Soil animals (macro-and microarthropods, annelids, nematodes) were sampled along an altitudinal gradient and over two years in holm oak (Quercus rotundifolia) forests of the Moroccan Atlas. We studied the influence of elevation and year on the vertical distribution of soil fauna. Whatever the elevation (1500, 1700 and 1900 m), the humus form was a Dysmull, with a thick litter horizon and a fine crumb A horizon. Thirty-six categories of fauna were found and classified at the group level. The influence of horizon, altitude and year was analysed by ANOVA (on seven broad zoological groups and on total fauna) and correspondence analysis (on thirty-six zoological groups). There was a decrease in the population size of most zoological groups from organic (OL, OF) to mineral horizons (A, S), but OL and OF horizons varied as the most populated horizon according to years and animal groups. More animals and more animal groups were present at higher elevation, following an increase in food and habitat availability.

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Summary Multivariate methods have been widely used for revealing the structures of communities, and in this paper we explore one particular method, namely correspondence analysis (also called reciprocal averaging), for studying humus profiles by the ‘method of small volumes’. The present study was done on humus profiles under holm oak (Quercus rotundifolia), an evergreen Mediterranean species, in the High Atlas of Morocco. Three sites (1500 m, 1700 m, 1900 m altitude) and 2 years (1999 and 2002) were compared. The humus form is Dysmull (mull with thick litter horizons), with variations in the thickness of the OL (entire leaves), OF (fragmented leaves with faecal pellets) and A (hemorganic) horizons according to altitude and year. The dead leaves are rapidly incorporated into holorganic (earthworm, insect) and hemorganic (enchytraeid) animal faeces, which form the bulk of the OF and A horizons. The S horizon (weathering parent rock) shows the greatest development of the root system. As altitude increases more fresh litter (OL) or more humified organic matter (OF, A) is accumulated. Variation from year to year is depicted by opposite differences in the amount of entire oak leaves and of dead roots. Humus components (classes) are used as active (main) variables, after standardization of their means and variances. The addition of numerous passive (additional) variables, standardized in the same way as active variables, enabled us to understand the influence of biological and climatic effects on the composition of humus profiles and soil trophic networks.