Humus profiles were sampled under the crown of a mature oak tree in a coppice with standards (Senart forest, 30 km south of Paris). The sampling design compared the composition of humus profiles at three distances of the trunk base (40, 140 and 240 cm) and in the four cardinal directions. An increase in the development of the OF layer (strongly decayed litter and faeces of epigeic fauna) was observed at 40 cm from the trunk base, paralleling an increase in soil titratable acidity. Since no significant change in litter composition occurred with distance to the trunk base and in the absence of stemflow reaching the ground during showers, diffusion of bark tannins from buried parts of the trunk and main lateral roots were suspected to negatively influence soil biological activity, particularly earthworm activity.