Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 102-113). Also available on the World Wide Web.

As human development occurs in forested areas, forests become fragmented into small islands in a matrix of urban land uses. This study examined the effect of surrounding urban land uses on the native and exotic plant species found on the edges of forest fragments in the Portland, OR metropolitan region.

This thesis examines the invasiveness of English ivy (Hedera helix L..: Araliaceae). an exotic horticultural species, in 14 near-urban parks in Greater Victoria. British Columbia. Using descriptive notes from field observations, the overall invasiveness of H. helix is assessed in each park, particularly near park boundaries. Land use associated with the fragmentation of natural habitat directly outside each park is characterised and related to invasion inside the park. Only three of the 14 representative parks examined are not invaded by P. helix. and four are very extensively invaded. ' The analysis of administrative park boundaries supports the hypothesis that H. helix begins invasion inside park boundaries that are adjacent to established residential areas. H. helix is found in moist forest communities of grand fir (Abies grandis) bigleaf maple (Acer macrophyllum) and western redcedar (Thuja plicata). Communities c f Garry oak (Quercus garryana) with black hawthorn (Crataegus douglasii) and English hawthorn (C. monogyna) are heavily invaded and vulnerable to invasion' However. Douglas-fir (Pseudotsuga menziesii) forest communities are most heavily invaded and especially at risk of invasion. H helix climbs at least 17 species of trees and tall shrubs. with Douglas-fir trees providing the tallest supports. Other areas in parks vulnerable to invasion by H. helix include woodlands with rich soils, slight canopy gaps. windthrown forest edges, park entranceways and accessways. During the growth season, H. helix shoots were monitored in both heavily and less invaded sites. On average, shoots on the forest floor grew 22 cm per month. and on host trees, shoots grew 17 cm per month. Another growth characteristic of H. helix is that where it is long established on host trees, its stems have radial growth rings viewable in cross-section. These rings are likely annual and sensitive to annual climatic variability. The spread of an introduced liana, a plant form not present in the indigenous flora, has several implications for near-urban forest ecology including altered physical forest structure, hastened tree death and suppression of understory species (e.g. seedlings and shrub species such as salal - Gaultheria shallon and possibly red huckleberry - Vaccinium parvifolium). The increased concentration and range of exotic, horticultural species such as H. helix, in near-urban park and forest fragments, signifies that an exotic species management strategy is urgently needed for habitat and ecosystem conservation.

Thesis (doctoral)--Universität Bern, 1913. / Includes bibliographical references.

Invasive species degrade ecosystems by altering natural processes and decreasing the abundance and diversity of native flora. Communities with major fluctuations in resource supply allow invasive species to exploit limiting resources making the community prone to invasion. In the Pacific Northwest, urban forests characterized with limited light and seasonally limited soil moisture are being dominated by nonnative English ivy (Hedera spp). Three observational studies were conducted in the Southern end of Forest Park within the Balch Creek Subwatershed in Portland, Oregon in order to understand 1) how English ivy changes over three growing seasons, 2) how the native understory composition responds to English ivy, 3) if the dominance of English ivy reduces soil moisture to neighboring plants, 4) how English ivy and two co-occurring native herbs (Hydrophyllum tenuipes and Vancouveria hexandra) physiologically respond to seasonal changes in soil moisture. Percent cover of the understory community was collected in both 2010 and 2013 growing seasons in 54 plots in order to understand the change in cover over time. Community response and the relationship with soil moisture was analyzed using percent cover of the understory community and associated environmental variables including soil moisture collected in 128 plots during the 2013 field season. Finally, 15 plots with co-occurring Hedera spp, H. tenuipes and V. hexandra were sampled for stomatal conductance, leaf water potential, and associated environmental variables. Results show ivy cover increases on average 14% between 2010 and 2013 while native understory cover increased on average < 1%. Once ivy forms dense cover over 44% there is a reduction of native richness, diversity and herb cover while also an increase in available soil moisture and deciduous canopy cover. There were disparate impacts to different functional groups and between species. As functional group, the herbaceous community was the most impacted by ivy invasion. The shrubs and fern community had a variable response to ivy invasion. Many of the fern and shrub species least impacted by ivy also had associations with high soil moisture and deciduous canopy cover. Finally, data suggests that ivy does not take advantage of seasonally limiting soil moisture to invade the understory community. This study indicates that English ivy is both efficient at water use and may have the ability to obtain water from distant locations throughout the forest. Once established, ivy has the ability to alter the community composition. Ivy removal and habitat restoration are essential in order to maintain and enhance biodiversity in Forest Park.

Increased nitrogen deposition has been shown to promote the dominance of invasive species, and nitrogen deposition rates have steadily increased in most of the Western United States in recent years due to population increases. The purpose of this study was to determine if increased rates of nitrogen deposition are contributing to the success of Hedera helix in the Pacific Northwest. Plots were established in Lesser Park in Portland, Oregon and received monthly treatments of ammonium nitrate for one year. Growth, measured as change in percent cover, was compared between treated and untreated plots for both H. helix and native species. Additionally, a greenhouse experiment was employed wherein H. helix was grown in shared pots with two native species, Fragaria vesca and Polystichum munitum, and three treatment levels were compared; none, low, and high nitrogen. Relative growth rate was compared between treatments for each species after twelve weeks of treatment. Results from the greenhouse experiment were highly variable and no general conclusions could be drawn about the effects of increased nitrogen deposition on competition between H. helix and native species. In the field, treatment did not have a significant effect on growth but species did have a significant effect, as H. helix had a greater increase in percent cover than native species regardless of treatment. However, a marginally significant interaction was found between species and treatment. Native species cover actually increased in plots that received nitrogen addition and decreased in control plots, while H. helix had a slightly higher increase in cover in control plots. Though the growth of H. helix was significantly higher than the growth of native species in control plots, nitrogen addition caused the growth rates to converge towards more similar means in treated plots. Results indicate that increased nitrogen deposition may actually have a positive effect on the growth of native species by reducing the invasive potential of H. helix.