Abstract:
Strategies to encourage urban biodiversity are becoming increasingly important as the population
of people living in cities continues to grow and urbanisation expands. Urban forests consist of all
the trees and lower vegetation within cities and play fundamentally important roles in supporting
urban biodiversity, improving the environmental quality of urban areas and the aesthetics of
urban landscapes. A key part of the urban forest is the large tree resource, as large trees are
known to be keystone structures in many landscapes, acting as ‘islands’ for associated
biodiversity.
Auckland, New Zealand, has a rapidly changing landscape, resulting in a reduction of green
spaces and habitat for ecologically important features such as large trees. Recent policy changes
have reduced blanket protection of large trees in urban areas, leading to an ongoing reduction in
large tree abundance in Auckland. This study characterized the large tree resource in urban
Auckland and determined the importance of large trees for a range of biodiversity. The Auckland
urban forest was compared using a 2013 LiDAR analysis across sixteen Local Board Areas and
the height distribution of trees located on various land types. Large trees were unequally
distributed across different LBAs, but this variation was not correlated with several social and
economic variables tested except for the time since urbanisation began. LBAs with younger
suburbs generally had more large trees, suggesting large trees are lost as urbanisation progresses.
Large trees were predominantly found on public parkland.
A subset of urban trees were then examined over a range of diameters, of four large tree species,
located in urban parks. These trees were categorised into size classes, and compared for bird
species interactions, presence of microhabitats, bat roost potential, epiphyte presence and habitat
features used by pest mammals. More birds (abundance) and more species of birds (species
richness) interacted with large trees than smaller tree size classes. There was no significant difference between native and exotic tree species. Microhabitat features and vascular epiphytes
were found predominantly on large trees compared to other tree sizes. Indications of pest
mammal presence, however, was not associated with tree size class.
This study provides evidence towards understanding why we require large trees to support the
urban ecosystem and the biodiversity found within it. Recommendations from this research
consider appropriate regulations around large trees that recognize their ecological importance for
associated biota and the urban environment. Therefore, this information can be used by a variety
of stakeholders in Auckland city to better manage and protect large trees and ensure a size
diverse urban forest.