Understanding the constraints on sexual reproduction of a plant species are important for predicting stability of the species in its natural habitat. In the absence of such data, any conservation efforts are arbitrary and remain ineffective. We investigated the reproductive ecology and breeding system of a rare and threatened tree species of central and western Himalaya, Ulmus wallichiana. The phenology, floral biology and pollination mechanism were observed in two populations for three consecutive years. In addition, breeding system and pollen limitation were examined using hand-pollination experiments. Flowering in U. wallichiana begins in early March during which trees are leafless and small, pinkish-white flowers cover the entire tree. Pollination is accomplished exclusively by wind with short average distances for pollen dispersal (~50 m). Pollination manipulation experiments showed high fruit set on outcrossing indicating U. wallichiana is xenogamous. However, a small percent of fruit set in autogamous and geitonogamous treatments indicate partial self-incompatibility. Fruit set augmented when pollen was supplemented indicating that natural fruit set in U. wallichiana is limited due to unavailability of adequate pollen. Thus, it can be concluded that reproductive success of U. wallichiana is compromised due to scarcity of conspecifics and partial SI may partly ensure reproductive assurance in the low pollen exchange environment. Present study emphasizes on establishment of minimum population size with optimum distance between trees for effective conservation interventions.