Abstract
A city’s planted trees, the great majority of which are in private gardens, play a fundamental role in shaping a city’s wild ecology, ecosystem functioning, and ecosystem services. However, studying tree diversity across a city’s many thousands of separate private gardens is logistically challenging. After the disastrous 2010–2011 earthquakes in Christchurch, New Zealand, over 7,000 homes were abandoned and a botanical survey of these gardens was contracted by the Government’s Canterbury Earthquake Recovery Authority (CERA) prior to buildings being demolished. This unprecedented access to private gardens across the 443.9 hectares ‘Residential Red Zone’ area of eastern Christchurch is a unique opportunity to explore the composition of trees in private gardens across a large area of a New Zealand city. We analysed these survey data to describe the effects of housing age, socio-economics, human population density, and general soil quality, on tree abundance, species richness, and the proportion of indigenous and exotic species. We found that while most of the tree species were exotic, about half of the individual trees were local native species. There is an increasing realisation of the native tree species values among Christchurch citizens and gardens in more recent areas of housing had a higher proportion of smaller/younger native trees. However, the same sites had proportionately more exotic trees, by species and individuals, amongst their larger planted trees than older areas of housing. The majority of the species, and individuals, of the larger (≥10 cm DBH) trees planted in gardens still tend to be exotic species. In newer suburbs, gardens in wealthy areas had more native trees than gardens from poorer areas, while in older suburbs, poorer areas had more native big trees than wealthy areas. In combination, these describe, in detail unparalleled for at least in New Zealand, how the tree infrastructure of the city varies in space and time. This lays the groundwork for better understanding of how wildlife distribution and abundance, wild plant regeneration, and ecosystem services, are affected by the city’s trees.
Highlights
Planted trees form a natural foundation for urban ecosystems (Lawton, 2007)
Avonloop had the most trees mapped per hectare (21.8 trees per hectare, including 7.9 native trees and 13.9 exotic trees), New Brighton followed with 19.5 trees per hectare and third was Linwood with 17.8 trees per hectare
Planting changes in the residential red zone Diameter at Breast Height (DBH) (Diameter at breast height) of exotic and native trees Comparing the DBH distributions of all exotic and all native trees showed that trees with large DBH were more likely to be exotic (Fig. 6)
Summary
Planted trees form a natural foundation for urban ecosystems (Lawton, 2007). The density, age, health, species traits, and spatial arrangement of a city’s trees all play important roles in determining a city’s wild biology, ecosystem functioning, and ecosystem services (Rowntree & Nowak, 1991; McPherson & Rowntree, 1993; Lawton, 2007). It has been suggested that using native plants in different urban habitats can make contribution to attract and conserve wildlife in cities (McKinney, 2002; Tallamy, 2007). These suggestions came from the historical co-evolution of native plants and native insects, which will help native insects feed or reproduce on native plants more effectively (Comba et al, 1999). Several studies demonstrated the correlations between native plants and native insects and birds such as Lepidoptera larvae (Burghardt, Tallamy & Gregory Shriver, 2009; Tallamy & Shropshire, 2009), pollen and nectar feeding bees (Hopwood, 2008) and native insectivorous and frugivorous birds (Burghardt, Tallamy & Gregory Shriver, 2009) to support the conclusion that enhancing the biomass and diversity of native plants will increase the diversity and abundance of insects, which will affect on bird communities (Tallamy, 2004; Burghardt, Tallamy & Gregory Shriver, 2009)
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