Influence of canopy cover and amount of open habitat in the surrounding landscape on proportion of alien plant species in forest sites

:Most alien plant species are open-habitat species. Therefore, at the site scale, canopy openness should favour the germination and growth of alien species, and at the landscape scale the presence of open habitat should increase the seed rain and therefore the colonization rate of forest sites by alien species. We tested these two hypotheses by quantifying the proportion of alien understory plant species in 192 forest sites ranging in canopy cover and amount of open habitat in the surrounding landscapes. Our results supported both hypotheses, although the total amount of variance explained was low (12%). The proportion of alien plant species in forest sites increased with decreasing canopy cover at the site and with increasing amount of open habitat in the landscapes surrounding the forest sites. The effect of canopy cover was much stronger than the effect of amount of open habitat in the surrounding landscapes, suggesting that conditions for germination and growth are a stronger limitation on alien species incursion into these forest sites than is the availability of alien species propagules.