Invasion of English ivy (Hedera spp., Araliaceae) into Pacific Northwest forests

Invasive plants present a major threat to native plant communities and agricultural systems. Despite potential ecological and economic impacts, many questions remain regarding the taxonomic identity of invasive plant populations, the horticultural sources of these plants, and the ecological factors underlying their demographic expansion. To address such issues, I investigated the invasion biology of English ivy occurring in a natural forest park near Seattle, Washington.; Ploidy analysis via flow cytometry revealed tetraploid H. hibernica to be the primary forest invader despite the frequent occurrence of diploid H. helix in nearby suburbs. Individuals of H. helix were small and rarely reproductive, in contrast to sampled plants of H. hibernica. Total fruit set by H. hibernica was 5 times higher on suburban than forest reproductives, suggesting that cultivated plants remain an important source for the invasion of natural areas. However, fruit set in both areas was greatly reduced from the possible maximum, due to a low proportion of developed fruits---a possible consequence of the tetraploid chromosome number of H. hibernica.; Surveys of the forest park suggested that recruitment of H. hibernica varied across environments. Seedlings occurred primarily in riparian and lakeside areas while juvenile and reproductive plants were encountered mostly in lakeside and upland habitats. Lakeside, riparian and upland habitats had distinct vegetation and soil composition, indicating, that the occurrence of H. hibernica was correlated with specific plant associations and soil moisture levels. Frugivorc occurrence did not vary significantly across forest environments, suggesting that environmental factors rather than dispersal limitation were responsible for differential recruitment at a local spatial scale.; Overall, the studied population of H. hibernica displayed demographic Features consistent with rapid population growth, including high levels of fruit production and seedling recruitment coupled with high survivorship across life history stages. Potential ecological impacts of this demographic expansion were revealed by vegetation surveys, which found a clear reduction in native plant species diversity in the proximity of H. hibernica . This trend was most apparent for plots placed within dense H. hibernica vegetation but was also observed in plots directly adjacent to H. hibernica patches.