Pollinator limitation and resource limitation of seed production in the Scotch broom, Cytisus scoparius (Leguminosae)

Pollinator limitation and resource limitation of seed production were examined in flowers of a natural population of Scotch broom, Cytisus scoparius , by taking advantage of the floral characteristics that flowers tripped open by an effective pollinator were easily distinguishable from unvisited flowers. In total, 40.26% of flowers were visited by effective pollinators, 28.91% were pollinated and 13.75% matured fruits. Therefore, 71.80% of visited flowers were pollinated and 47.58% of pollinated flowers matured fruits. Thus, the most limiting factor for fruit production was pollinator visitation rate (pollinator limitation), and secondarily the process from pollination to fruiting further constrained fruit production (resource limitation). The pollinator visitation rate was significantly higher for plants growing in a sunny habitat than in a shady one, and pollinator limitation was more severe in the shady habitat than the sunny one. The proportion of fruit produced to pollinated flowers was not significantly different between the sunny and shady habitats, but was lower on larger plants, indicating that resource limitation was more severe on larger plants producing many more flowers.