Pollen limitation and fruiting failure related to canopy closure in Calypso bulbosa (Orchidaceae), a northern food-deceptive orchid with a single flower

Natural fruit set is constrained by pollen limitation and fruiting failure, and pollen limitation is expected to be especially severe in deceptive orchids. We performed hand cross-pollinations in ten populations of a food-deceptive orchid, Calypso bulbosa, under sparse and dense canopies in three non-consecutive years. We explored the relationships between natural fruit set, pollen limitation and fruiting failure. Mean natural fruit set over the years was 60%, which is exceptionally high for a deceptive orchid. On average, hand cross-pollination increased fruit set by 23%. Among open-pollinated plants that did not set a fruit, 55.5% were estimated to be pollen limited and 44.5% to be limited by fruiting failure, i.e. inability to set a fruit after pollination. In species with high natural fruit set, hand cross-pollination experiments may not always detect statistically significant pollen limitation. In our case, pollen limitation tended to become significant when the natural fruit set dropped below 60%. Canopy cover had a significant effect on fruiting failure, which was more severe under a dense canopy. Although our results demonstrate pollen limitation in many cases, they also highlight the fact that food deception can be a very effective pollination strategy.