Many temperate and boreal tree species have small and scattered populations near the low-latitude margins of their range that have persisted over extended periods of time in microrefugia. It remains poorly understood how patterns of mating and gene flow influence key components for their long-term persistence such as effective population size or genetic diversity. Yet such information is critical for designing effective protection measures and informing programs for the conservation of forest genetic resources. Here we investigate the mating system and pollen dispersal of a long-term refugial population of beech (Fagus sylvatica), a major European forest tree. This population stretches over ca. 7 km along the ravines of a small lowland river in SW France. We exhaustively mapped the adult population (n = 932 trees) and collected seed families (n = 450 seeds) from 30 mother trees spread across the stand. All individuals were genotyped at 113 SNP markers and submitted to paternity analyses. We estimated various mating parameters and tested whether among-individual variation was related with the ecological neighbourhood of the mother tree. A unique father could be identified for all seeds analysed. We detected neither pollen immigration nor selfing events. Instead, we observed extensive mating between neighbouring trees that resulted in a very steep decline of the pollen dispersal kernel and a low effective number of fathers (median N-ep = 7.9). In turn, male fecundity was tightly related with the distance between mates. The adult population showed an exceptionally strong and far-reaching spatial genetic structure indicating that neighbouring trees are most often sibs. The predominant mating between these neighbours suggests that patterns of reproduction and pollen flow are not constrained by cross-compatibility issues. Instead, dense conspecific neighbourhoods even tend to bar long-distance pollen flow by saturating the stigmas of mother trees through pollen swamping. The observed patterns are a direct consequence of the refugial habitat that imposes the population's linear distribution and whose dense vegetation represents a further obstacle to pollen dispersal. The elevated capacity of sib-mating might be a microevolutionary consequence of the population's long-term persistence in isolation. The resulting spatial genetic structure and the critical role of tree density in assuring an effective reproduction have important consequences for the design of forest genetic resources programs as well as for the conservation management of refugial and other riparian tree populations.Read less <