sudden transition between the litter and the underlying mineral horizon are characteristic of the mor form of humus in contrast with the moder forms (including dysmoder), regardless of the thickness of the accumulated organic layer which cannot in itself constitute the distinguishing characteristic.