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Prioritising ecological restoration sites and strategies in Hamilton City, New Zealand

Urban ecological restoration faces distinct challenges due to the highly modified biophysical conditions that characterise cities. To overcome these difficulties and assist in decision-making, ecological principles should underpin comprehensive analyses at both the landscape and site-specific scale. With increasing recognition of the role that the environment plays in social and economic prosperity, there are a growing number of strategies focused on increasing native habitat in urban centres. This thesis presents three integrated studies in support of Hamilton City Council’s goal under the Nature in the City Strategy to increase indigenous vegetation cover from about 1.8% to a minimum of 10% by 2050. Landform and ecological unit representativeness analyses in the city reveal the extent of under-represented environments, but it also presents opportunities to improve this criterion rapidly. Then, a multicriteria ecological prioritisation tool specifically developed for Hamilton City identified the most ecologically intact sites that could support the selection of potential gully and reserve restoration sites following the Nature in the City Strategy. Finally, at the site-specific scale, assessments of the vegetation composition and structure of three kahikatea forest remnants determined appropriate restoration strategies that may guide restoration projects in other cities. Landform and ecological unit areas in Hamilton City that would need to be restored to meet ecological representativeness thresholds under the 10% goal are presented. The peatlands, alluvial plains and hills landform and their respective ecological units are severely under-represented, while the gullies are sufficiently represented. However, gullies tend to contain more micro-environments and diversity than the other landforms, so restoring them may provide greater biodiversity rewards. Compared to previous representativeness studies of Hamilton City, this assessment provides more refined information available on a geographic information system (GIS) and highlights opportunities for its rapid improvement. For example, adequately representing some of the most under-represented ecological units, such as peat domes, would require the least restoration effort by extent (14 hectares (ha)). While improving representativeness could enhance the city’s pool of indigenous species, this study demonstrated that it should not be used alone to prioritise restoration sites in highly modified urban environments. Rather, representativeness could be included in a wider range of criteria and more appropriately employed for regional or national strategies. About 198 ha of Hamilton City is dominated by native vegetation, requiring the restoration of 886 ha or approximately 32 ha annually from 2023 to achieve the 10% goal by 2050. A prioritisation tool specifically developed for the city comprising eight ecological criteria identified 873 gully and reserve sites with the most intact ecological integrity that satisfy this area. These sites were allocated into five prioritisation categories to support the adoption of a staged approach for this city-wide restoration project. Sites that received high ecological scores included sections within Hamilton Gardens, Hammond Park, Minogue Park, Kirikiriroa gully, the major riverside gully and Mangaonua gully. Most high-ranking sites were found in the city’s east, although restoring sites on both sides of Hamilton City would enhance ecological connectivity on a regional scale. As the city expands, restoring large undeveloped areas would likely require fewer resources than restoring many dispersed isolated patches. Similarly, restoring whole systems, such as the high-ranking Kirikiriroa gully, is suggested to provide greater biodiversity rewards. Further application of this tool could incorporate social, economic and cultural criteria for a more holistic prioritisation of sites. The condition of kahikatea (Dacrycarpus dacrydioides, white pine) forest remnants at Totara Park, Hillcrest Park and Grove Park was examined and compared with Te Papanui (Claudelands Bush/Jubilee Park) as the reference site to identify restoration strategies. Age structure analyses found that Hillcrest Park comprises the oldest kahikatea population with an average age of 82 years, followed by Grove Park (70 years), Te Papanui (60 years) and Totara Park (32 years). The Kahikatea Green Wheel, life form and epiphyte analyses highlighted the importance of Totara Park’s high water table and sheltered conditions. While Te Papanui was found to support the most native vascular plants (64 species), Totara Park’s conditions have contributed to its greater species richness (41 species) than Hillcrest Park (15 species) and Grove Park (eight species). More native epiphytes were also identified at Totara Park (nine) than Te Papanui (six), Hillcrest Park (one) and Grove Park (none). Epiphytes absent from Te Papanui found at Totara Park may be due to the loss of the once abundant Dicksonia squarrosa (whekī), a prominent host. Differences between the native vascular plants found at Te Papanui and the case studies signal gaps in characteristic species of kahikatea forest that could be filled by relevant planting at each site. While Totara Park’s remnant requires a careful manipulation restoration strategy to gradually remove troublesome plants without disturbing its locally rare native flora, the ecological integrity of Hillcrest Park and Grove Park could improve most from buffer, ground cover and shrub tier plantings.
Type of thesis
The University of Waikato
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