In order to manage forest systems effectively, understand biological invasions, and predict community trajectories, it is important to understand the abiotic and biotic factors that influence establishment of introduced and native woody forest species. Pairs of introduced and native congeners are particularly useful in this context because they help us to distinguish establishment patterns associated with species introductions from those associated with common evolutionary history. We investigated establishment patterns of the introduced shrub Lonicera maackii, its native congener Lonicera canadensis, and the introduced shrub Rosa multiflora in a forest remnant in northeastern Ohio. We hypothesized that the introduced species show stronger growth responses to high light availability, have a stronger competitive response, and “escape” mammalian herbivory compared to the native species. In a split-plot design, we planted individuals propagated from stems under two light level treatments, gap and interior, at the whole plot level. At the subplot level, we applied herbivory and competition treatments in a factorial design. After two growing seasons, we harvested all plants and took growth measures. We here report height responses analyzed using quantile regression.
Rosa multiflora individuals had the largest mean final height (52.6 ± 32.0 cm), followed by L. maackii (20.4cm ± 16.1 cm) and L. canadensis (12.6 ± 8.3 cm). Gaps promoted growth of tall Rosa individuals, but did not affect the height of L. maackii or L. canadensis individuals. Smaller Rosa and intermediate L. canadensis individuals responded negatively to interior plots, but L. maackii did not respond to interior plots. Competition did not affect any of the focal species, and herbivory did not affect R. multiflora or L. maackii.
The ability of Rosa to grow quickly in gap sites suggests a possible mechanism for this species to spread within forest remnants. Growth of the other introduced species. L. maackii, was not promoted in gaps; however, this response was consistent with the response of its native congener, L. canadensis. Other mechanisms such as broad dispersal, high propagule output, and extended phenology may better explain establishment of this species. None of the focal species was affected by the presence of a competitor plant. However, in an environment with many competitors, the ability of plants like R. multiflora to quickly outgrow competitors may improve its chances of long-term survival.