Slow plants in a fast forest: local dispersal as a predictor of species frequencies in a dynamic landscape

1 Regional distributions of plants could be determined by local dispersal processes or by extreme long-range dispersal. The importance of dispersal scale was examined by comparing regional frequencies of 36 forest herbs with predicted frequencies generated by a dynamic landscape model that only allowed local dispersal. The model was parameterized to land use turnover in south-eastern Pennsylvania and northern Delaware, USA, for the period of 300 years since initial forest clearance. 2 The model successfully predicted the frequencies of fleshy fruited and ant-dispersed forest herbs in the modern landscape, but failed to predict frequencies of adhesive species or of all species considered together. No attempt was made to predict the frequency of wind-dispersed species, which were infrequent in these forests. 3 Frequencies of species in the modern landscape appeared to be controlled by the time available for spread after emerging from a period of low habitat frequency in the 19th century. 4 Species dispersed by ingestion of fruits were more abundant than predicted, probably due to gap-crossing by animal dispersal vectors. Ingested species appear to have spread at a rate only achievable by modelled species in an artificially long time frame. 5 The predictive power of a model based on local dispersal implies that extreme long-range dispersal is rare on the time-scale of human land use and does not significantly influence regional distributions. 6 Predicted frequencies were less than or equal to 20% for all species, consistent with an observed decline in forest species diversity. Herb species impoverishment can be interpreted as a distinct vegetational condition created by 18th-century forest clearance, which will probably be detectable for at least another century. 7 Seed dispersal ability appears to be critical to the regional survival of forest herb species. At the spatial and temporal scales of human land use, slow-migrating species, and those unable to cross gaps, may be in danger of regional extinction.