Dust seed production and dispersal in Swedish Pyroleae species

Dust seeds are the smallest seeds in angiosperms weighing just about a few micrograms. Th ese seeds are characteristic of most orchids, and several studies have been performed on seed features, fecundity and dispersal of orchid dust seeds. In this study we examine seed features, seed production and seed dispersal in another plant group with dust seeds, the Pyroleae (Monotropoideae, Ericaceae), focusing on six species: Pyrola chlorantha , P. minor , P. rotundifolia , Chimaphila umbellata , Moneses unifl ora and Orthilia secunda . Seed production per capsule among these species was in the range between ca 1000 and 7800, and seed production per capsule bearing shoot was in the range between ca 7000 and 60 000. Combining our results with published information on pollen-ovule ratios suggests that these Pyroleae species have a generally effi cient pollination system. Th e most fecund species was P. minor , the only species among the investigated that is probably largely self-pollinating. Th e investigated Pyroleae species have a seed production comparable to the less fecund orchid species. We studied seed dispersal in the fi eld in one of the species, P. chlorantha . Despite the extremely small and potentially buoyant seeds, the vast majority of seeds are deposited close to the seed source, within a few meters. Further studies on the recruitment ecology of the investigated Pyroleae species are currently under way.