Seed Dispersal and the Holocene Migration of Woodland Herbs

The distribution of many woodland herbs extends 1000–2000 km in a north–south direction, yet the majority of these species grow clonally, have little recruitment by seed, and possess no obvious mechanism for long-distance seed dispersal. Although aware that woodland herbs disperse poorly, ecologists have tacitly assumed that, given long periods of time, even small dispersal distances would allow woodland herbs to colonize the vast geographic region they now occupy. We examined this assumption for the understory herb Asarum canadense. To estimate long-term rates of spread by seed, we calibrated seed-dispersal diffusion models with life history data and with data on seed carries by ants. We supplemented our field observations and modeling results for A. canadense with a literature survey on the dispersal capabilities of other plant species. Ants transported A. canadense seeds up to 35 m, the largest distance ants are known to move the seeds of any woodland herb. Empirically calibrated diffusion models indicated that over the last 16000 yr A. canadense should only have traveled 10–11 km from its glacial refugia. In actuality, A. canadense moved hundreds of kilometers during this time. Models that examined the tail of A. canadense's seed-dispersal curve indicated that occasional dispersal events had to have a high frequency (≥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 most woodland herbs and many other forest, desert, coastal, and open-habitat plant species have limited seed-dispersal capabilities, similar to those in A. canadense. We conclude that woodland herbs, as well as many other plants, disperse so slowly that there is no documented mechanism by which most of these species could have reached their present geographical range since the last glacial maximum. This suggests that occasional events leading to long-distance dispersal dominate the Holocene colonization of northern temperate forest by woodland herbs, and this, in turn, has implications for issues ranging from the importance of genetic analyses to the structure of metapopulation models.