SEED DISPERSAL AND THE HOLOCENE MIGRATION OF WOODLAND HERBS

The distribution of many woodland herbs extends 1000-2000 km in a nort h-south direction, yet the majority of these species grow clonally, ha ve little recruitment by seed, and possess no obvious mechanism for lo ng-distance seed dispersal. Although aware that woodland herbs dispers e poorly, ecologists have tacitly assumed that, given long periods of time, even small dispersal distances would allow woodland herbs to col onize the vast geographic region they now occupy. We examined this ass umption for the understory herb Asarum canadense. To estimate long-ter m rates of spread by seed, we calibrated seed-dispersal diffusion mode ls with life history data and with data on seed carries by ants. We su pplemented our field observations and modelling results for A. canaden se with a literature survey on the dispersal capabilities of other pla nt species. Ants transported A. canadense seeds up to 35 m, the larges t distance ants are known to move the seeds of any woodland herb. Empi rically calibrated diffusion models indicated that over the last 16000 yr A. canadense should only have traveled 10-11 km from its glacial r efugia. In actuality, A. canadense moved hundreds of kilometers during this time. Models that examined the tail of A. canadense's seed-dispe rsal curve indicated that occasional dispersal events had to have a hi gh frequency (greater than or equal to 0.001 on a per seed basis) and a large magnitude (dispersal distance >1 km) for A. canadense to have traveled over 200 km in 16000 yr. The literature survey showed that mo st woodland herbs and many other forest, desert, coastal, and open-hab itat plant species have limited seed-dispersal capabilities, similar t o those in A. canadense. We conclude that woodland herbs, as well as m any other plants, disperse so slowly that there is no documented mecha nism by which most of these species could have reached their present g eographical range since the last glacial maximum. This suggests that o ccasional events leading to long-distance dispersal dominate the Holoc ene colonization of northern temperate forest by woodland herbs, and t his, in turn, has implications for issues ranging from the importance of genetic analyses to the structure of metapopulation models.