The aim of this study was to investigate whether genetically different pollen donors (Betula pendula clones)
differed in pollen-tube growth rate across 11 maternal plants and in vitro, and whether the differences between the
donors were consistent across the recipients. To compare the seed-siring success of competing pollen donors, a
two-donor hand-pollination experiment with six donors and six recipients was conducted. The experiments were
performed at a plastic-house seed orchard. The donors showed significant variation in pollen-tube growth rate on
all the 11 recipients. The rankings of the pollen donors were statistically consistent across different maternal
plants. A significant positive correlation between pollen tube growth in vivo and in vitro was found. The seed-siring success of two competing pollen donors was unequal in 20 of 29 cases and there was a significant positive
correlation between seed-siring success and pollen-tube growth rate in vivo and in vitro. The results show that
fertilizations are not random and pollen competition operates in a B. pendula seed orchard population.