The results of a long-term study over 32 years in the fungus reserve La Chanéaz reveal striking changes in the species composition of the macromycetes: the mycorrhizal species considerably decreased in relation to the other species. This development is worrying, taking into account the crucial role these fungi play in forest tree health by enhancing nutrient acquisition, drought tolerance and pathogen resistance of their hosts. We search for causes of this development and raise the question about possible effects of a decrease of the mycorrhizal fungi in terms of tree health of the associated forest trees.