Pollination biology of Oreocereus celsianus (Cactaceae), a columnar cactus inhabiting the high subtropical Andes

The geographical dichotomy hypothesis suggests that columnar cacti in the tropics depend primarily on bats for pollination. This dependence may to be less in the outer tropics where many columnar cactus species (or their populations) show a relatively generalized pollination system with both nocturnal (moths and bats) and diurnal pollinators (bees and hummingbirds) (geographical dichotomy hypothesis). This hypothesis has been mostly tested in the northern tropics; nonetheless, our knowledge of columnar cactus species inhabiting the southern tropics is still scarce. The aim of this project was to evaluate the pollination biology of Oreocereus celsianus, a columnar cactus with restricted distribution in the subtropical Andes, to determine if the pollination system of this cactus tends to be more generalized than specialized because of the geographical position where it occurs. Observations of frequency of visit showed that Patagona gigas (Giant Hummingbird) is the main pollinator of the flowers, visiting them when they are opening (afternoon of the first day). Bees, wasps and moths were occasional visitors of the flowers. None of them seem to act as pollinator. Autogamy, geitonogamy and xenogamy treatments produced high fruit set, showing that O. celsianus has an unusual mixed mating system. The results suggest that this Andean columnar cactus is partially specialized on hummingbirds, with most pollination service performed by a single species, and it has the capacity of selfing (??fail-safe?? pollination system). This mixed mating system (both outcrossing and selfing) may be a response to the unpredictable environment of the Prepuna in the subtropical Andes.