The species structure of an ectomycorrhizal (ECM) community was assessed monthly for 15 months in the two horizons (A1 and A2) of an oak temperate forest in northeastern France. Ectomycorrhizal species were identified each month by internal transcribed spacer sequencing. Seventy-five fungal symbionts were identified. The community was dominated by Tomentellaceae, Russulaceae, Cortinariaceae, and Boletales. Four species are abundant in the study site: Lactarius quietus, Tomentella sublilacina, Cenococcum geophilum, and Russula sp1. The relative abundance of each species varied depending on the soil horizon and over time. Some species, such as L. quietus, were present in the A1 and A2 horizons. C. geophilum was located particularly in the A2 horizon, whereas T. sublilacina was more abundant in A1. Some species, such as Clavulina sp., were detected in winter, while T. sublilacina and L. quietus were present all year long. Our results support the hypothesis that a rapid turnover of species composition of the ECM community occurs over the course of a month. The spatial and temporal unequal distribution of ECM species could be explained by their ecological preferences, driven by such factors as root longevity, competition for resources, and resistance to environmental variability.The fine roots of social tree species in temperate and boreal forests are symbiotically associated with fungi (52), forming composite organs called ectomycorrhizas (ECM). The ECM fungi play a crucial role in tree health by enhancing the nutrient acquisition, drought tolerance, and pathogen resistance of their hosts. ECMs efficiently take up water and organic and inorganic nutrients from the soil via the extramatricial mycelium and translocate these to colonized tree roots, receiving carbohydrates from the host in return (52). Most of the ectomycorrhizal roots are located in the top 20 centimeters of the soil, an area which is enriched in organic matter and where nutrients are concentrated (50). The ECM fungal community is species rich at the forest stand level, where hundreds of different fungal symbionts can be identified by morphotyping and DNA-based molecular methods (13,17,36,56).Beside its species composition, the structure is an important characteristic of the ECM community. Differences in ECM community structure on different scales are well documented: on the ecosystem scale (postdisturbance or postplanting successions) and along forest dynamics (4, 28, 56, 65), on the seasonal scale (6,8,20,33,54), and along spatial dimensions (vertical scale [13,14,24,49] and horizontal scale [36,58]). In a microsite or on a forest strand scale, species are distributed neither uniformly nor randomly but rather are aggregated in patches or distributed along gradients (9,12,19,24,44). The spatial heterogeneity of communities is important in terms of succession, adaptation, maintenance of species diversity, interspecific competition, and community stability (38). A spatial niche differentiation of ECM species and of ECM exploration types (1) could be du...