Alnus nepalensis is a medium-sized, deciduous tree that occurs in the Indian sub-continent, South America, Hawaii, and China. It is a prolific pioneer species in freshly exposed soil in landslide areas of the western Himalayas and has the potential of fixing nitrogen. A study was conducted to assess the reproductive phenology, pollen production, pollen release, and pollen-mediated gene flow of Alnus nepalensis by considering a patch of trees as a pollen source in the temperate forest of Garhwal Himalaya to develop sustainable management strategies relating to the plantation geometry in seed orchards. Staminate flowers of A. nepalensis are composed of “cymules”. The presence of bifid stigma and protandry condition were the unique features of the species. Flowering in the male phase was initiated in the last week of September and continued till November. Peak shedding of pollen generally proceeds peak receptivity by 1–2 weeks. The time between onset and peak flowering was 2 weeks 4 days and the total average duration of the flowering period was about 24.8 days. Temperature and relative humidity played a major role in pollen release and the maximum pollen release occurred at 29.2 °C at 13.00 hrs of the day. Pollen production per catkin varied significantly among trees. The average pollen grains per tree were 2.20 × 1010. The pollen-ovule ratio suggests that the breeding system of A. nepalensis falls under the class xenogamy. Pollen mediated gene flow revealed that the significant pollen which can cause pollination of A. nepalensis can travel up to 40 m uphill and 80 m in downhill directions. Thus, an isolation strip of 80 m is sufficient to manage the seed orchard of A. nepalensis in the western Himalayan region.
