Print PageSpecies distribution patterns represent the outcome of an ecological filtering process, including limitations imposed by dispersal, abiotic conditions, and biotic interactions. For plants, most models of species distributions focus almost exclusively on abiotic factors, like soils and climate, with the implicit assumption that distribution limits resulting from dispersal are likely only as transitory phenomena. However, recent studies focused on the forests of eastern North America and western Europe indicate that the large-scale geographic distributions of some plant species may be dispersal-limited over long periods of time, likely due to low seed production and diaspores specialized for small-scale dispersal. In this study, we investigated the role of dispersal limitation in the local and large-scale distribution of the ant-dispersed forest herb Jeffersonia diphylla in the Northeast U.S. At a local scale, seed sowing transects were established along the margins of three populations in central New York to assess the relative importance of dispersal vs. abiotic limitation in determining the locations of population edges. At a regional scale, seeds were planted at three study sites ~300 km beyond the species central New York range edge to test for the existence of suitable, but unoccupied, habitat in western Massachusetts.
Results/Conclusions
Seed sowing beyond local population margins resulted in the establishment of juveniles outside population areas, at distances up to 100 times the mean distance from naturally occurring seedlings to mature plants. There were no declines in germination or growth rates relative to distance from population margins, suggesting that local population expansion is significantly limited by dispersal. At the regional scale, seed sowing beyond the range edge resulted in the establishment of juvenile plants that have survived and grown at significantly higher rates than plants in control plots within the range over a 5-year period (e.g., mean total leaf area per plant: 50.0 vs. 18.0 cm2 in 2010). Our results suggest that highly suitable, but unoccupied, habitat for J. diphylla exists at a range of spatial scales, and that dispersal limitation is a key factor influencing the species local and regional distribution. Further, the species northeastern range edge does not appear to be linked to clear abiotic limits; rather, the species absence from large areas of apparently suitable habitat may trace to limited post-glacial range expansion. These findings underscore the potential for dispersal to substantially limit some species abilities to respond to rapid climate change in the future.
