Propagule size, dispersal ability, and seedling performance in Asclepias syriaca.

Using the propagules of common milkweed (Asclepias syriaca L.) we tested the predictions that decreasing size may increase dispersal ability, but also decrease the probability of seedling success, of wind-dispersed seeds. In 1982 and 1983 we released seeds from four milkweed clones at two heights in an open field and measured their dispersal distances. In the laboratory we measured falling times in a dead air space, seed mass and area, and coma mass and length of the same seeds. The seeds were later planted in a greenhouse and germination, mortality, and seedling dry mass were recorded.Seed mass was negatively correlated with dispersal distance in 1982, but not in 1983 under highly variable wind conditions. Coma mass/seed mass ratio was positively correlated with dispersal distance in 1982. During both years seed mass and coma mass/seed mass ratio were highly significantly correlated with falling time in dead air space. However, heavy seeds had superior germination, survivorship, and seedling mass at harvest, and seeds that germinated fell faster in dead air than those that did not. We found substantial variation in seed morphology both among clones and among pods within clones. Propagule falling times in dead air and (in 1982) dispersal distances in the field also differed significantly among clones and pods. The potential therefore exists for differential dispersal and establishment of milkweed genotypes.