Comparative Rates of Pollination and Fruit Set in Widely Separated Populations of a Rare Orchid (Cypripedium fasciculatum)

Abstract

The rare orchid Cypripedium fasciculatum offers no reward to the small wasps that pollinate its flowers. Nonrewarding species typically suffer from low rates of pollination and fruit set, yet they appear frequently in the Orchidaceae. In order to understand the dynamics of pollination and fruit set in nonrewarding orchids and to evaluate whether a conservation strategy for C. fasciculatum should include managing for pollinator service, we evaluated factors influencing reproductive success in three widely separated populations. The percentage of open‐pollinated flowers maturing fruit varied greatly among populations, equaling 69% in Oregon, 29% in Idaho, and 18% in Colorado. These values are greater than is typical for a nonrewarding orchid. Thus, managing for pollinator service may be unnecessary, especially in the Oregon population. Like many nonrewarding orchids, the species appears to have a self‐compatible breeding system; fruit set did not differ between flowers receiving self pollen and flowers receiving cross pollen ($$Z=0.81$$, $$P> 0.1$$). Within‐season fruit set may be pollen limited, however, since supplemental hand pollination with cross pollen significantly increased fruit production by ca. 4.5 times in Colorado and 1.5 times in Idaho in comparison with open pollination. Even so, many pollinated flowers failed to mature fruit. Pollen tube growth was studied using fluorescence microscopy, and the percentage of pistils with germinated pollen at the end of the flowering season was found to exceed significantly final fruit set by a rate of approximately two in both Idaho and Colorado.