| The sand dunes of the Great Lakes are exposed to continuous natural disturbance and may be vulnerable to invasion by exotic species. In this study, the effects of an introduction of more than 25,000 Austrian pines (Pinus nigra), planted in 1956-1972 as a stabilization measure, were investigated on the Lake Michigan sand dunes in Allegan County, Michigan. The post-planting establishment success of viable trees, their biotic and abiotic effects on the dune system, and their reproductive capacity, and recruitment success were examined in four seral stages: foredunes, forest edges, wetpannes (ephemeral dune ponds), and inland blowouts.;Pinus nigra inhibits the growth of native dune vegetation in all four seral stages, and depresses species richness by up to 80% relative to sites lacking P. nigra in all stages but foredunes. It provides colonization sites for forest species absent from the adjacent dune communities, however, and a higher density of woody seedlings and saplings such as Sassafras albidum occur under pine canopy at the edge of native forest than in comparable sites lacking P. nigra. Additionally, wetpannes surrounded by P. nigra were drier than the native Pinus banksiana wetpannes and were characterized by a higher density of shrubs.;Pinus nigra foredune seed production per tree is 10-40 times that of other seral stages, but survivorship of seedlings is five times higher in blowouts and wetpannes. Light availability is positively correlated with survivorship in all stages (Spearman Rho = 0.52) and is postulated to regulate recruitment of seedlings after initial germination. Demographic projection models indicate that population growth rates are highest in wetpannes (lambda = 0.998 to 1.201), despite the higher understory cover and species richness of this seral stage. These findings suggest that P. nigra has made the transition from introduction to invasion on the sand dunes of Lake Michigan.;Measures of establishment success (height, dbh, and stem volume) indicate that P. nigra grows as well or better on the dunes than in its native range. Tree size differs more within seral stages than among them, indicating that the coarse-scale differences in environmental factors among stages, such as soil moisture and wind exposure, account for only some of the variation in growth. The broad ecological tolerance of P. nigra appears to be a major attribute in its successful establishment in a variety of dune conditions. |
