Factors that influence invasion success of two woody invaders of forest understories

I examined factors that affect the establishment of two woody invaders of forest communities: Rhamnus cathartica (native to Europe and invasive in North America) and Prunus serotina (native to North America and invasive in Europe).; Surveys of invasion patterns of both species at the forest stand scale showed positive relationships between invasion and resident understory diversity and negative relationships between invasion and resident understory cover. This suggests that sites that are favorable for colonization by many native species are also favorable for colonization by invasives. However, cover-related competition from native plants may inhibit colonization. The relationship between diversity and invasion was scale-dependent: in small patches, both native diversity and native cover had negative relationships with invasion of R. cathartica.; A field experiment showed effects of canopy-tree and herbaceous-species competition. Growth and survival of R. cathartica was greater in canopy gaps than in closed-canopy areas. Herbaceous species decreased growth of R. cathartica in canopy gaps, decreasing light and soil NO3 for R. cathartica seedlings. These results, combined with the survey results, suggest that shady forests may be less invisible and that an increase in the cover of herbaceous plants at any particular site should decrease invisibility.; A second field experiment and a greenhouse experiment showed that mature R. cathartica trees facilitate their seedlings through their affects on N cycling and possibly on soil biota. These trees also had positive effects on herbaceous species, casting doubt on the ecological importance of allelopathy of R. cathartica.; In a growth chamber experiment, P. serotina showed growth patterns consistent with escape-from-natural-enemies: inhibition by soil pathogens in North American soils but not in European soils. However, R. cathartica showed the opposite pattern, encountering new enemies, or biotic resistance, in its invaded North American range. Its growth was inhibited by soil pathogens in North America at all tested light levels while its growth in European soils only showed inhibition at low light. R. cathartica roots showed greater infection by pathogens or saprophytes in soils from North America, corroborating these results. These results show that escape from soil pathogens may not explain the success of some invasive species.