EFFECT OF STRUCTURAL CHARACTERISTICS OF S UBSOIL ON THE FRUITING OF TUBER MELANOSPORUM AND OTHER MYCORRHIZAL FUNGI

The collection sites for ascocarps of Tuber melanosporum Vitt were not ed over four consecutive years in a truffle bed of live oaks (Quercus ilex) mycorrhized by T melanosporum. The mycorrhizas of Tuber and othe r fungi were checked I I years after planting. At the same time, a det ailed soil survey made it possible to distinguish different soil units within the truffle bed. Cartographic analysis of the data set showed that the mycorrhizal status and the location of the fungus collection sites were related to the structural characteristics of the subsoil. T he presence of a slightly permeable clay subsoil discourages fruiting in both epigeal and hypogeal fungi. Soils presenting a shallow calcare ous crust discourage the growth of hypogeal Tuber-type fungi, but enco urage that of epigeal fungi. T melanosporum grows best in soils with a very porous horizon developed in contact with the calcareous bedrock. This horizon (termed B-bio by the authors) is characterized by an int ensive biological activity.