Aims We investigated whether tree species growing in mixtures and under different water supply would segregate their fine roots vertically, produce more fine roots overall, or only in specific soil layers. Methods We examined the biomass, morphology, and distribution of fine roots down to 90 cm (forest floor, 0-5, 5-15, 15-30, 30-60, 60-90 cm) in pure and mixed stands of 10-year-old birch and pine trees, planted on a sandy podzol with discontinuous hardpan and seasonal high water table, following a randomized block design with four blocks receiving irrigation and four blocks left unirrigated during summer. Results Our results did not show any vertical root segregation between birch and pine in mixed plots. None of the species overyielded belowground throughout, but pine developed more roots in the top soil layer under irrigation. Both species had shallower fine root distributions in wet conditions, especially birch that was more plastic than pine in response to irrigation. Conclusions Both species followed similar ecological strategies, occupying and competing for the same layers of the soil profile, under both control and irrigated conditions. However, the greater allocation of pine roots at the top soil horizons under irrigated conditions suggests locally favourable niches can lead to depth-specific asymmetric competition. This sheds new light on vertical niche partitioning of young tree mixtures under varying environmental conditions.