The reproductive system of hybrids is an important factor shaping introgression dynamics within species complexes. We combined paternity and parentage analyses with previous species characterization by genetic assignment, to directly identify reproductive events that occurred within a stand comprising four European white oak species. Comparing species status of parent pairs provided a precise quantification of hybridization rate, backcrosses, and intraspecific matings in two life stages. The detailed mating system analysis revealed new findings on the dynamics of interspecific gene flow. First, hybrids acted successfully as both male and female during reproduction. They produced acorns and seedlings that were as viable as those sired by purebreds. Second, species maintenance could be due to a relatively low level of interspecific mating contrasting with a large proportion of intraspecific crosses and backcrosses. Despite a high proportion of hybrids and extensive interspecific gene flow, partial species integrity is maintained by genetically controlled pollen discrimination, ensuring preferential matings within purebreds and high parental species fidelity in hybrid reproduction, which impedes complete collapse into a continuous hybrid swarm. Finally, we showed that pollen from the different species had unequal contributions to reproduction suggesting that introgression processes could ultimately lead to extirpation or expansion of some species.