A current long-term study on fungal reserve in western Switzerland started in 1977 aims at determining the effects of various factors on the diversity and productivity of wild forest mushrooms. Here the effect of an increment thinning of an old-growth forest is described . Fruit bodies of all the epigeous macro-fungal species were identified and counted at weekly intervals between 1997 and 2006 in 5 experimental plots . In one of the plots an increment thinning was performed in early spring 1987 in order to lighten the dense old-growth forest and to favour the understory . The thinning induced a significant increase of fungal species richness , as well as of fruit body numbers, especially of the ectomycorrhizal species. Comparing the period before ( 1977 - 1986) with the period after the thinning (1987 - 2006) , seven times more species and 23 times more fruit bodies were counted on average per year . The mycorrhizal species produced muchmore, up to 9 times more species and 32 times more fruit bodies on average after the thinning. Obligatory beech-specific mycorrhizal fungi clearly increased their dominance after the thinning , their species number increasing from 36% to 54% and the fruit body number from 19% to 70% . These changes are discussed in relation to the possible growth reactions of the remaining host trees after the artificial intervention.