Clear-cutting,a management practice applied to many beech forests in the North of Spain,modifies microclimate and,consequently,the composition of the understory plant community in the disturbed areas.The objectives of this study were to asses8 if changes in the understory vegetation caused by altered light microclimate after clear-cutting affect the infectivity of arbusculax mycorrhizal fungi(AMF)on herbaceous plant species in beech(Fagus sylvatica L.)forests naturally regenerating from clear-cutting and to test if the use of bioassays for studying the infectivity of native AMF could provide useful information to improve the management of clear-cut areas.Three nearby beech forests in northwest Navarra,Spain,a region in the northwest part of the Pyrenees,were selected:an unmanaged forest,a forest clear-cut in 1996,and another forest clear-cut in 2001.High stem density in the forest clear-cut in 1996(44 000 trees ha-1)attenuated photosynthetic active radiation(PAR)and impaired the growth of herbaceous species within the ecosystem.The percentage of AMF colonization of plants in bioassays performed on soil samples collected from the forest clear-cut in 1996 was always lower than 10%.In the forest clear-cut in 2001,where soil was covered by perennial grasses,PAR was high and the infectivity of native AMF achieved minimum values in spring and autumn and a maximum value in summer.In contrast,the infectivity of native AMF in the unmanaged forest remained similar across the seasons.Our results demonstrated that changes in the composition of understory vegetation within beech forests strongly affected the infectivity of native AMF in clear-cut areas and suggested that the assessment of the infectivity of native AMF through bioassays could provide helpful information for planning either the removal of overstory when the tree density is so high that it impairs the correct development of herbaceous species or the plantation of new seedlings when high light intensity negatively affects the establishment of shade species.