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Auckland’s urban forest canopy cover: state and change (2013-2016/2018)


Author:  
Nancy Golubiewski, Grant Lawrence, Joe Zhao, Craig Bishop
Source:  
Auckland Council Research and Evaluation Unit, RIMU
Publication date:  
2020
Topics:  
Environment

The structure and function of Auckland’s forests are important to the region for both the ecosystem services they provide and their intrinsic value. In recent years, a building boom resulting from economic growth and housing demand has resulted in land-use change that includes noticeable tree removals, especially in urban areas where development activities are both intensifying and expanding the built environment. Across Auckland, urban areas expanded eight per cent between 1996 and 2012 and a further four per cent between 2012 and 2018/19.

This project quantified the extent of tree canopy cover across 16 local boards in the central and mostly urban part of the Auckland region: Papakura, Manurewa, Ōtara-Papatoetoe, Māngere-Ōtāhuhu, Howick, Maungakiekie-Tāmaki, Ōrākei, Waitematā, Albert-Eden, Puketāpapa, Whau, Henderson-Massey, Upper Harbour, Kaipātiki, Devonport-Takapuna, and Hibiscus and Bays. Data collected between 2016 and 2018 were assessed and analysed to identify the characteristics of the canopy in this period and to detect any changes that have occurred since 2013. A canopy height model was produced from Auckland Council’s 2016/18 LiDAR data to serve as a comparable tree canopy cover to the 2013 one.

The extent of tree canopy is an issue of scale: describing coverage depends on the size of the area under consideration. That is, while regional canopy cover is 18 per cent, canopy cover ranges from 8 to 31 per cent across the 16 urban local boards. The composition of land cover and/or land use, and whether one dominates, in the particular boundary area (from parcel through suburb and local board to region) influences the aggregate tree cover. While 11 of the 16 local boards meet the minimum of 15 per cent canopy cover in Auckland’s Urban Ngahere (Forest) Strategy, the five south Auckland local boards in this study are under the 15 per cent minimum threshold for tree canopy cover.

A slight increase (0.5%) in urban forest cover compared to 2013 was detected across all the local boards. Gains and losses across the 16 urban local board areas largely balance each other out. At finer scales, however, more distinct net changes are detectable; for example, the net changes ranged from -8% to +14% at the local board level. Changes also vary according to land cover, land use, and tenure. Importantly, the characteristics of gains and losses differ: gains in canopy cover largely consist of biomass growth of existing vegetation throughout the entire tree canopy – small but ubiquitous instances of tree growth and crown expansion, whereas, tree canopy losses were a combination of small, widespread instances (e.g., from maintenance such as pruning and trimming) and discrete events, usually larger in area than the dispersed new growth. Discrete loss events are far more noticeable, and ecologically different, than small, dispersed growth.

This report on the tree canopy presents early findings of the first comprehensive, regional assessment of Auckland’s urban forests and its change over a three to five-year period. It provides an overall assessment, from which more detailed studies can build in the future.

Auckland Council technical report, TR2020/009

July 2020