Potential for multiple lag phases during biotic invasions: reconstructing an invasion of the exotic tree Acer platanoides.
Perennial woody invaders often form persistent patches that significantly alter the structure and composition of native plant communities. Given their long generation times compared with ruderal invaders, these species may experience prolonged establishment phases between successful introduction and spread. Gap dynamics of shade-tolerant invaders could lead to multiple lag phases during the invasion process. In order to investigate the potential for long or multiple lag phases, we reconstructed the invasion of Acer platanoides (a shade-tolerant, invasive, exotic tree) on an 1130-ha temperate forested island in Lake Huron, USA. We measured and mapped the spatial location of every A. platanoides ≥0.5 m in height that had successfully established within a 728-ha forested park on the island. A simple age-diameter relationship, developed from a randomly selected subsample of the population, was used to assign an establishment date to each individual. Following a 34-year establishment phase, the area occupied by ≥1 A. platanoides ha-1 increased linearly at a rate of 5.6 ha year-1 for 35 years, after which range expansion slowed. Population growth lagged behind range expansion, with rapid population growth associated with infill between parents. During the expansion phase, numerous satellite populations established but contributed little to population growth and spatial expansion because of the long time required for them to become reproductive. These satellite populations will most probably accelerate population growth and spread once they reach reproductive age. Roads and trails provided important corridors for propagule movement away from developed areas over the course of the invasion. They also appeared to facilitate longer distance dispersals than would be expected given the biology of the species. Synthesis and application. Our results suggest that shade-tolerant invaders with long generation times may undergo long establishment phases as well as periodic lags during the expansion phase. These lags may provide windows of opportunity for control but could easily be misinterpreted as signs that the population has reached an equilibrium density or the geographical extent of its spread. Additionally, roads and trails may provide important corridors for movement of propagules via non-standard means of dispersal.