Forest disturbances promote invasion of alien herbaceous plants: a comparison of abundance and plant traits between alien and native species in thinned and unthinned stands

Invasion of alien plant species can have irreversible effects on ecosystems. Although alien plants often distribute in disturbed area, understanding of the initial invasion process soon after disturbance is poor. We compared forest floor vegetation between thinning treatment plots and intact control plots in a red pine (Pinus densiflora) forest in central Japan to test whether the thinning treatment (i.e. disturbance) induced invasion of alien plants. We also examined how invasion of aliens is influenced by the environment, plant traits, and buried seeds in the soil. In control plots, few alien plants emerged at the herbaceous layer and only a few buried seeds of alien plants were found. In treatment plots, on the other hand, many alien plants were observed both at the herbaceous layer and in the soil seed bank. Treatment plots had a larger percentage of canopy openness than control plots. In treatment plots, the cover of alien plants was negatively correlated with the distance from the nearest primary road. Alien plants had taller maximum plant height, larger leaves, and greater leaf nitrogen concentration than native species in treatment plots. This indicates that alien plants were superior to native plants in growth, competition, and carbon assimilation abilities. Alien plants tended to have seeds with long dispersal abilities, such as anemochory and zoochory, and short life histories, such as annual and biennial histories. These findings suggest that thinning promoted invasion of alien plants and that alien plants had the potential for further invasion by rapid dispersion, establishment, and growth.