POPULATION-SIZE, POLLINATOR VISITATION AND FRUIT PRODUCTION IN THE DECEPTIVE ORCHID CALYPSO-BULBOSA

It has been proposed that in non-rewarding animal-pollinated plants th e pollination intensity should decrease with increasing population siz e and should increase with increasing local abundance of reward-produc ing plants. To test these hypotheses, we examined how population size, local abundance of Salix caprea, and tree cover were related to polle n removal and fruit production in 16 populations of the deceptive, ear ly-flowering and bumblebee-pollinated orchid Calypso bulbosa in northe rn Sweden in 3 consecutive years. To determine whether fruit productio n was limited by pollinator visitation, supplemental hand-pollinations were performed in three populations in 3 years. Finally, to examine w hether increased fruit production was associated with a reduction in f uture flower production, vegetative growth or survival, supplemental h and-pollination was repeated for 5 years in one population. The levels of pollen export, pollen deposition, and fruit set of C. bulbosa vari ed considerably among years and among populations. The proportion of p lants exporting pollen was negatively related to population size, and positively related to density of S. caprea and to tree cover in 1 of t he 3 years. In the other 2 years, no significant relationship was dete cted between proportion of plants exporting pollen and the latter thre e variables. In no year was there a significant relationship between f ruit set and population size, density of S. caprea and tree cover. The re was substantial among-year variation in the extent to which fruit p roduction was limited by insufficient pollen deposition and in the amo unt of weather-induced damage to flowers and developing fruits. Fruit set was consistently higher in hand-pollinated than in open-pollinated plants, but this difference was statistically significant in only one of 3 years. Supplemental hand-pollination in 5 consecutive years incr eased cumulative fruit production 1.8 times, but did not affect flower production, plant size, or survival. Tree cover was negatively correl ated with the incidence of frost damage in 1 year. The results indicat e that life-time seed production may be pollen limited in C. bulbosa, and that variation in population size and local abundance of the early -flowering, nectar-producing S. caprea can only partly explain the ext ensive variation in pollinator visitation among populations of this sp ecies.