Composition and structure of the urban forest, North Shore, Auckland

Abstract

Urban forests can enhance both biodiversity values and human quality of life. For example, they provide resources for local fauna, regulate the urban heat island effect and improve air quality through the removal of pollutants. Human contact with the urban forest has also been associated with higher levels of physical and psychological well-being. Despite the important role of urban forests in improving the environmental quality of urban areas, relatively little ecological research has been undertaken in this field. Private residential gardens in particular have been largely neglected by researchers, even though these areas constitute a major component of green space in many towns and cities. The overall aim of this research was to determine the composition and structure of the Auckland urban forest within residential gardens, and elucidate key factors affecting variation in forest characteristics. This was achieved by describing the woody vegetation in 66 North Shore gardens and investigating how factors such as section size, suburb age and proximity to native forest fragments influence plant composition, tree size, basal area and density. The North Shore urban forest was found to have high levels of species richness and a high beta diversity. Although most exotic species are rare, they dominate the garden flora. Important structural characteristics of this forest include low tree density, a consistently low canopy height of around 4.5 m, a high proportion of small, multistemmed trees and a lack of large, tall individuals. The newest suburbs contain fewer weeds, smaller trees and a higher density of native stems. No significant effect was detected in the composition of the urban forest in relation to proximity to native forest fragments. Larger sections – with an increased area and proportion of greenspace – contain more plant species, support a higher basal area, and include larger and taller trees than smaller sections. This key finding implies that continuance of the current trend towards more intensively developed residential areas with smaller section size and lower proportions of greenspace per section will lead to a reduction in the size and species richness of the urban forest, resulting in reductions in its associated environmental benefits. Such changes must be of concern to urban planners.