Effects of roads and forest successional age on experimental plant invasions

Anthropogenic activities can promote plant invasions by changing habitat conditions and the dispersal of invasive plant propagules. Invasions are often more common near roads and in young habitats than in interior and mature habitats, but it is unknown if these patterns are due to differences in habitat conditions or to dispersal limitation. We tested if reduced colonization of interior and mature forests by invasive plants is due to unfavorable habitat conditions. We experimentally introduced seeds and seedlings of three invasive and three native species at six distances from roads in a series of young and mature forests. Seed germination and seedling survival, growth, and herbivore damage were quantified over three years. Exotics had higher germination rates than natives with the greatest difference in mature forests. Both native and exotic seedling growth declined with increasing distance from roads. Overall, seedling growth was much greater in young compared to mature forests. Despite growth rate differences, more than 75% of exotic and native seedlings survived over the experiment. Native species suffered significantly more herbivory than exotic species, suggesting that their growth may be suppressed by herbivory. In general, conditions along roads and in young forests promoted invasive shrub growth but invasions were not entirely limited by conditions in interior and mature forests. Our results show that two common landscape features, roads, and successional age heterogeneity, can influence the distribution of plant invasions. To better manage these invasive species, mature forests should be maintained and disturbances such as road–building should be avoided.