Abstract <p>Changes in the functional structure of communities under the influence of alien plants are one of the important but insufficiently studied consequences of plant invasions. We tested the hypothesis about the change in the probability of mycorrhiza formation in plants of the grass-shrub layer under the influence of the invasion of alien box elder (<i>Acer negundo</i> L.): the participation of mycorrhizal and non-mycorrhizal herbs in communities changes equally or differently depending on the degree of dominance of <i>A. negundo</i>. The results of 118 geobotanical relevés carried out in the Belarusian Polesia, the Middle Volga region and the Middle Urals were analyzed. In each region, communities with dominance of <i>A. negundo</i> and without <i>A. negundo</i> were described. The ability of plant species to form mycorrhiza was defined as a continuously varying value of the probability of the formation of mycorrhiza in a species based on the information presented in the FungalRoot Database. Accordingly, mycorrhiza formation was analyzed as a functional trait in the sense used in the analysis of the functional diversity of communities. It was found that the community weighted mean probability of mycorrhiza formation in communities with dominance of <i>A. negundo</i> is lower than in similar communities dominated by other tree species. This indicates a simplification of the community structure and a slowdown in their successional development under the influence of alien box elder. The results confirm that the analysis of the functional structure of communities can be a useful tool for improving the understanding of the mechanisms and conditions that facilitate the occurrence of invasions.</p>