Blurred park boundaries and the spread of English Ivy (Hedera helix L.): case studies from Greater Victoria, British Columbia
Author(s)Larocque, Krystal Lynn
Contributor(s)Edgell, M. C. R.
UVic Subject Index::Humanities and Social Sciences::Social Sciences::Geography
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AbstractThis 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.